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BBC Microcomputer Advanced User Guide

1. 10 REM Enable RS423 input 20 AEX 272 30 REM Set baud rates to 4800 40 XEX 7 6 50 XEX 8 6 60 REM Disable escape 70 FX 229 1 80 REM This to be an 80 column VDU 90 MODE 3 100 OSBYTE amp FFF4 110 REM Main loop 120 REPEAT 130 REM Set next OSBYTE to insert in buffer 140 A S 138 X S 2 150 REM If character in input buffer 160 IF ADVAL 1 gt 0 AND ADVAL 3 gt 0 THEN Y GET CALL OSBYTE 170 REM Set next input to read RS423 180 FX 2 1 190 REM Check RS423 buffer 200 IF ADVAL 2 gt 0 THEN VDU GET 210 REM Restore old state 220 FX 2 2 230 REM Forever so 240 UNTIL FALSE This program continually scans the RS423 input and keyboard input buffers Whenever the keyboard input buffer is not empty and the RS423 buffer not full the character is transferred to the RS423 output buffer Whenever the RS423 input buffer is not empty that character is transferred to the VDU Note that for a real application this program would usually be translated into machine code 14 2 4 Writing to the cassette port The user may wish to have direct control over the cassette port for doing such things as reading tapes from other computers or writing protected tapes for the BBC When writing to the cassette port using the RS423 system it is not as simple as merely directing the RS423 to transfer to the cassette port as the cassette port is controlled differently to the RS423 port The main difference applies to the RTS line see
2. amp amp O m Gy amp 08 8 Variable Variable Variable MODE 2 Tao EE ESE Era EA 83001 83009 LLL se ee to Me CS ete oa ca ad Tee tt he Sa gee ae il nee ee aed As gate een es She EE oe ees es GS RO ek en ee OE ne DES eh rate See amp 3280 amp 3281 amp 7B06 BO Mr a ae pees es es Se ee amp 7D80 amp 7D88 o 22L A E TE ee See a E a ms Da da US a Ole En ei pe can eae alee a ee ae Le ee A ta ee apes gece meena STi A amp 7087 amp 7D8F 2 PIXELS 4 BITS PIXEL N B The screen layout is only as shown after a clear screen it will change when the screen is hard scrolled 467 MODE 3 Screen layout Graphics Colours Text Registers RO R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 468 Not available 2 80 x 25 Horizontal total Characters per line Horizontal sync position Horizontal sync vvidth Vertical sync time Vertical total Vertical total adjust Vertical displayed characters Vertical sync position Interlace mode bits 0 1 Display delay bits 4 5 Cursor delay bits 6 7 Scan lines per character Cursor start blink type Cursor start bits 0 4 Cursor blink bit 6 Cursor type bit 5 Cursor end Screen start address Cursor position Light pen position amp 50 Dom QO amp 02 N amp o N II amp 00 amp amp om 5 6S amp 67 amp 07
3. EAST selects printer strobe on printer connector selects 6522 CA2 connected to printer strobe circuit board track not fitted to issues 1 3 enables Econet NMI disables Econet NMI wire link NEVER fit this link with 1C91 fitted Selects the clock base frequency for Econet This bank of 9 links is only fitted on issues 1 3 5 25inch DISC select 8 inch DISC select circuit board track enable Econet clock disable Econet clock only fitted to issues 1 3 divide Econet clock by 2 divide Econet clock by 4 only fitted to issues 1 3 connects pin 30 of 8271 FDC chip to 5 volts connects pin 30 of 8271 FDC chip to 0 volts 483 10 11 12 13 14 484 OPEN CLOSED OPEN CLOSED Note WEST EAST CLOSED OPEN Note CLOSED OPEN Note OPEN CLOSED Note disconnects disc head load signal from PL8 connects disc head load signal to PL8 enables disc NMI disables Disc NMI this link must be OPEN when IC78 is fitted select 5 25inch disc select 8inch disc This block of 8 links selects the Econet ID The NORTH connection is the least significant bit A different Econet ID will be assigned to each station on an Econet system Refer to the Econet manual for more details connects ROM select line A to Ov model A ROM select line is driven by IC76 model B do NOT make this link with IC76 fitted connects ROM select line B to Ov model A ROM select line is
4. Figure 22 19 Interrupt Flag Register 414 REG 14 INTERRUPT ENABLE REGISTER ATELE CA2 CA1 SHIFT REG c82 0 INTERRUPT DISABLED CB1 1 INTERRUPT ENABLED TIMER 2 TIMER 1 SET CLEAR NOTES 1 IF BIT 7 IS A 0 THEN EACH 1 IN BITS 0 6 DISABLES THE CORRESPONDING INTERRUPT 2 IF BIT 7 IS A 1 THEN EACH 1 IN BITS 0 6 ENABLES THE CORRESPONDING INTERRUPT 3 IF A READ OF THIS REGISTER IS DONE BIT 7 WILL BE 1 AND ALL OTHER BITS WILL REFLECT THEIR ENABLE DISABLE STATE Figure 22 20 Interrupt Enable Register 415 416 23 The System VIA Sheila addresses amp 40 amp 4F The System VIA is responsible for a large amount of control within the BBC Micro itself It controls the speech system sound system and keyboard Also several other sections can be partially controlled from this VIA These are the hardware scrolling vertical sync pulse interrupt joysticks input end of conversion input from the ADC and a light pen strobe input 23 1 System VIA line allocation PAO PA7 The 6502 CPU does not talk to the speech system sound generator or keyboard directly over its data bus Instead it writes to and reads from the 8 bit port A I O lines This forms a slow databus over which the CPU can communicate To write to this databus the data direction register A at Sheila amp 43 should set all lines as outputs The 6502 can then write directly into output register A at Sheila am
5. LINE THIS IS SOME TEXT results in THIS IS SOME TEXT being printed out See also CODE 17 2 12 LOAD lt filename gt lt address gt L A file may be loaded into memory from the selected filing system using the LOAD command If the load address is not specified in the command line then the file will load at its start address this is usually the address from which it was saved 2 13 MOTORn M OSBYTE with A amp 89 137 This command executed with n 0 opens the cassette relay i e switches the motor off and with n 1 closes the cassette relay i e motor on 2 14 OPTx y O OSBYTE with A amp 8B 139 This command is highly filing system specific and although the general protocol of the cassette filing system is usually adhered to other filing systems may interpret this command differently and expand upon it For the cassette and ROM filing systems OPT 0 0 restore OPT default values OPT 1 0 turn off filing system messages OPT 1 1 turn on filing system messages non extended OPT 1 2 turn on extended messages OPT 2 0 errors ignored though messages may be given OPT 2 1 on error prompt for re try OPT 2 2 on error abort OPT 3 n _ set interblock gaps to n 10 seconds only relevant to cassette SAVE operations When extended messages have been selected the following information is printed on the screen on completion of the filing system operation file name block no file lengt
6. 3 N NA N N Oy ll tl 417 l LL LE amp amp O ea Gy amp 09 9 Variable Variable Variable MODE 3 EES ee eee Cesena as ara ROL SE ES Rage SALES NESE SERS ee ENEA wo a EE DRET ESE La MEG ee eg oe eee ae ee i 84281 beta Site ee En 7 eL amp 7E78 ESE eee es SE EES EE eee eR ES ree ey bate et ts DE een yee eaten eA Lee een cen te RON ep ee amp 7co6 87COE LL 8 amp 7co7 _ amp rco ae ETE N EES ae 8 PIXELS 1 BIT PIXEL N B The screen layout is only as shown after a clear screen it will change when the screen is hard scrolled 469 MODE 4 Screen layout Graphics Colours Text Registers RO R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 470 320 x 256 2 40 x 32 Horizontal total Characters per line Horizontal sync position Horizontal syne width Vertical sync time Vertical total Vertical total adjust Vertical displayed characters Vertical sync position Interlace mode bits 0 1 Display delay bits 4 5 Cursor delay bits 6 7 Scan lines per character Cursor start blink type Cursor start bits 0 4 Cursor blink bit 6 Cursor type bit 5 Cursor end Screen start address Cursor position Light pen position R Q Es gon D amp 28 A wa A Hi MN R GC oOo Oo Ner amp 26 amp 00
7. FX 11 0 X 0 Disables auto repeat facility FX 11n X n Sets delay to n centiseconds n 32 is the default setting After call A is preserved X contains the old setting Y and C are undefined 127 OSBYTE amp 0C 12 FX 12 Set keyboard auto repeat rate Entry parameters X determines auto repeat periodic interval Y 0 FX 12 0 X 0 Resets delay and repeat to default values FX 12 n X n Sets repeat interval to n centiseconds n 8 is the default value After call A is preserved X contains the old FX 12 setting Y and C are undefined 128 OSBYTE amp 0D 13 FX 13 Disable events Entry parameters X contains the event code Y 0 FX 13 0 X 0 Disable output buffer empty event FX 13 1 X 1 Disable input buffer full event BX 13 2 X 2 Disable character entering buffer event FX 133 X 3 Disable ADC conversion complete event FX 13 4 X 4 Disable start of vertical sync event FX 135 X 5 Disable interval timer crossing 0 event FX 13 6 X 6 Disable ESCAPE pressed event FX 13 77 X 7 Disable RS423 error event FX 13 8 X 8 Disable network error event FX 13 9 X 9 Disable user event For more information about events see chapter 12 After call A is preserved X and Y contain the old enable state 0 disabled C is undefined 129 OSBYTE amp 0E 14 FX 14 Enable events Entry parameters X contains the event code Y not important FX 14 0 X 0 Enable output buffer empty event PX 14 1 X 1 Enable
8. OSBYTEs B8 184 and amp B9 185 Read video processor ULA registers OS copies only lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y See OSBYTE calls with A amp 9A and A amp 9B and the Video Hardware chapter 19 The last value written to the ULA registers can be read using this method These calls should not be used to write to these locations because to do so would make the internal operating system copy of the registers inconsistent with the actual register contents 193 OSBYTE amp BA 186 Read ROM number active at last BRK error lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains the ROM number of the paged ROM that was in use at the last BRK 194 OSBYTE amp BB 187 Read number of ROM socket containing BASIC lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y Basic is recognised by the fact that it is a language ROM which does not possess a service entry This ROM is then selected by the BASIC command see section 2 4 If no BASIC ROM is present then this location contains amp FF See Paged ROMs chapter 15 195 OSBYTE amp BC 188 Read current ADC channel lt NEW VALUE gt lt OLD VALUES AND Y EOR X The ol
9. Select printer destination Entry parameters X determines print destination FX 5 0 FX 5 1 FX 5 2 FX 5 3 FX 5 4 PX 5 5 255 After call A is preserved nen Ne x PE XK 4 5 Printer sink printer output ignored Parallel output default setting RS423 output will act as sink if RS423 is enabled using OSBYTE with A 3 User printer routine see Vectors 10 6 Net printer see Vectors 10 7 5 255 User printer routine see Vectors 10 6 X contains the previous FX 5 setting Y and C are undefined Interrupts are enabled by this call This call is not reset to default by a soft break 121 OSBYTE amp 06 6 FX 6 Set character ignored by printer Entry parameters X contains the character value to be ignored FX 6 10 X 10 This prevents LINE FEED characters being sent to the printer unless preceded by VDU 1 this is the default setting after call A is preserved X contains the previous FX 6 setting Y and C are undefined 122 OSBYTE 8 07 7 FX 7 Set RS423 baud rate for receiving data Entry parameters X determines transmission rate EX 7 1 X 1 FX 7 2 2 EX 7 3 X 3 EX 7 4 X 4 FX 7 5 X 5 EX 7 6 X 6 EX 7 7 X 7 EX 7 8 X 8 After call A is preserved X and Y contain the old C is undefined 5 150 300 1200 2400 4800 9600 19200 baud transmit baud transmit baud transmit baud transmit baud transmit baud transmit baud transmit baud transmit 123 OSB
10. 11 3 Page two amp 200 amp 2FF Page two is the main work zone of the operating system It contains all of the main vectors and user accessible operating system variables Page two is laid out thus amp 200 amp 235 are the vectors See the vectors chapter 10 amp 236 amp 28F are the main system variables accessed by OSBYTE calls amp A6 through amp FF amp 290 is the VDU vertical adjust as set by TV OSBYTE amp 90 amp 291 is the interlace toggle flag as set by TV OSBYTE amp 90 amp 292 amp 296 and amp 297 amp 27B are the two stored values of the system clock as read by TIME Two values are kept so one can be read while the other is being updated by the interrupt routines 272 amp 29C amp 2A0 are the countdown interval timer value This is used to cause an event after a certain time has elapsed See the chapters on events number 12 and on OSWORD number 9 for more details of using the countdown timer amp 2A1 amp 2B0 form the paged ROM type table as pointed to by value read by OSBYTEs amp AA and amp AB Each byte contains the ROM type of the corresponding ROM or zero if there is no ROM in that socket For details of ROM types see the Paged ROMs chapter number 15 amp 2B1 and amp 2B2 are the INKEY countdown timer This is used to time out an INKEY call amp 2C5 amp 2C7 are used as work locations by OSWORD 1 amp 2B6 amp 2B9 are the low bytes of the most recent analogue converter
11. WO O Wv xx OO Aa amp 00 amp amp oS io O N amp 67 amp 07 3 N 41 l SL E a E LS LL LS LL LAN LO LL ee E s N OO amp amp O m Gy El amp 08 8 Variable Variable Variable MODE 4 85800 EX DS 85938 De petar a ESE EES EES Cerne eee DES DEU TEA CEE nara ESC ENS EEES NCR amp 593E amp 593F esy J a EA I amp 7D86 Eco nena Patent can Cee a aan eee nee PEE dec ier ern ear a die LY any ad eae E ern See ee IC a DE ee ae oer ee eee E ng T E RSI hee Bor aed Seka AE 8 PIXELS 1 BIT PIXEL N B The screen layout is only as shown after a CLS it will change when the screen is hard scrolled amp 4000 should be subtracted for each screen location on a model A 471 MODE 5 Screen layout Graphics Colours Text Registers RO R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 472 160 x 256 4 20 x 32 Horizontal total Characters per line Horizontal sync position Horizontal syne width Vertical sync time Vertical total Vertical total adjust Vertical displayed characters Vertical sync position Interlace mode bits 0 1 Display delay bits 4 5 Cursor delay bits 6 7 Scan lines per character Cursor start blink type Cursor start bits 0 4 Cursor blink bit 6 Cursor type bit 5 Cursor end Screen start add
12. Zero flag Z Bit 1 Set if the result of an operation is zero otherwise cleared Interrupt disable I Bit 2 When set IRQ interrupts are disabled Set by the processor during interrupts 41 Decimal mode flag D Bit 3 When set the add and subtract instructions work in binary coded decimal arithmetic When clear these operations are performed using binary arithmetic Break flag B Bit 4 This flag is set by the processor during a BRK interrupt Otherwise this flag is clear Unused flag Bit 5 Unused by the processor Overflow flag V Bit 6 If during an operation there is a carry from bit 6 to bit 7 and no external carry then the overflow flag is set This flag is also set if there is no carry from bit 6 to bit 7 but there is an external carry Negative flag N Bit 7 Set if bit 7 of a result is set otherwise cleared Stack Pointer SP An 8 bit register which forms the low order byte of the address of the next free stack location the high order byte of this address is always amp 1 Program Counter PC PCL PCH low byte high byte A 16 bit register which always contains the address of the next instruction to be executed 42 6 2 The Assembler Mnemonics The following section contains a detailed description of each of the operation codes or instructions in the 6502 instruction set The assembler recognises three letter mnemonics which it translates into the 8 bit values which the microprocessor actua
13. amp 10 amp 1F Serial ULA Serial system chip amp 20 amp 2F Video ULA Video system chip amp 30 amp 3F 74LS161 Paged ROM selector amp 40 amp 5F 6522 VIA SYSTEM VIA amp 60 amp 7F 6522 VIA USER VIA amp 80 amp 9F 8271 FDC Floppy disc controller amp A0 amp BF 68B54 ADLC ECONET controller amp C0 amp DF uPD7002 Analogue to digital converter amp E0 amp FF Tube ULA Tube system interface Section number 18 20 3 20 9 19 21 23 24 25 1 25 2 26 27 Note Some Sheila addresses are not normally used This is because the same devices appear at several different Sheila addresses For example the paged ROM select register is normally addressed at location amp 30 but it could equally well be addressed at any one of the fifteen other locations amp 31 amp 3F 357 358 18 THE 6845 CRTC Sheila address amp 00 amp 07 18 1 General introduction to the 6845 The 6845 cathode ray tube controller chip CRTC forms the heart of the BBC Micro s video display circuitry Its major function is that of displaying the video data in memory on a raster scan display device a television or monitor As an extra bonus the 6845 also refreshes all of the random access memory so that the data stored there is not lost This refreshing process is inherent in the sequential nature of accessing memory for the video display The 6845 does not interfere with processor access to the memory since the processor and 6845 operate on alternat
14. not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of the result is set Addressing mode bytesused cycles immediate 2 2 Zero page 2 3 absolute 3 4 Example Branch if Y amp 0D CPY amp 0D compare Y with amp 0D 13 BEQ cr if Y 13 goto cr 64 DEC Decrement memory by one M M 1 This instruction decrements the value contained in the specified memory location Processor Status after use C carry flag not affected Z zero flag set if memory contents become 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of the result is set Addressing mode bytesused cycles Zero page 2 5 Zero page X 2 6 absolute 3 6 absolute X 3 7 Example Decrement location amp 2900 DEC amp 2900 amp 2900 amp 2900 1 65 DEX Decrement X register by one M M 1 This instruction decrements the contents of the X register by one Processor Status after use C carry flag not affected Z zero flag set if X becomes 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of X becomes set Addressing mode bytesused cycles implied 1 2 Example Decrement X register DEX X X 1 66 DEY Decrement
15. 124 7C 123 7D 126 7E 200 C8 220 DC 229 ES 230 E4 13 D 14 E 20 14 182 B6 files flashing input Reyboard brief description close EXEC or SPOOL files check for EOF of open file fast Tube BPUT read write CFS timeout counter TAPE ROM switch read write EXEC file handle read write SPOOL file handle flashing col mark duration flashing col space duration read write flash counter read write mark period count read write space period count select I P stream read write input source auto repeat delay auto repeat rate reflect keyboard status in LEDs write current keys pressed perform keyboard scan perform keyboard scan from 16 read key with time limit read key translate address read write keyboard semaphore read write auto repeat delay read write auto repeat period read write net keyboard disable read write keyboard status read write TAB key value read write ESCAPE key value dec hex 119 77 127 7F 157 9D 176 BO 183 B7 198 C6 199 C7 10 A 193 C1 194 C2 195 C3 177 B1 11 B 12 C 118 76 120 78 121 79 122 7A 129 81 172 AC 173 AD 178 B2 196 C4 197 C5 201 C9 202 CA 219 DB 220 DC 451 memory O S output paged ROM printer 452 brief description character definition RAM read high order address read top of OS RAM OSHWM read bottom of display HIMEM read hypothetical HIMEM read write primary OSHWM read write current OSHWM read write ch explosion
16. 219 OSBYTE amp D9 217 FX 217 Read write number of lines since last halt in page mode lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains the number of lines printed since the last page halt 220 OSBYTE amp DA 218 FX 218 Read write number of items in the V DU queue lt NEW VALUE gt lt OLD VALUES AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This contains the 2 s complement negative number of bytes still required for the execution of a VDU command Writing 0 to this location can be a useful way of abandoning a VDU queue otherwise writing to this location is not recommended 221 OSBYTE amp DB 219 FX 219 Read write character value returned by pressing TAB key lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains the value to be returned by the TAB key It is possible to use the TAB key as a soft key by setting this location to amp 80 n where n is the soft key number Default value is 9 forward cursor 1 char 222 OSBYTE amp DC 220 FX 220 Read write Escape character lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location cont
17. 26825600 26835641 LDX 0 set X to 0 LDA amp 80 X A amp 4000 address in amp 80 X amp 81 X INX X X 1l i e 1 INX X X 1 LDA amp 80 X A amp 4100 address in amp 82 amp 83 5 10 Post indexed indirect addressing using an indirect address in zero page plus offset in Y This indexed indirect addressing mode uses a single address held in zero page The contents of the Y register are then added to that address held in zero page to give the effective address used N B The X register cannot be used for this addressing mode e g Set 256 bytes of memory to 0 starting at the address contained in locations amp 80 low byte and amp 81 high byte 26805640 284815472 LDY 0 set loop index to 0 TYA A 0 loop STA amp 80 Y 2 amp 7240 Y 0 base addr in amp 80 and 881 INY Y Y 1 CPY 0 Y 0 comparison not needed after INY BNE loop if Y lt gt 0 goto loop 39 5 11 Relative addressing The 6502 instruction set contains 8 branch instructions which cause a jump if a certain condition is met In the example above a BNE instruction is used to cause the loop to be executed again if the loop index Y register does not equal 0 These branch instructions can only be used with relative addressing If the condition of the branch is satisfied the byte following the branch instruction is added to the program counter as an 8 bit two s complement number This method of relative
18. 298 413 272 271 291 272 249 249 70 71 258 298 258 299 J JIM expansion bus reading and writing JMP Jump to new location Joysticks connections fire buttons JSR Jump subroutine K Key read with time limit Key number table Key numbers summary Keyboard auto repeat delay auto repeat rate buffer status control vector disable empty keyboard buffer event function keys f0 f9 input buffer storage input select insert character in buffer interrupts LEDs links locking read status scan from 16 scanning selection for input semaphore translation table write status Keyboard auto repeat count page two location Keyboard auto repeat timer zero page location Keyboard auto repeat delay read write Keyboard auto repeat rate Keyboard scan ignore character zero page location Keys function keys f0 f9 Keys pressed information KEYV 438 170 72 432 418 73 153 142 456 127 128 151 262 206 138 172 225 279 186 172 187 139 246 206 207 145 144 18 187 183 207 273 270 202 202 225 142 262 L Label in assembler programs Language zero page work space Language ROM entering Language ROM number Language workspace Languages in paged ROMs paged ROM entry point workspace available Latch LDA Load A from memory LDX Load X from memory LDY load Y from memory LED cassette motor LEDs on the keyboard Lightpens construction software strobe unit Line in
19. 3 for the 8K mode and 4 for teletext Note that this table overlaps the last 284 amp C447 amp C458 contain various VDU section control numbers of no direct relation to any screen parameters Note that this zone overlaps the previous table amp C459 amp C45D contain one byte per memory map type see above giving the most significant byte of the number of bytes taken up by the screen The least significant byte is always zero 8 C45E amp C462 contain one byte per memory map type see above giving the most significant byte of the address of the first location used by the screen The least significant byte is always zero amp C463 amp C46D contain tables used by the VDU section to index into the other tables amp C46E amp C4A9 contain tables of values to be sent to the various 6845 registers There is one block of 12 bytes for each of the five memory map types see above amp C46E amp C479 6845 registers 0 11 for memory map type 0 modes 0 2 amp C47A amp C485 6845 registers 0 11 for memory map type 1 mode 3 amp C486 amp C491 6845 registers 0 11 for memory map type 2 modes 4 5 amp C492 amp C49D 6845 registers 0 11 for memory map type 3 mode 6 amp C49E amp C4A9 6845 registers 0 11 for memory map type 4 mode 7 amp C4AA amp C4B5 are used by the VDU driver as Jump table addresses into its internal plotting routines They should not be used by the
20. 30a 30b 486 NORTH SOUTH OPEN CLOSED Note OPEN CLOSED Note NORTH SOUTH WEST EAST WEST EAST WEST EAST Note WEST EAST Note ROMSEL provides the LOW ROM select bit to IC20 A12 provides the LOW ROM select bit to IC20 RS423 DATA line not terminated RS423 DATA line is terminated see Note for link 24 RS423 CTS line not terminated RS423 CTS line is terminated It is not normally necessary to terminate the RS423 lines unless high baud rates or long interconnecting wires are being used 32K RAM select model B 16K RAM select model A NORMAL video output INVERTED video output 5 25inch disc 8MHz clock select 8inch disc 16MHz clock select base baud rate select 1200 baud rate select RS423 rate is affected with link 28 set EAST 1200 baud rate select base baud rate select RS423 rate is affected with link 29 set WEST used to WIRE OR two or more ROM select signals Note see links 34 38 and the circuit diagram 31 32 33 34 35 36 37 VVEST EAST VVEST EAST VVEST EAST OPEN CLOSED Note OPEN CLOSED Note OPEN CLOSED Note OPEN CLOSED Note positive CSYNC to RGB video output negative CSYNC to RGB video output connects A13 to A13 pin of IC52 and IC88 connects 5 volts to A13 pin of IC52 and IC88 connects A13 to A13 pin of IC100 and IC101 connects 5 volts to A13 pin of IC100 and IC101 allows the OS
21. 7 Operating System calls Input Output The input device from which characters may be fetched can be selected using the OSBYTE call with A 2 FX 2 Input may be selected from the keyboard and or RS423 Output may be channelled to any combination of the following destinations Screen Printer RS423 or Spooled file Selection of destination is achieved using OSBYTE call with A 3 FX 3 See the OSBYTE call section chapter 8 for a full description of these OSBYTE calls 7 1 OSWRCH Write character to currently selected O P stream Call address amp FFEE Indirected through amp 20E This routine writes the character given in the accumulator to the currently selected output stream or streams NB Unrecognised VDU commands are passed to a vector at location amp 226 See Vectors section 10 8 After an OSWRCH call A X and Y are preserved C N V and Z are undefined The interrupt status is preserved though it may be enabled during a call 101 7 2 Non Vectored OSVVRCH Call address amp FFCB This routine is normally used by OSWRCH and the call address is contained in the OSWRCH vector on reset The non vectored OSWRCH routine is believed to be used by the Tube system This routine has not been documented by Acorn and should be used with caution 7 3 OSRDCH Read character from currently selected I P stream Call address amp FFEO Indirected through amp 210 This routine reads a character from the currently selecte
22. 8 120 IF A 138 THEN PROCMOVE 640 130 IF A 139 THEN PROCMOVE 640 140 UNTIL FALSE 150DEF PROCMOVE offset 100START STARTtoffset 170REM IF ABOVE OFFICIAL START THEN SUBTRACT SCREEN LENGTH 181IF START gt amp 8000 THEN START STARI amp 5000 190REM IF BELOW OFFICIAL START ADDRESS ADD SCREEN LENGTH 200IF START lt amp 3000 THEN START START amp 5000 190REM MODIFY 6845 MEMORY START ADDRESS REGISTER 220VDU23 12 START DIV 2048 231VDU23 13 START MOD 2048 240ENDPROC LA LA 0 0 DIV 8 0 0 0 374 18 15 6845 REGISTER SUMMARY TABLE Register number AR RO R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 x not used Register name Address register Horizontal total Horizontal displayed Horizontal sync position Horizontal sync width Vertical sync width Vertical total Vertical total adjust Vertical displayed Vert sync position Interlace mode Display enable delay Cursor enable delay Scan lines character Cursor start Cursor blink rate Cursor blink ON OFF Cursor end Screen start address H Screen start address L Cursor address H Cursor address L Light pen H Light pen L Program unit Character Character Character Character Scan line Char row Scan line Char row Char row Character Charac
23. Each digit is stored as a binary value in 4 bits 1 nibble Normally 4 bits can be used to represent numbers in the range 0 to 15 In BCD arithmetic 6 of the values that could be represented in 4 bits are not used Adding 1 to 9 in BCD will cause the low nibble to be set to 0 and the high nibble to be set to 1 The carry flag is used to store the carry from the high nibble This is an example of a program which uses BCD arithmetic 10 DIM MC 100 20 OSWRCH 6FFEE 30 OSRDCH amp FFEO 40 OSNEWL amp FFE7 50 FORopt 0 TO 3 STEP3 60 PS MC 70 80 OPT opts 90 start SED set flag for BCD arithmetic 100 CLC clear carry flag 110 LDA amp 80 A amp 80 120 ADC 1 A A 1 C 130 STA amp 80 replace valu 140 LDA 681 A amp 81 150 ADC 0 A A 0 C 160 STA amp 81 replace valu 170 CLD clear flag no more BCD 180 CLC clear carry flag 190 LDX 2 set loop index 200 loop DEX decrement index 210 LDA amp F0 mask for high nibble 220 AND amp 80 X A A AND X amp 80 230 LSR A LSR A LSR A LSR A 240 move high nibble to low nibble 250 ADC amp 30 add value to ASCO 260 JSR OSWRCH print value 270 LDA amp F mask for low nibble 280 AND amp 80 X A A AND X amp 80 290 ADC amp 30 add value to ASC 0 300 JSR OSWRCH print number 310 CPX 0 has index reached 0 320 BNE loop if not go round again 330 LDA amp D A carriage return value 3
24. It may be used to initialise peripherals whenever a power up or a BREAK causes a reset 441 Analogue Input This is an input to the audio amplifier on pin 16 the main computer The amplified signal is produced over the speaker on the keyboard Its input impedance is 9K Ohms and a 3 volt RMS signal will produce maximum volume on the speaker Note however that signals as large as this will cause distortion if the sound or speech is used at the same time DO D7 pins 18 24 This is a bi directional 8 bit data bus which is connected via a 74LS245 buffer IC72 to the CPU The direction of data transfer is determined by the R W line signal The buffer is enabled whenever FRED or JIM are accessed AQ A7 pins 27 34 These connected directly to the lower 8 CPU address lines via a 74LS244 buffer 1C71 which is always enabled 28 5 Cleaning up FRED and JIM s page selects All 1MHz peripherals are clocked by a 1MHz 50 duty cycle square wave designated as 1MHZE in figure 28 2 This clock rate was chosen to allow chips such as 6522 VIAs to use their internal timing elements correctly The system 6502 CPU is normally clocked at twice the speed of the peripherals and so it operates at 2MHz However if the CPU wishes to access any device on the 1MHz bus the processor has to be slowed down The effect of this slow down circuit is illustrated in figure 28 2 After generating a valid 1MHz address the slow down circuit stretches the c
25. LDA amp FF load the accumulator with value amp FF LDX count load X with value of the constant count 5 4 Absolute addressing using a fixed address When the address required for an instruction is known at the time of assembly then absolute addressing may be used Absolute addressing is the default addressing mode used by the assembler If a number or variable is placed in the operand field of the assembler it will be treated as a 16 bit effective address e g CMP amp 1900 compare A with contents of location amp 1900 JMP label goto address specified by label 5 5 Zero page addressing using a fixed zero page address This mode is the same as absolute addressing except that an 8 bit address is specified This 8 bit addressing limits use to the first amp 100 bytes of memory zero page The assembler will automatically select zero page addressing when the operand value is less than 256 amp 100 e g CPY amp 80 compare y with contents of location amp 80 ASL amp 81 shift left contents of location amp 81 one bit 36 5 6 Indirect addressing using an address stored in memory Using this addressing mode an instruction can use an address which is actually computed when the program runs The JMP instruction may use this addressing mode The address used for the jump is taken from the two bytes in memory starting at the address specified in the operand field low byte first high byte second Indirect add
26. STA temp reload memory 68 INC Increment memory by one M M 1 This instruction increments the value contained in the specified memory location Processor Status after use C carry flag not affected Z zero flag set if memory contents become 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of memory becomes set Addressing mode bytesused cycles Zero page 2 5 zero page X 2 6 absolute 3 6 absolute X 3 7 Example Increment location amp 80 INC amp 80 amp 80 amp 8041 69 INX Increment X register by one X X 1 This instruction increments the contents of the X register by one Processor Status after use C carry flag not affected Z zero flag set if X becomes 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of X becomes set Addressing mode bytesused cycles implied 1 2 Example Increment X register INX X X 1 70 INY Increment the Y register by one Y Y 1 This instruction increments the contents of the Y register by one Processor Status after use C carry flag not affected Z zero flag set if Y becomes 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V
27. The graphics cursor is then moved left until the first non background pixel is encountered The graphics cursor is then moved right until the first non background coloured pixel is encountered on the right hand side If the PLOT number is 73 or 77 then a line will be drawn between these two points in the current foreground colour If the PLOT number is 72 or 76 then no line is drawn but the cursor movements are made these may be read using OSWORD call with A amp D 13 see chapter 9 460 Horizontal line blanlcing right These PLOT numbers can be used to undravv an object on the screen They have an the opposite effect to those of the horizontal line filling functions except that the graphics cursor is moved right only PLOT numbers 91 and 95 will cause a line to be drawn from the specified co ordinates to the nearest background coloured pixel to the right in the background colour PLOT numbers 89 and 93 move the graphics cursor but do not cause the line to be blanked 461 Appendix F Screen mode layouts MODE 0 Screen layout Graphics 640 x 256 Colours 2 Text 80 x 32 Registers RO Horizontal total R1 Characters per line R2 Horizontal sync position R3 Horizontal sync vvidth Vertical sync time R4 Vertical total R5 Vertical total adjust R6 Vertical displayed characters R7 Vertical sync position R8 Interlace mode bits 0 1 Display delay bits 4 5 Cursor delay bits 6 7 R9 Scan lines per character R10 Cursor star
28. address contained in P P should then be incremented by the appropriate amount before entering the assembler e g to incorporate a string into a machine code program 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 DIM MC 100 OSRDCH amp FFEO OSASCI amp FFE3 FOR opt 0 TO 3 STEP3 PS MC OPT opts entry LDY 0 zero loop index loop LDA string Y load accumulator with Y string JSR OSASCI write the character INY increment loop index CMP amp 0D is the current character a CR BNE loop if not get the next character JSR OSRDCH get character from keyboard CMP 9 is it the TAB key BNE error if not flag an error RTS return to BASIC string SPS Please press the TAB key P P LEN P 1 OPT opts error BRK cause an error NEXT opt 2 PS amp FF P P 1 SPS Wrong key pressed P3 LEN P 0 CALL en try This program prompts the user to press the TAB key by printing out a message If the wrong key is pressed an error is flagged 27 3 7 Handling errors with BRK In the example program above the BRK instruction is used to generate an error The BRK instruction forces an interrupt which is interpreted by the operating system as an error As part of the error handling in BASIC the programmer can incorporate an error number a
29. amp FFD1 Indirected through amp 21A This routine transfers a number of bytes to or from an open file It can also be used to transfer filing system information On entry X and Y point to a control block in memory A defines the information to be transferred 339 The control block format is 00 File handle 01 Pointer to data in either I O processor or Tube 02 processor 03 Low byte first 04 05 Number of bytes to transfer 06 Low byte first 07 08 09 Sequential pointer value to be used for transfer 0A Low byte first 0B 0C The sequential pointer value given replaces the old value of the sequential pointer The real utility of the OSGBPB call comes in the Econet system where there is a considerable time overhead for the transfer of each piece of data If single bytes are transferred using OSBPUT and OSBGET the overhead incurred for each transfer has a marked effect on performance times The greater the number of bytes that can be read or written the more efficient that transfer is a single OSGBPB call can replace an OSARGS call and a large number of OSBGET or OSBPUT calls The accumulator value before the OSGBPB call has the following effects A 1 Put bytes to media using the new sequential pointer A 2 Put bytes to media ignoring the new sequential pointer 340 A 3 A 4 A 5 A 6 Get bytes from media using the new sequential pointer Get bytes from media ignoring the new sequential
30. applications 438 BOTTOM A7 A6 A5 Ad AB A2 Al AO ov D7 D6 DS D4 D3 D2 DI Do ov ANALOG IN ov RST ov NPGFD ov NPGFC ov NIRQ OV NNMI ov 1MHzE ov R W ov TMHz BUS CONNECTOR LOOKING INTO SOCKET MOUNTED ON MAIN CIRCUIT BOARD Note Pins 1 and 34 connect to the wires at the edge of the ribbon cable connected to the IDC header Figure 27 1 The 1MHz bus connector 439 8 4 Bus signal definitions The one megahertz bus connector is illustrated in figure 28 1 The specification for the signals on the one megahertz bus are 0 volts R W pin 2 IMHZE pin 4 NNMI pin 6 440 This is connected to the main system 0 volts line The reason for putting OV lines between the active signal lines is to reduce the interference between different signals This is the read not write signal from the 6502 CPU buffered by two 74LS04 inverters This is the 1MHz system timing clock It is a 50 duty cycle square wave The 6502 CPU is operating at 2MHz so the main processor clock is stretched whenever 1MHz bus peripherals are being accessed The trailing edges of the IMHZzE and 2MHz processor clock are then coincidental The processor clock is only ever truly 2MHz when accessing main memory Not Non Maskable Interrupt This is connected directly to the 6502 NMI input It is pulled up to 5 volts with a 3K3 resistor Use of Non Maskable Interrupts on the BBC microcomputer is only advisable after the
31. by one of the interval timers The second timer can be programmed to count pulses on the PB6 input These outputs are capable of sourcing up to 1 mA at 1 5 volts in the output mode This allows direct drive of Darlington transistor circuits Note that only the port B lines can provide 1mA the port A lines cannot CB1 CB2 port B control lines The port B control lines act as interrupt inputs or as handshake outputs just like port A They can also be programmed to act as a serial port under the control of the shift register These lines cannot source 1mA either 22 1 2 ELECTRICAL SPECIFICATION Inputs Input voltage for logic 1 2 4 VDC minimum Input voltage for logic 0 0 4 VDC maximum Maximum required input 1 8mA current Outputs Output logic 1 voltage 2 4 VDC minimum at a load of 100 uA maximum except PBO PB7 Output logic 0 voltage 0 4 VDC maximum when sinking 1 6 mA Current sinking capability 1 6 mA minimum 398 22 2 FUNCTIONAL DESCRIPTION Register RS Coding Number S3 S2 o ojojo S 1 S 0 Register Desig ORB IRB ORA IRA DDRB ola w i wn Description Write Output Register B Output Register A Data Direction Register B Read Input Register B Input Register A 0 0 0 1 0 1 2 0 1 0 5 4 0 0 T1C H o DDRA T1C L Data Direction Register A T1 Low Order Latches T1High Order Counter T1 Low Order Counter ojo ejo T1L L T
32. screen bottom Program space 15 3 Filing systems Filing systems are discussed fully in the Filing Systems chapter number 16 This section covers filing systems and their relations with paged ROMs 15 3 1 Initialising a filing system Filing systems that are ROM resident can be initialised in three ways all of which should be catered for in a filing system ROM An operating system command is issued naming the filing system The filing system should watch for its selection command coming through a service call A service call amp 12 is issued with Y equal to the filing system number Auto booting on system reset If a filing system receives a service call amp 03 it should initialise if appropriate see service call amp 03 details Note that at this stage the language has not been initialised so errors will be treated oddly If Y 0 at this point a boot file should be searched for if appropriate 328 On initialisation a filing system should do the follovving a Call OSFSC vvith A 6 see filing systems section to warn the old vector owner that the vectors are about to be redefined b Set up the extended vectors as previously specified c Issue service call amp 0F to warn all other paged ROMs of the vector change d Restore all open files from the last activation of the filing system Filing systems should be able to keep files open on hold whilst inactive 15 3 2 Other considerations for filing syste
33. with all the registers preserved If however the ROM performs the requested service and wishes to prevent other ROMs also performing the service the accumulator should be zero on exit The service types are 00 No operation All ROMs are to ignore this service request a higher priority ROM has already provided it 01 Absolute workspace claim On break each ROM is asked to stake a claim for absolute workspace Absolute workspace is a single block of memory which is only allocated to one ROM at any one time Being absolute this memory runs from amp E00 to the highest address asked for by any of the ROMs On entry the Y register contains the current upper limit of the absolute workspace This starts off as amp 0E the upper byte of the first address of absolute workspace Each ROM should compare the value in the Y register with the upper limit of absolute memory required by the ROM and if necessary replace it by the level required by the ROM This memory is shared between all the ROMs and should be claimed with service call amp 0A before use The accumulator should be preserved during this call 320 02 03 04 Private workspace claim On break after absolute workspace allocation each ROM is offered the chance to take some private workspace This memory is exclusive to the ROM claiming it The ROM is entered with the first page number of the workspace available to it in the Y register It should save this value in the
34. 0D 13 when an OSNEWL call is performed After an OSASCI call A X and Y are preserved C N V and Z are undefined Interrupt status is preserved though interrupts may be enabled during a call 7 7 Main VDU character output entry point Call address amp FFBC This is the entry point for raw VDU character processing On entry the accumulator contains the character to be written to the VDU drivers Any settings of OSBYTE amp 3 FX 3 are totally ignored and no output will go to any other destination except characters preceded by VDU 1 which will be sent to the printer if the printer has previously been enabled with a VDU2 This call has not been documented by Acorn There will normally be no need to use this routine as OSWRCH with the appropriate FX3 call can be used to the same effect 104 7 8 GSINIT General string input initialise routine Call address amp FFC2 The GSINIT and GSREAD routines are used by the operating system to process strings used for commands such as KEY and LOAD The advantage of using this system for reading strings is that an escape sequence can be used to introduce control characters which would otherwise be difficult to type in directly from the keyboard the escape character is see section 2 10 for details of its use This routine should be used to initialise a string which is to be used for input using the GSREAD routine see below The routine requires locations amp F2 and amp F3 plus
35. 1 for the 16K mode 2 for 10K modes 3 for the 8K mode and 4 for teletext The bottom two bits of the number in this location are sent to the addressable latch on the system VIA to control the hardware wrap around on the display amp 357 amp 35A contain the current colours These are stored as the value that would be stored in a byte in screen memory to completely colour that byte to the colour required The locations are amp 357 Foreground text colour amp 358 Background text colour amp 359 Foreground graphics colour amp 35A Background graphics colour amp 35B and amp 35C contain the graphics plot mode for the foreground and background plotting respectively These are set by the GCOL first parameter amp 35D and amp 35E are used as a general jump vector The vector is used for decoding VDU control codes and PLOT numbers amp 35F contains a record of the last setting of the 6845 cursor start register even if changed through VDU 23 0 so that the cursor can be turned off and back on tidily using VDU 23 1 amp 360 contains the number of logical colours in the current mode minus one amp 361 contains the number of pixels per byte minus one for the current mode or zero if text only mode 277 amp 362 and amp 363 contain the left and right colour masks respectively These bytes contain a bit set in each bit position corresponding to the leftmost or rightmost pixel For example in a two colour mode these bytes
36. 100 256 bytes long There are therefore 256 pages in the entire address space 256 pages of 256 bytes each account for the 65536 bytes of addressable memory Parallel parallel data transfers occur when data is sent along two or more lines at once The system data bus for example has eight lines operating in parallel Peripheral any device connected to the 6502 central processor unit such as the analogue port printer port econet interface disc interface etc but not including the memory Poll most of the hardware devices on the BBC microcomputer generate interrupts to the 6502 CPU If interrupts have been enabled the CPU has to find out which device generated the interrupt It does this by successively reading status bytes from each of the hardware devices which could have caused an interrupt This successive reading of devices is called polling RAM Random access memory the main memory in the BBC microcomputer is RAM because it can be both written to and read from Refresh all of the memories in the BBC microcomputer are dynamic memories This means that they have to be refreshed every few milliseconds so that their data is not lost The refreshing function is performed by the 6845 as it accesses memory regularly for video output Register the 6502 and many of the peripheral devices in the BBC microcomputer contain registers These are effectively one byte memory locations which do not necessarily reside in the main
37. 115 73 t 116 74 u 117 75 v 118 76 VV 119 77 x 120 78 y 121 79 Z 122 7A 123 7B 124 7C 125 7D 126 7E 457 Rey ASCII INREY Internal Rey No character dec hex dec hex dec hex ESCAPE 27 1B 113 8F 112 70 TAB 9 9 97 OF 96 60 CAPSLOCK 65 BF 64 40 CTRL 2 FE 1 1 SHIFT LOCR 81 AF 80 50 SHIFT 1 FF 0 0 DELETE 127 7F 90 A6 89 59 COPY 106 96 105 69 RETURN 13 D 74 B6 73 49 UP CURSOR 58 C6 57 39 DOWN CURSOR 42 D6 41 29 LEFT CURSOR 26 E6 25 19 RIGHT CURSOR 122 86 121 79 f0 33 DF 32 20 f1 114 8E 113 71 f2 115 8D 114 72 f3 116 8C 115 73 f4 21 EB 20 14 f5 117 8B 116 74 f6 118 8A 117 75 7 23 E9 22 16 f8 119 89 118 76 f9 120 88 119 77 Start up option switch on front of keyboard bit 0 9 9 bit 1 8 8 bit 2 7 7 bit 3 6 6 bit 4 5 5 bit 5 4 4 bit 6 3 3 bit 7 2 2 458 Appendix D VDU Code Summary dec XD CO IJ DC OI BB CO N H DO 127 hex MMONAWSFOMNDUARWNFO RRR UNEO BPRPRP EEE EEE PY TMMOADASOCMONDOSA 7F CTRL PT TTNK ASA CHMADVOZZMAT TV AOFAOONS So J tH T bytes ONRNOOBBEOCO COI oC OC OCoOCIIN Co CDC DC OC CCC OC DC OC DC OC CX OX HH DO function Does nothing Send next character to printer only Enable printer Disable printer Write text at text cursor Write text at graphics cursor Enable VDU drivers Make a short bleep BEL Move cursor back one character Move cursor forward one character Move cursor down one line Move cursor
38. 16 for FILEV to FSCV 10 5 The event vector EVNTV amp 220 1 For entry conditions to the event vector refer to the chapter on events number 12 10 6 The user print vector UPTV amp 222 3 This vector contains the address of the user provided printer driver The facility for providing a user print driver is important since not all printers have Centronics parallel or standard serial interfaces Using this vector allows specialised code for driving the hardware on a particular printer to be inserted 258 The user printer driver acts as an interface betvveen the printer buffer in memory and the printer hardvvare The driver is responsible for removing the characters from the printer buffer when the printer is ready to accept them and passing them on to the printer hardware The printer driver is regularly entered allowing it to poll its hardware and the printer buffer and informing it of relevant changes in printer conditions The user printer driver should preserve all registers On entry the X register contains the buffer number that should be tapped to claim bytes for output The Y register contains the printer type selected by FX 5 The driver should recognise calls to itself by checking the Y register against the printer type it recognises The normal number to recognise is 3 but it is possible to design a printer driver that will run two types of printer The driver can also use any number from 5 to 255 4 being reserv
39. 257 493 264 171 131 138 224 172 263 162 151 263 169 208 280 274 274 289 149 37 393 355 437 355 355 355 355 55 56 494 28 32 41 89 280 393 18 Cassette critical flag zero page location 270 Cassette filing system see CFS Cassette LED 393 Cassette motor control of 161 Cassette Port example input program 314 example output program 312 reading form user software 313 software control 311 Cassette relay 18 393 Cassette tape format 347 Cassette RS423 selection flag 210 393 Catalogue CAT 13 CFS 346 page three work space 278 page two work space 274 software select switch 192 tape format 347 timeout counter 185 CFS options byte zero page location 269 CFS status byte 269 Character definition OSVVORD 251 Chip 494 CLC Clear carry flag 57 CLD Clear decimal flag 58 CLI Clear interrupt disable flag 59 Clock see system clock Clock electronic 494 Clocks in software 237 Closing files OSFIND 342 CLV Clear the overflow flag 60 CMP Compare memory with A 61 CNPV 264 Colour code selection in ULA 378 Colour palette read OSWORD 251 selection in ULA 379 write OSWORD 252 Colours actual 382 flashing 125 126 logical 380 Command line interpreter CLI 107 Command line interpreter 11 Control codes insertion into text 16 Countdown timer see interval timer CPU Central processing unit 494 CPX Compare memory vvith X 63 CPY Compare memory vvith Y 64 CRC 348 CRTC video
40. 4 otherwise it is returned containing the value 4 Y and C are undefined 133 OSBYTE amp 12 18 FX 18 Reset soft keys No parameters This call clears the soft key buffer so the character strings are no longer available After call A and Y are preserved X and C are undefined 134 OSBYTE amp 13 19 FX 19 Wait for vertical sync No parameters This call forces the machine to wait until the start of the next frame of the display This occurs 50 times per second on the UK BBC Microcomputer and can be used for timing or animation N B User trapping of IRQ1 may stop this call from working After call A is preserved X Y and C are undefined 135 OSBYTE amp 14 20 FX 20 Explode soft character RAM allocation Entry parameters X value explodes implodes memory allocation In the default state 32 characters may be user defined using the VDU 23 statement from BASIC or the OSWRCH call in machine code These characters use memory from amp C00 to amp CFF Printing ASCII codes in the range 128 amp 80 to 159 amp 9F will cause these user defined characters to be printed up these characters will also be printed out for characters in the range amp A0 amp BF amp CO amp DF amp E0 amp FF In this state the character definitions are said to be IMPLODED If the character definitions are EXPLODED then ASCII characters 128 amp 80 to 159 amp 9F can be defined as before using VDU 23 and memory at amp C00
41. 6502 registers are restored to their original state before returning to the main program If they are modified the main program will suddenly find garbage in its registers in the middle of some important processing It is probable that a total system crash would result from this 13 1 2 Maskable Interrupts Maskable interrupts are very similar to non maskable interrupts in most respects A hardware device can generate a maskable interrupt to which the 6502 must normally respond The difference comes from the fact that the 6502 can choose to ignore all maskable interrupts under software control if it so desires To disable interrupts only the maskable ones though an SEI set interrupt disable flag instruction is executed Interrupts can be re enabled at a later time using the CLI clear interrupt disable flag instruction When an interrupt is generated the processor knows that an interrupt has occured somewhere in the system Initially it doesn t know where the interrupt has come from If there were only one device that could have caused the interrupt then there would be no problem However since there is more than one device causing interrupts in the BBC microcomputer each device must be interrogated That is that each device is asked whether it caused the interrupt When the interrupt processing routine has discovered the source of a maskable interrupt it must decide what type of action is required This usually involves tr
42. 6502 writes or reads the shift register The interrupt flag is set after 8 shift pulses The 6502 can then load the next byte of data into the shift register RITE SR OPERATION CBUINPUT SHIFT CLOCK Sasa comma OO Pe ora OMSK X l Figure 22 18 Output Mode 7 Timing 22 2 11 Interrupt operation Interrupt flags are set either by an interrupt condition in the chip eg from a counter or an interrupt condition on an input to the chip Interrupt flags normally remain in the set condition until the interrupt has been serviced The source of an interrupt can be determined by reading these interrupt flags in order from highest priority to lowest priority This is best performed by reading the flag register into the processor accumulator shifting either right or left and using conditional branch instructions to detect an active interrupt There is an interrupt enable bit associated with each interrupt flag If this enable bit is set to a logic 1 and the associated interrupt occurs then the 6502 will be interrupted If the enable bit is set to 0 then the 6502 will not be interrupted All interrupt flags are contained in the interrupt flag register IFR see figure 22 19 To enable the 6502 to check the 6522 without checking each bit in the IFR bit 7 will be set to a logic 413 1 if the 6522 has generated the interrupt In addition to reading the IFR individual bits may be cleared by vvriting a 1 into the appropriate bit o
43. 7 Lower 4 bits of 8 8 8 8 4 4 4 4 R3 18 4 2 Vertical sync pulse width The upper 4 bits contain the number of scan line times for the vertical sync pulse This is set to 2 in all modes 18 5 THE VERTICAL TIMING REGISTERS The point of reference for vertical registers is the top displayed character position Vertical registers are programmed in character row times or scan line times 362 18 5 1 Vertical total register R4 The vertical sync frequency is determined by both R4 and R5 In order to obtain an exact 50Hz or 60Hz vertical refresh rate the required number of character line times is usually an integer plus a fraction The integer number of character lines minus one Nvt on figure 18 1 is programmed into this 7 bit write only register Mode 0 1 2 3 4 5 6 7 R4 38 38 38 30 38 38 30 30 18 5 2 Vertical total adjust register R5 This 5 bit write only register is programmed with the fraction for use in conjunction with register R4 It is programmed with a number of scan lines Nadj on figure 18 1 It can be varied slightly in conjunction with R4 to move the whole display area up or down a little on the screen It is usually set to 0 except when using modes 3 6 and 7 in which it is set to 2 TV OSBYTE amp 90 controls the vertical positioning of a display on the screen Refer to the OSBYTE section for more details 18 5 3 Vertical displayed register R6 This 7 bit write only register determines the number of dis
44. 8 bitl amp 48 72 amp 09 9 bit 0 amp 49 73 RETURN amp 10 16 Q amp 50 80 SHIFT LOCK amp 11 17 3 amp 51 81 S amp 12 18 4 amp 52 82 C amp 13 19 5 amp 53 83 G amp 14 20 f4 amp 54 84 H amp 15 21 8 amp 55 85 N amp 16 22 f7 amp 56 86 L amp 17 23 amp 57 87 amp 18 24 amp 58 88 amp 19 25 LEFT CURSOR amp 59 89 DELETE 142 amp 20 amp 21 amp 22 amp 23 amp 24 amp 25 amp 26 amp 27 amp 28 amp 29 amp 30 amp 31 amp 32 amp 33 amp 34 amp 35 amp 36 amp 37 amp 38 amp 39 55 56 57 OWN CURSOR UP CURSOR amp 60 amp 61 amp 62 amp 63 amp 64 amp 65 amp 66 amp 67 amp 68 amp 69 amp 70 amp 71 amp 72 amp 73 amp 74 amp 75 amp 76 amp 77 amp 78 amp 79 96 97 98 99 100 101 102 103 104 105 112 113 114 115 116 117 118 119 120 121 COPY ESCAPE f1 f2 f3 f5 f6 f8 f9 RIGHT CURSOR Bits 0 to 7 refer to the links at the front of the keyboard circuit board on the right hand side See OSBYTE amp FE for further information about these links To convert these internal key numbers to the INKEY numbers they should be EOR Exclusive ORed with amp FF 255 Entry parameters X and Y contain values to be written Value in X is stored in amp ED old key Value in Y is stored in amp EC new key See also OSBYTE calls with A amp AC and A amp AD After call A X and Y are preserved C is undefined 143
45. A 2 Read length of file BASIC EXT A amp FF Update this file to media Note the control block always resides in the I O processor s memory regardless of the existence of a Tube processor After an OSARGS call X and Y are preserved A is preserved except when entered with Y 0 A 0 C N V and Z undefined D 0 Interrupt state is preserved but may be enabled during the call 16 4 OSBGET Get one byte from an open file Call address amp FFD7 Indirected through amp 216 This routine reads a single byte from a file On entry Y contains the file handle as provided by OSFIND The byte is obtained from the point in the file designated by the sequential pointer 338 On exit X and Y are preserved A contains the byte read C is set if the end of the file has been reached and indicates that the byte obtained is invalid N V and Z are undefined Interrupt state is preserved but may be enabled during the call 16 5 OSBPUT Write a single byte to an open file Call address amp FFD4 Indirected through amp 218 OSBPUT writes a single byte to a file On entry Y contains the file handle as provided by OSFIND A contains the byte to be written The byte is placed at the point in the file designated by the sequential pointer On exit X Y and A are preserved C N V and Z are undefined Interrupt state is preserved but may be enabled during the call 16 6 OSGBPB Read or write a group of bytes Call address
46. A X This instruction is used to copy the contents of the X register to the accumulator Processor Status after use C carry flag not affected Z zero flag set if A becomes 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of A is set Addressing mode Bytes used Cycles implied 2 example Transfer contents of X to A TXA A X 98 TXS Transfer X to S S X This instruction is used to copy the contents of the X register to the stack pointer Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode Bytes used Cycles implied 1 2 Example Transfer contents of X to S TXS S X 99 IYA Transfer Y to A A Y This instruction is used to copy the contents of the Y register to the accumulator Processor Status after use C carry flag not affected Z zero flag set if A becomes 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of A is set Addressing mode Bytes used Cycles implied 1 2 Example Transfer contents of Y to A TYA A Y 100
47. As shown in figure 28 2 CNPGFC2 will go low a time lag 40nS after IMHZzE goes high and it will remain valid until 40nS after IMHZE has gone low again Take care if any of the lines on the 1MHz bus are buffered because delays introduced by buffers could make data invalid when it is latched Refer to the precise timing information in section 28 6 5 for more details 446 5V NPGFC CNPGFC2 or NPGFD or CNPGFD2 1MHZE Figure 28 4 Clean up circuit 2 28 6 Hardware requirements for 1MHz bus peripherals All additional hardware designed to operate from the IMHz bus must conform to the following standards 28 6 1 Power supply No power should be drawn from the BBC microcomputer All peripherals should have their own integral power supply or use a separate power supply unit 28 6 2 Logic line loading No more than one low power Schottky TTL load should be presented to any of the logic lines by a peripheral In most instances this means that all logic lines will have to be buffered for each peripheral 28 6 3 Connection to the BBC microcomputer Connection to the BBC microcomputer should be via a 600mm length of 34 way ribbon cable terminated with a 34 way IDC socket The 1MHz bus connections should feed through the unit ie a 34 way output header plug connector should be provided so that more devices can be connected as required 28 6 4 Bus termination All bus lines except NRST NNMI and NIRQ shoul
48. CPU but this can be stretched into a 1MHz clock when slow peripherals such as the 6522s are being accessed See chapter 28 on the 1MHz bus for more details about cycle stretching CPU Central processing unit the 6502A in the BBC microcomputer It is this chip which does all of the computing work associated with running programs Cycle this is usually applied to the system clock A complete clock cycle is the period between a clock going high low then high again See clock Data bus a set of eight connections over which all data transactions between devices in the BBC microcomputer take place Field a space allocated for some data in a register or ina program listing For example in an Assembly language program the first few spaces are allocated to the line number field the next few spaces are allocated to the label field and so on Handshaking this type of communications protocol is used when data is being transferred between two asynchronous devices Two handshaking lines are normally required One of these is a data ready signal from the originating device to the receiving device When the receiving device has accepted the data it sends a data taken signal back to the originating device which then knows that it can send the second lot of data and so on 494 High sometimes used to designate logic 1 Interrupt this signal is produced by peripheral devices and is always directed to the 6502A
49. Clear BCD BCS Branch on Carry Set BELL channel duration frequency sound BEQ Branch on result zero BGETV OSBGET Bibliography Bidirectional Binary Binary Coded Decimal Bit BIT Test memory hits BMI Branch if negative BNE Branch if not zero BPL Branch on positive result BPUT fast for Tube BPUTV OSBPUT 26 24 25 21 23 25 24 25 29 43 23 24 23 25 493 270 273 127 128 245 23 21 21 26 21 195 493 123 124 47 33 48 214 217 216 215 49 338 491 493 31 33 493 50 51 52 53 176 339 BREAR effect control intercept code status of last BREAK Break flag Break vector BRK handling errors paged ROM service call pointer in zero page BRK errors reading active ROM after BRK Forced interrupt BRK vector BRKV Buffer count purge vector examine status flush class of buffer flush specific buffer input code interpretation insert character insert vector insertion into read status remove vector removing characters from RS423 buffer limit Buffer page eight addresses Buffer busy flags page two locations Buffer indices Buffers events flushing at ESCAPE Bug in the 6502 in the cassette system Bus 1MHz Bus signals clock interrupt read write reset BVC Branch if overflow clear BVS Branch if overflow set Byte Cc CALL from BASIC Carry flag Cassette buffer storage bug interblock gaps 166 205 241 244 42 257 28 322 272 194 54 257
50. Econet Manual and OSBYTES amp C9 amp CE amp CF amp D0 for more information The addresses of registers within the 68B54 are given here for reference Sheila address Write function Read function amp AO Control Register 1 Status Register 1 amp Al Control Registers 2 3 Status Register 2 amp A2 Transmit FIFO Receive FIFO Frame Continue amp A3 Transmit FIFO Receive FIFO Frame Terminate 25 3 The Econet station ID register Sheila amp 20 Read only This will only be valid for users on an Econet system Reading from this register will return the station ID number This is set via links S11 to any number between 0 and 255 The Econet data link controller circuit produces NMIs to the CPU These interrupts are automatically enabled by the hardware every time when the station ID is read 428 26 The Analogue to Digital converter Sheila addresses amp C0 amp C2 The analogue to digital converter ADC chip provided in the BBC microcomputer is a 10 bit integrating converter It has four input channels which can be selected under software control By applying a voltage of between 0 volts and Vref to the channel inputs a 10 bit binary number will be generated which is directly proportional to the applied voltage For example applying a voltage Vref 2 would produce a 10 bit value of approximately 511 Vref itself corresponds to about 1023 26 1 Programming the Analogue to Digital converter The analogue to digital c
51. Exploding the character set definitions enables the user to uniquely define characters 32 amp 20 to 255 amp FF in steps of 32 extra characters at a time The operating system must allocate memory for this which it does using memory starting at the operating system high water mark OSHWM This is the value to which the BASIC variable PAGE is usually set and so if a totally exploded character set is to be used in BASIC then PAGE must be reset to OSHWM amp 600 i e PAGE PAGE 82600 ASCII characters 32 amp 20 to 128 amp 7F are defined by memory within the operating system ROM when the character definitions are imploded See OSBYTE amp 83 131 for details about reading OSHWM from machine code The memory allocation for ASCII codes in the expanded state is as follows ASCII code Memory allocation FX 20 0 X 0 amp 80 amp 8F amp C00 amp CFF imploded 136 FX 20 1 FX 20 2 FX 20 3 FX 20 4 FX 20 5 FX 20 6 2 X 3 X 4 5 amp A0 amp BF amp CO0 amp DF amp E0 amp FF amp 20 amp 3F amp 40 amp 5F amp 60 amp 7F OSHWM OSHWM amp FF A above OSHWM amp 100 OSHWM amp 1FF A above OSHWM amp 200 OSHWM amp 2FF A above OSHWM amp 300 OSHWM 8 amp 3FF A above OSHWM amp 400 OSHWM amp 4FF A above OSHWM amp 500 OSHWMG amp 5FF A above See also OSBYTE call with A amp B6 182 after call A is preserved X contains the new OSHWM high byte Y an
52. OSBYTE 8 79 121 Reyboard scan Entry parameters X determines the Rey to be detected and also determines the range of Reys to be scanned Rey numbers refer to internal Rey numbers in the table above To scan a particular Rey X key number EOR amp 80 on exit X lt 0 if the key is pressed To scan the matrix starting from a particular key number X key number on exit X key number of any key pressed or amp FF if no key pressed During the keyboard scan the key whose value is stored in location amp EE is ignored The contents of this location are set to 0 by the operating system on reset After call A is preserved Y and C are undefined 144 OSBYTE amp 7A 122 Reyboard scan from 16 decimal No entry parameters Internal Rey number see table above of the Rey pressed is returned in X This call is directly equivalent to an OSBYTE call with A amp 79 and X 16 After call A is preserved Y and C are undefined 145 OSBYTE amp 7B 123 Inform operating system of printer driver going dormant Entry parameters X should contain the value 3 print buffer i d This OSBYTE call should be used by user printer drivers when they go dormant The operating system will need to wake up the printer driver if more characters are placed in the printer buffer See Vectors Section user printer drivers 10 6 After call A X and Y are preserved C is undefined 146 OSBYTE amp 7C 124 FX 124 Clear ESCAPE
53. ROM to be WIRE OR ed use ordinary OS ROM select from IC20 see full circuit diagram for more details allows IC52 ROM to be VVIRE OR ed use ordinary IC52 ROM select from IC20 Link only available on issue 4 Replaced by R125 in issues 1 3 allows IC88 ROM to be VVIRE OR ed use ordinary IC88 ROM select from IC20 Link only available on issue 4 Replaced by R142 in issues 1 3 allows IC100 ROM to be WIRE OR ed uses IC100 ROM select from IC20 Link only available on issue 4 Replaced by R149 in issues 1 3 487 38 39 488 OPEN CLOSED Note OPEN CLOSED allows IC101 ROM to be WIRE OR ed uses IC101 ROM select from IC20 Link only available on issue 4 Replaced by R153 in issues 1 3 standard monochrome monitor output on BNC socket colour output on BNC video socket Appendix J The keyboard circuit The Reyboard circuit is connected to the main printed circuit board by tvvo ribbon cables The larger of these is fitted to all BBC microcomputers and carries all Rey pressed and start up option data The smaller one is connected directly to the speech processor With the speech system installed this extra connector allows serial ROMs to be plugged in to the hole on the left hand side of the keyboard In the lower right hand corner of the keyboard printed circuit board there are two rows of eight holes These are the keyboard link options The operation of these links is defined by a mad
54. This instruction causes a relative branch if the zero flag is set when the instruction is executed The assembler automatically calculates the relative address from the address given and will cause an error if the address is out of range Used after a CMP instruction this branch occurs if A data Used after an LDA instruction this branch occurs if A 0 Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused_ cycles relative 2 2 1 if branch succeeds 2 if to new page Example Subroutine not used when A 3 CMP 3 A 3 comparison BEQ over if A 3 goto over JSN anything subroutine to be missed if A 0 SOD Agari 49 BIT Test memory bits with accumulator A AND M N M7 V M6 This instruction can be used to test whether one or more specified bits are set The zero flag is set if the result is 0 otherwise the zero flag is cleared Bits 7 and 6 of the memory location are transferred to the status register The BIT instruction performs an AND operation without storing the result but setting the status flags Processor Status after use C carry flag not affected Z zero flag set if the result 0 I interrupt disable not affected D decimal mode flag not affected B break comm
55. UNTIL X lt gt 1 OR amp 70 lt gt 0 470 REM if something else found such as a DCD or another 480 REM character then await another DCD interrupt 490 500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 IF X lt gt ASC OR 2 REM Sync found and REPEAT X3 GET VDUX3 UNTIL X 13 All done so X20 Turn motor off OTOR 0 Disable event x37 Restore 6850 X 156727252 EM Restore RS423 XEX 205 0 El ty El El i El Hi D W D x PU GI amp 70 lt gt 0 THEN 320 carrier present reset input route so can now input 315 316 15 Paged ROMs Paged ROMs are ROMs that fit in the four rightmost ROM sockets on the BBC microcomputer circuit board They all have the following features in common They exist in the address space amp 8000 to amp BFFF thus only one can be active at a time the rest being paged out They are scanned by the operating system under certain circumstances usually associated with an occurrence which is not understood by the operating system eg inexplicable interrupts unrecognised operating system commands The operating system has the software to handle up to 16 paged ROMs although the hardware can only handle four A paged ROM is used in the following applications Filing systems such as disc operating systems or the Econet system Languages such as BASIC FORTH BCPL etc Utilities such as word processor
56. a Y register offset to specify the string address The string need not be enclosed by quotation marks GSINIT must be used not only to strip off leading spaces but also to set up an information byte in zero page which indicates the termination character and if the string is surrounded by quotation marks If the carry flag is clear on entry then the first space or carriage return or second quotation mark will be considered as the terminating character for the string If the carry flag is set then only a carriage return or a second quotation mark will be considered as the terminal character On exit Y contains the offset of the first non blank character from the address contained in amp F2 and amp F3 A contains the first non blank character as returned by the first call of GSINIT Z flag is set if the string is a null string e g a BEQ instruction will cause a branch This routine has not been documented by Acorn but has been used in applications software 105 79 GSREAD Read character from string input Call address amp FFC5 This routine should only be used following a GSINIT call GSREAD should be entered with Y set to either the Y value following a GSINIT call or following a previous GSREAD call Locations amp F2 and amp F3 should contain the address of the start of the string i e should not have been altered since the last GSINIT call On exit A contains the character read from the string Y contains the ind
57. access When opening a file for output only an attempt is made to delete the file before opening On exit X and Y are preserved A is preserved on closing and on opening contains the file handle assigned to the file If A 0 on exit the file could not be opened C N V and Z are undefined Interrupt state is preserved but may be enabled during the call 16 8 OSFSC Various filing system control functions This has no direct call address Indirected through amp 21E This entry point is used for miscellaneous filing system control actions The accumulator on entry contains a code defining the action to be performed A 0 A OPT command has been used X and Y are the two parameters See operating system commands section 2 14 for details of how OPT is interpreted for the cassette and ROM filing systems Other filing systems should follow the same pattern as nearly as is appropriate A 1 EOF is being checked On entry X is the file handle of the file being checked On exit X amp FF if an end of file condition exists X 0 otherwise A 2 A command has been used The character is not part of the command name The filing system should attempt to RUN the file whose name follows the character This gives the user an 343 A 3 A 4 A A 6 A 7 344 abbreviated way of RUNning a file when specifying a directory in the file name e g L FORMS80 L FORM80 would load the program FORM80 from the
58. address may be in the form length where the second field is preceded by a and the size of memory to be saved is specified in hexadecimal 19 2 18 SPOOL lt filename gt SP The SPOOL command causes all screen output to be repeated into a file The file is opened by SPOOL sfile name gt and closed by repeating this command or by typing SPOOL alone See OSBYTEs amp 03 and amp C7 199 for more information 2 19 TAPEn T OSBYTE with A amp 8C 140 TAPE without any number selects cassette filing system and sets the default baud rate 1200 T APE3 selects tape with 300 baud and TAPE12 selects 1200 baud 2 20 TVx y no abbreviation OSBYTE with A amp 90 144 The TV command allows the vertical position of the screen to be altered and interlace to be switched on or off The first parameter causes the vertical position to be altered a value of 0 causes no change a value of 1 would cause the screen to be moved up one line and a value of 255 would cause the screen to be moved down one line The second parameter should be 0 or 1 a value of 0 causes interlace to be enabled and a value of 1 causes interlace to be switched off Any change of interlace or screen position will only come into effect at the next mode change and will remain until a further IV command or a hard reset Interlace cannot be turned off in mode 7 It is possible to switch off interlace in mode 7 but the character set stored in the SAA 5050
59. addressing allows a branch forward 127 bytes or back 128 bytes from the program counter value after the branch instruction has been executed The calculation of the relative branch value is normally quite transparent to the programmer using the BASIC assembler When writing in assembly language the programmer follows the branch instruction with a label or absolute address and the assembler performs the necessary calculations The use of relative addressing will only become apparent when a label or absolute address is specified outside the relative addressing range When this occurs the assembler will flag an Out of range error to the user OPT 0 is used to suppress this error from forward references on the first assembler pass 40 6 The 6502 Instruction Set 6 1 The 6502 registers and abbreviations Accumulator A An 8 bit general purpose register used for all the arithmetic and logical operations X Index Register X An 8 bit register used as the offset in indexed and pre indexed indirect addressing modes or as a counter Y Index Register Y An 8 bit register used as the offset in indexed and post indexed indirect addressing modes or as a counter Status Register An 8 bit register containing various status flags and an interrupt mask These are Carry flag C Bit 0 Set if a carry occurs during an add operation and cleared if a borrow occurs during subtraction Used as a 9th bit in rotate and shift operations
60. addressing below e g LDA amp 2800 X load accumulator from amp 2800 X ADC table Y A A tablety 5 8 Zero page X addressing using zero page address X This mode is the same as the absolute X addressing mode except that an 8 bit base address is used The assembler automatically uses this mode where available if a zero page address is specified in the operand field If a variable is used to describe the address of the zero page location it should be set up before the first pass of the assembler This is because the assembler will assume 16 bit addressing on the first pass if the variable is unrecognised and allocate two bytes for the address On the second pass the zero page opcode and one byte of address will be assembled causing all further label values to be wrong N B For the LDX instruction a zero page Y addressing mode is provided e g LDX amp 72 Y load X with contents of amp 72 Y LSR amp 80 X one bit right shift contents of amp 80 X 5 9 Pre indexed indirect addressing using a table of indirect addresses in zero page This addressing mode is designed for use with a table of addresses in zero page locations The operation is performed on a memory location the address of which is contained within the zero page locations specified by an 8 bit base address plus the contents of the X register 38 N B The Y register cannot be used for this addressing mode 26805600 26815640
61. amp D3 211 FX 211 Read write BELL CTRL G channel lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains the channel number to be used for the BELL sound Default value is 3 214 OSBYTE amp D4 212 FX 212 Read write BELL CTRL G SOUND information lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains a byte which determines either the amplitude or the ENVELOPE number to be used by the BELL sound If an ENVELOPE is specified then the value should be set to ENVELOPE no 1 8 Similarly an amplitude in the range 15 to 0 must be translated by subtracting 1 and multiplying by 8 The least significant three bits of this location contain the H and S parameters of the SOUND command see User Guide e g Try FX 212 216 for a softer BELL sound amplitude 4 Default value 144 amp 90 215 OSBYTE amp D5 213 FX 213 Read write bell CTRL G frequency lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This value contains the pitch parameter as used by SOUND command third parameter used for the BELL sound Default value 101 amp 65 216 OSBYTE amp D6 214 FX 214 Read write bell CTRL G duration lt NE
62. amp FF and a negative INKEY value in X see appendix C this call performs a keyboard scan On exit X and Y contain amp FF if the key being scanned is pressed 153 OSBYTE 2 82 130 Read machine high order address No entry parameters This call provides a 16 bit high order address for filing system addresses which require 32 bits As the BBC microcomputer uses 16 bit addresses internally a padding value must be provided which associates a given address to that machine On exit X and Y contain the padding address X high Y low This address is amp FFFF for the BBC microcomputer I O processor After call A is preserved C is undefined 154 OSBYTE amp 83 131 Read top of operating system RAM address OSHWM No entry parameters On exit X and Y contain the OSHWM address X low byte Y high byte This call is used by BASIC to initialise the value of PAGE After call A is preserved C is undefined 155 OSBYTE amp 84 132 Read bottom of display RAM address HIMEM No entry parameters On exit X and Y contain the HIMEM address X lovv Y high A is preserved C is undefined 156 OSBYTE 8 85 133 Read bottom of display RAM address for a specified mode Entry parameters X determines mode number On exit X and Y contain the address X low byte Y high byte This call may be used to investigate the consequences of a particular mode s selection After call A is preserved C is undef
63. as BASIC FORTH BCPL or VIEW 279 11 6 Page eight amp 800 amp 8FF This page is primarily buffers but does also contain the sound processing workspace This page is laid out thus amp 800 amp 83F Sound workspace amp 840 amp 84F Sound channel 0 buffer amp 850 amp 85F Sound channel 1 buffer amp 860 amp 86F Sound channel 2 buffer amp 870 amp 87F Sound channel 3 buffer amp 880 amp 8BF Printer buffer amp 8C0 amp 8FF Envelope storage area envelopes 1 4 11 7 Page nine amp 900 amp 9FF This page can be used in one of three basic ways a As an extended envelope storage area amp 900 amp 9BF Envelope storage area envelopes 5 16 amp 9C0 amp 9FF Speech buffer b As an RS423 output buffer amp 900 amp 9BF RS423 output buffer amp 9CO0 amp 9FF Speech buffer c As a cassette output buffer amp 900 amp 9FF Cassette output buffer Uses b and c are largely compatible apart from speech as the 6850 can only be used by either the cassette or the RS423 system at any one time and the cassette system waits until the RS423 output has timed out before taking control of the 6850 At time out the RS423 output buffer is usually clear 11 8 Page ten amp A00 amp AFF This page is used for either the cassette input buffer or for the RS423 input buffer 280 11 9 Page eleven amp B00 amp BFF This page is the soft key buffer The first sev
64. asynchronous communications interface adapter see serial system chapter 20 will produce three types of interrupt 1 Receiver interrupt a character has been received De Transmitter interrupt a character has been transmitted 3 Data Carrier Detect DCD interrupt a 2400Hz tone has been discontinued at the end of a cassette block The 6850 contains a status byte that enables the 6502 to locate the cause of the interrupt This byte is organised as bit 0 bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 bit 7 This bit is set on a receiver interrupt Bits four five and six are valid after this interrupt This bit is set on a transmit interrupt This bit is set on a DCD data carrier detect interrupt This bit is set if the 6850 is not CLEAR TO SEND Framing error Receive error Receiver overrun Receive error Parity error Receive error Set if the 6850 was the source of the current interrupt This bit is the first to be checked by the operating system interrupt handling routine If it isn t set the routine moves on to checking the system VIA to see if it generated the interrupt Serial processing can be split into two parts that done for the cassette filing system and that done for the RS423 system 300 13 8 1 The cassette serial system The cassette system uses the interrupts in the follovving vays A transmitter interrupt causes the next byte of output data to be sent to the 6850 A receive
65. available for use by any other part of the assembler or BASIC program 3 5 Forward Referencing and Two Pass Assembly In the construction of a machine code program using the BASIC assembler a large number of labels may be generated It is often the case that one part of the program needs to jump forward over another part of the program Labels provide a convenient way of marking that point in the program to 25 which the processor is to jump When assembling the machine code the assembler works sequentially through the program and in the case of a forward reference the assembler will encounter the reference before the label In the normal course of events an error will be flagged No such variable In order to resolve forward references two passes of the assembler are required The first pass should be performed with error trapping switched off and during this pass all the labels will be initialised A second pass will provide all the correct values required for forward referencing During this second pass error trapping should be enabled to pick up any genuine programming mistakes The most convenient way of performing the two passes is to use a FOR NEXT loop The programmer should make sure that P is reinitialised at the beginning of the second pass It is often convenient to set up the pseudo operation OPT using the FOR loop variable errors and listing disabled for the first pass errors enabled and listing as required for the sec
66. connector Chapter 22 explains how port B can be programmed The diagram below figure 24 1 illustrates the connector The view is shown looking into the board mounted connector from outside Note that wires 1 and 20 are the two outermost wires on the ribbon cable The female part to the connector is a standard 20 way IDC connector IDC means insulation displacement connector The plug is normally connected to users circuits via a length of special 20 way ribbon cable which is available from most good computing shops This cable can be connected to a circuit directly by soldering the wires to the circuit board or indirectly by another IDC plug and header or a DIL header A DIL header will plug into any ordinary integrated circuit socket 425 24 3 The USER PORT connector PB7 ov PB6 ov PB5 ov PB4 ov PB3 ov PB2 ov PB1 ov PBO ov CB2 5V CB1 5V USER PORT CONNECTOR LOOKING INTO SOCKET MOUNTED ON THE MAIN CIRCUIT BOARD Note Pins 1 and 20 connect to the wires at the edge of the ribbon cable connected to the IDC header Figure 24 1 User port connector 426 25 Floppy Disc and Econet Sheila addresses amp 80 amp BF 25 1 The 8271 floppy disc controller Sheila amp 80 amp 9F The 8271 floppy disc controller chip and its associated hardware must be fitted to the standard model B before discs can be used The upgrade information is in Appendix H The function of this chip is to extract data from a dis
67. controller see 6845 501 CTRL G channel duration frequency sound Cursor editing status position position of text cursor positions storage in page 3 reading graphics cursors shape Cursor control in video ULA Cursor editing Cursor keys defining as soft keys Cycle Clock Cycle numbers Cyclic redundancy checking D Data bus Data carrier detect DCD see data carrier detect DEC Decrement memory by one Decimal flag Decimal mode flag Default messages Default vector table DEX Decrement X by one DEY Decrement Y by one Directories Disc drive timings Disc filing system Disc upgrades E Econet event hardware OS call interception status read character status write character status zero page work space Econet filing system Econet vector Editing status of cursor Editing using the cursor Electrical specification for 6522 End of conversion for ADC End of file check Envelope command OSWORD 214 217 216 215 233 367 158 275 252 366 379 120 120 494 334 348 494 313 65 33 90 42 13 265 66 67 333 246 350 480 293 427 211 211 211 267 351 260 233 120 398 418 150 250 EOR exclusive OR memory with A 68 EQU in assembler programs 502 26 Error handling after a BRK BRK vectoring using BRK ESCAPE effect control event Escape flag ESCAPE providing escape action read write flags returning an ASCII value Escape character insertion into text ESCAPE character re
68. counted For a count operation if the carry flag is set the amount of space left in the buffer is returned otherwise the number of entries in the buffer is returned On exit For purge X and Y are preserved For count X low byte of result Y high byte of result A is undefined V C are preserved 10 13 The spare vectors IND1V IND3V amp 230 235 These vectors are not used by OS 1 20 Future versions of the operating system may use these vectors or Acorn may allocate them for use by application software 264 10 14 The default vector table There exists within the operating system OS 1 2 onwards a lookup table of the default vector contents This table is useful for restoring the vectors after changing them or for software protection some people use events to get past software protection The information on the table is given thus Location amp FFB6 length of lookup table in bytes amp FFB7 low byte of the address of the table amp FFB8 high byte of the address of the table 265 266 11 Memory usage This chapter describes how the memory in the BBC microcomputer is allocated between the different contenders The area allocated to the operating system is described in some detail but that used by the language and user programs is only outlined as it varies from language to language Some of the information in this section is also to be found elsewhere see chapters on filing systems and paged ROMs num
69. current directory An unrecognised operating system command has been used the current filing system is offered the command last after all the paged ROMs this may include the filing system itself which will be offered the command initially as a filing system command via the service entry point See paged ROMs section 15 1 1 The filing system should normally attempt to RUN unrecognised commands If the currently selected filing system is unable to RUN a file quickly i e within a few seconds it should not attempt to RUN the file but issue a Bad Command message instead The cassette filing system is unable to execute a tape file quickly and so does not try to RUN unrecognised operating system commands On entry X and Y point to the command name A RUN command has been used Load and execute the file whose name is pointed to by X and b A CAT command has been used Produce a catalogue X and Y point to the rest of the command line for any parameters required A new filing system is about to take over so shut down gracefully close the SPOOL and EXEC files using OSBYTE amp 77 and do anything else considered necessary A request has been issued for the range of file handles usable by the filing system Return in X the lowest handle issued and in Y the highest handle possible Most filing system handles do not overlap A 8 This call is issued by the operating system each time it is about to process an operati
70. decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused_ cycles relative 2 2 1 if branch succeeds 2 if to new page Example A loop which shifts A left until bit 7 is set loop ASL A shift accumulator 1 hit left BPL loop if bit 7 not set then go round again N B This will be an endless loop if A 0 on entry 53 BRR Forced Interrupt PC and P pushed on stack PCL amp FFFE PCH amp FFFF This instruction forces an interrupt to occur The processor jumps to the location stored at amp FFFE The program counter is pushed onto the stack followed by the status register A BRK instruction usually represents an error condition and the BRK handling code is usually an error handling routine Using machine code in a BASIC environment it is possible to use BASIC s error handling facilities see section 3 7 A user BRK handling routine may be implemented see Vectors section section 10 2 Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command set V overflow flag not affected N negative flag not affected Addressing mode bytesused_ cycles implied 1 7 N B A BRK instruction cannot be disabled by setting the interrupt disable flag Example Cause an error if A is greater than 4 C
71. filing systems only 2 1 An operating system command line with a string escape character as its first non blank character will be ignored by the operating system This could be used to put comment lines into a series of operating system commands placed in an EXEC file for example 22r This command is directly equivalent to the CAT command 2 3 lt file name gt This command is treated exactly the same as typing RUN lt file name gt or lt file name gt The file name is offered directly to the filing system and will not be interpreted as a command 2 4 BASIC B While the operating system is independent of any particular language and only serves to provide an interface between languages or utility software and the machine hardware BASIC has been accorded special status The BASIC command is resident in the operating system command table and it enables the operating system to select a language in paged ROM not possessing a service entry point for further details about paged ROMs see chapter 15 The operating system scans the paged ROMs and keeps a record of which paged ROM contains BASIC see OSBYTE amp BB 187 If a BASIC ROM is not present this command is offered to other paged ROMs 12 2 5 CAT This command displays a catalogue of files from the selected filing system When the cassette or ROM filing systems are selected the name of each file encountered is printed on the screen along with the b
72. from the command line parameters Any OSBYTE call may be made using the FX command but it is not always appropriate to make an OSBYTE call using this direct method e g OSBYTE calls that return values in any of the registers The FX command is a useful way of making those OSBYTE calls which have a direct effect from a BASIC program or from the command line interpreter For further information on specific PX OSBYTE calls refer to chapter 8 2 9 HELP FH Typed in on a machine containing only the operating system and BASIC ROMs this command causes the version number of the operating system to be printed out i e OS 1 20 Each HELP call is offered to any paged ROMs that are resident and these may be able to respond to further command line parameters e g HELP VIEW HELP UTILS For more information about HELP handling in paged ROMs see section 15 1 1 service call 9 2 10 KEYn lt string gt K The ten red topped function keys the BREAK key the COPY key and the four cursor control keys may be set up using this command Using KEYn with n in the range 0 to 9 sets up the function keys KEY10 can be used to program the BREAK key Before the remaining programmable keys can be used a FX4 2 must be performed This disables cursor editing and enables the following soft keys 15 KEY 11 COPY KEY 12 left cursor KEY 13 right cursor KEY 14 down cursor KEY 15 up cursor Each time a soft key is pressed a soft key ch
73. generated in this way 406 22 2 6 Timer 2 operation Timer 2 operates either as an interval timer in the one shot mode only or as a counter for counting negative pulses on the PB6 pin A single control bit in the Auxiliary Control Register selects between these two modes Timer 2 comprises a write only low order latch T2L L a read only low order counter and a read write high order counter The counter register contents are decremented at 1 MHz Figure 22 9 illustrates the timer 2 counter registers EG 8 TIMER 2 LOW ORDER COUNTER REG 9 TIMER 2 HIGH ORDER COUNTER re 1 256 a 2 512 s E 4 1024 you a COUNT fod VALUE 2048 coun k ALUE i 32 8192 t 16384 32788 WAITE DA AEEA aS INTO T2 LOW ORDER WRITE 8 BIT LOADED INTO T HIGH ORDER COUNTER ALSO LOW ORDER LATCHES READ 8 BITS FROM T2 LOW ORDER COUNTER TRANSFERRED TO LOW ORDER TRANSFERRED TO MPU T2 INTERRUPT COUNTER IN ADDITION T2 INTERRUPT FLAG IS RESET FLAG IS RESET READ 8 BITS FROM T2 HIGH ORDER COUNTER TRANSFERRED TO MPU Figure 22 9 T2 Counter Registers 22 2 7 Timer 2 one shot mode In the one shot mode the operation of timer 2 is similar to that of timer 1 T2 provides a single interrupt for each time out after T2C H had been set The counter continues to decrement after time out but the interrupt is disabled after the initial time out so that it will not be set again each time that the timer decrements through zero T
74. in page 3 GSINIT GSREAD H Handshaking Hard BRK Hardware introduction screen wrap around 209 199 210 238 117 235 228 378 173 125 126 201 201 201 427 480 131 251 281 278 325 191 437 170 225 225 225 225 338 282 268 268 269 252 275 105 106 494 244 353 419 503 High order address HIMEM Host processor I I O processor memory read OSWORD write OSWORD Immediate addressing Implicit addressing INC Increment memory by one Index registers indexed addressing Indirect addressing INKEY INKEY countdown timer page two location Input buffer OSVVORD 1 zero page location Input line OSWORD input stream selection Instruction 6502 cycle Instruction set for the 6502 INSV Interlace Internal key numbers table Interrupt bit masks disable flag forced return from unrecognised paged ROM Interrupt vectors Interrupts example program interception keyboard rnaskable non maskable OS processing serial processing system VIA user VIA vectors VIAs zero page accumulator storage Interval timer crossing zero event page two address read OSWORD write OSWORD INX Increment X by one INY Increment Y by one IRQ1V IRQ2V 504 154 156 433 249 249 36 35 69 41 37 37 153 273 270 248 118 43 43 41 263 168 20 142 495 229 41 59 91 54 86 322 258 297 306 307 305 187 296 296 299 300 302 304
75. in the User Guide FX155 can be used to write colour data into the Palette It will automatically EOR the physical colour with 7 see later Usually it is better to use VDU19 or OSWORD amp 0C to program logical and actual colours 379 The palette register consists of two 4 bit fields Bits 0 3 are the actual colour field Bits 4 7 are the logical colour field as illustrated in figure 19 2 7 6 5 2 1 LOGICAL COLOUR ACTUAL COLOUR REGISTER REGISTER Fig 19 2 The Palette register 19 2 1 Logical colour field The following description of programming the palette only applies to direct programming using FX155 If OSWORD amp 0C or VDU19 are used none of the problems which are about to be outlined will be relevant Programming the logical colour directly is easy in mode 2 The logical colour number then occupies the entire 4 bit field In two colour modes 0 3 4 and 6 programming the logical colour directly is more complex Bit 7 defines the logical colour but bits 4 5 and 6 must be programmed to all their possible values In other words in order to set logical colour 1 to actual colour 5 it is necessary to program logical colours 9 10 11 12 13 14 and 15 to 5 If this is not done some parts of characters will be in one colour and other parts will be in a different colour Programming the logical colours in a four colour mode is slightly more complex Bits 7 and 5 together contain the logical colour number
76. input buffer full event FX 14 2 X 2 Enable character entering buffer event PX 14 3 X 3 Enable ADC conversion complete event FX 14 4 X 4 Enable start of vertical sync event FX 14 5 X 5 Enable interval timer crossing 0 event FX 14 6 X 6 Enable ESCAPE pressed event FX 14 7 X 7 Enable RS423 error event FX 14 8 X 8 Enable network error event FX 14 9 X 9 Enable user event For more information about events see chapter 12 After call A is preserved X and Y contain the old enable state gt O enabled C is undefined 130 OSBYTE amp 0F 15 FX 15 Flush selected buffer class Entry parameters X value selects class of buffer X 0 All buffers flushed X lt gt 0 Input buffer flushed only See OSBYTE call amp 16 FX 21 After call Buffer contents are discarded A is preserved X Y and C are undefined 131 OSBYTE amp 1C 16 FX 16 Select ADC channels vvhich are to be sampled Entry parameters X value selects number of channels sampled FX 16 0 X 0 Sampling disabled FX 16 n X n Number of channels to be sampled n must be in the range 0 to 4 if greater then set to 4 After call A is preserved X contains the old FX 16 value 132 OSBYTE 8 11 17 FX 17 Force an ADC conversion Entry parameters X value specifies ADC channel FX 17n X n Force ADC conversion on channel n if n gt 4 then n 4 See OSBYTE with A amp 80 128 also After call A is preserved X is preserved if it is in the range 0 to
77. is designed to be used with interlace on Type in VDU23 0 8 amp 90 0 0 0 23 0 9 809 0 0 0 and you will see why the operating system disallows this See chapter 18 for more information about programming the 6845 video controller chip 20 3 The BASIC Assembler One of the many attractive features of BBC BASIC is the incorporation of a mnemonic assembler within the language itself This provides a powerful environment for the assembler and allows machine code to be easily incorporated within BASIC programs Hybrid BASIC machine code programs may often lead to the use of the best features of each language the speed of machine code when it is required coupled with the increased power of BASIC when speed is not of paramount importance The assembler facilities available to users are dependent on the version of BASIC that is resident in the machine To ascertain which version of BASIC is present type REPORT following a BREAK If the copyright message is dated 1981 then this is old BASIC which will henceforth be referred to as Level 1 BASIC and if the message is dated 1982 then this is new BASIC which will be referred to as Level 2 BASIC Below is an example of a simple machine code program written using the BASIC assembler 10 OSWRCH amp FFE3 20 DIM MC 100 25 DIM data amp 20 30 FOR opt S 0 TO 3 STEP 3 40 PS MC 50 60 OPT opts 70 entry LDX 0 set index count in X reg to 0 ES LDA data load first ite
78. is preserved X Y and C are undefined 176 OSBYTE amp 9E 158 Read from speech processor No entry parameters This call may be used to read data from the serial speech ROM or to read the status register of the speech processor In order to read from the speech ROM a read byte command must have previously been sent to the speech processor using OSBYTE call with A amp 9F FX 159 If the speech processor has not been primed in this way then a copy of the speech processor s status register is returned in the Y register After call A is preserved X and C are undefined 177 OSBYTE amp 9F 159 FX 159 Write to speech processor Entry parameters Data command in Y This call enables the user to pass opcodes commands or bytes of data to the speech processor After call A is preserved X Y and C are undefined 178 OSBYTE amp A0 160 Read VDU variable value Entry parameters X contains the number of the number to be read On exit X contains low byte of number and Y contains the high byte This call reads locations amp 300 X and amp 301 X See memory usage section 11 4 After call A is preserved X Y and C are undefined 179 OSBYTEs amp A6 166 and amp A7 167 Read start address of OS variables lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This call returns the start address of the memory used by
79. memory map All software on the 6502 makes extensive use of the internal registers for programming The bits in most peripheral registers define the operation of a particular piece of hardware or tell the processor something about that peripheral s state 496 Rollover this is a function provided on the keyboard to cope with fast typists Two keys can be pressed at once The previous key with a finger being removed and the next key with the finger hitting the key The software in the operating system ensures that rollover normally operates correctly It doesn t operate at all when the shift key is held down ROM Read only memory as the name implies ROM can only be read from and cannot be modified by being written to The MOS and BASIC plus any resident software in the BBC microcomputer are held in ROMs Serial data transmitted along only one line is transmitted serially Serial data transmission is normally slower than parallel data transmission because only one bit instead of several bits are transferred at a time Stack a page of memory in the 6502 used for temporary storage of data Data is pushed onto a stack in sequence then removed by pulling the data off the stack The last byte to be pushed is the first byte to be pulled off again The stack is used to store return addresses from subroutines Page amp 01 is used for the stack in the BBC microcomputer Transducer a device which converts some analogu
80. not affected V overflow flag set if the sign of the result is wrong N negative flag set if bit 7 of the result is set Addressing mode bytesused cycles immediate 2 2 Zero page 2 3 zero page X 2 4 absolute 3 4 absolute X 3 4 1 if page crossed absolute Y 3 4 1 if page crossed indirect X 2 6 indirect Y 2 5 1 if page crossed Example 16 bit value at locations amp 80 and amp 81 subtracted from 16 bit value at locations amp 82 and amp 83 result at locations amp 82 and amp 83 SEC ready for any borrow LDA amp 80 low order byte of first value SBC amp 82 A A amp 82 borrow may occur STA amp 82 place result in amp 82 LDA amp 81 high order byte of first value SBC amp 83 A A amp 83 1 C STA amp 83 place result in amp 83 88 SEC Set carry flag C71 This instruction is used to set the carry flag This instruction should be used to set the carry flag prior to a subtraction unless the carry flag has been deliberately left as a borrow from a previous subtraction Processor Status after use C carry flag set Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode Bytes used Cycles implied 1 2 Example Explicit setting of the carry flag SEC C l 89 SED Set decimal mode D 1 This instructio
81. of having latches which remember the initial value put into the counter is that the initial value can be restored after the counter has decremented to zero If this is done automatically then the timer enters a free running mode In the free running mode PB7 is inverted and the interrupt flag is set each time the counter has decremented to zero The contents of the 16 bit latch are then transferred to the counter which decrements to zero again and so on This produces a true square wave of variable frequency on the PB7 output The interrupt flag can be cleared by writing T1C H by reading T1C L or by writing directly into the flag as will be described All of the timers in the 6522 can be retriggered This means that rewriting the value in the counter will always re initialise the time out period Time out will therefore be completely inhibited if the processor continues to rewrite the timer before it reaches zero T1 operates in this way if the 6502 writes into the high order counter T1C H If the 6502 only loads the latches this will not affect the counter until the next time zero is reached The timer can be read without affecting its value This can be very useful because the new timer time doesn t come into effect until zero is reached If the 6502 responds to each interrupt by programming a new value into the latches the period of the next half cycle on the PB7 output will be determined Waveforms with complex mark space ratios can be
82. on an output line every time its count reaches zero Figure 22 7 and figure 22 8 illustrate the T1 counter and latches 404 REG 4 TIMER 1 LOW ORDER COUNTER REG 5 TIMER 1 HIGH ORDER COUNTER 1 256 2 512 4 1024 8 COUNT 2048 LL COUNT 1 VALUE 4096 32 8192 64 16384 128 32768 WRITE 8 BITS LOADED INTO Tt LOW ORDER WRITE 8 BITS LOADED INTO T1 HIGH ORDER LATCHES LATCH CONTENTS ARE LATCHES ALSO AT THIS TIME BOTH TRANSFERRED INTO LOW ORDER HIGH AND LOW ORDER LATCHES COUNTER AT THE TIME THE HIGH TRANSFERRED INTO TI COUNTER ORDER COUNTER IS LOADED REG 5 T1 INTERRUPT FLAG ALSO IS RESET READ 8 BITS FROM Tt LOW ORDER COUNTER READ 8 BITS FROM T1 HIGH ORDER COUNTER TRANSFERRED TO MPU IN ADDITION TRANSFERRED TO MPU T1 INTERRUPT FLAG IS RESET BIT 6 IN INTERRUPT FLAG REGISTER Figure 22 7 T1 Counter Registers REG 6 TIMER 1 LOW ORDER LATCHES REG 7 TIMER 1 HIGH ORDER LATCHES a 2 512 4 1024 8 COUNT 2048 COUNT 16 VALSE agag VALUE 32 8192 64 16384 128 32768 WRITE 8 BITS LOADED INTO T1 LOW ORDER WAITE B BITS LOADED INTO T1 HIGH ORDER LATCHES THIS OPERATION IS NO LATCHES UNLIKE REG 4 OPERATION DIFFERENT THAT A WRITE INTO NO LATCH TO COUNTER TRANSFERS REG 4 TAKE PLACE READ 8 BITS FROM T1 LOW ORDER LATCHES READ 8 BITS FROM Ti HIGH ORDER LATCHES TRANSFEARED TO MPU UNLIKE REG 4 TRANSFERRED TO MPU OPERATION THIS DOES NOT CAUSE RESET OF T1 INTERRUPT FLAG Figure 22 8 T
83. operands This would be a very tedious business if it had to be done by looking up the opcode values in tables and poking in the values by hand It is much faster easier and more efficient to get the computer to do most of the work A program which analyses text input representing opcode symbols converts these to opcode values and inserts these values into memory is called an assembler The text input consists of opcode mnemonics three letter words which specify the opcode type followed by numbers variable names or expressions which give the values of the operand to be used by the opcode Like BASIC the assembler requires the program to be written in a defined way according to a syntax The language that an assembler understands is called assembler or assembly language In the BBC Microcomputer an assembler is available as part of the BASIC language and a description of how this assembler can be used is contained in the chapter on the BASIC assembler Many of the following sections include descriptions of the various operating system routines and facilities which are available to the machine code programmer 10 2 Operating System commands The command line interpreter resident vvithin the operating system will recognise a number of commands and act upon receiving them These commands are most appropriately often used from the keyboard or in BASIC programs using the prefix Commands may also be passed to the command line interpreter
84. out of the CB2 pin under the control of an internal modulo 8 counter Pulses from an external source can be applied to CB1 to shift a bit into or out of CB2 Alternatively with proper mode selection shift pulses generated internally will appear on the CB1 pin for controlling external devices The control bits which select the various shift register operating modes are located in the Auxiliary Control Register The configuration of the SR data bits and the SR control bits of the ACR are illustrated in figure 22 10 and figure 22 11 408 REG 10 SHIFT REGISTER BORAGE SHIFT REGISTER BITS NOTES 1 WHEN SHIFTING OUT BIT 7 S THE FIRST BIT OUT AND SIMULTANEOUSLY IS ROTATED BACK INTO BIT 0 2 WHEN SHIFTING IN BITS INITIALLY ENTER BIT O AND ARE SHIFTED TOWARDS BIT 7 Figure 22 11 Shift Register Control Bits 22 2 10 Shift register modes of operation Shift Register Disabled SRMODE 0 In this mode the SR is disabled The 6502 can however write or read the SR and the SR will shift one bit left on each CB1 positive edge The logic level present on CB2 is shifted into bit 0 The SR interrupt flag is always disabled in this mode Shift in under control of T2 SRMODE 1 In mode 1 the shifting rate is controlled by the 8 low order bits of T2 Shift pulses are generated on the CB1 pin to control shifting in external devices The time between transitions of this output clock is controlled by the low order T2 latch
85. overflow flag not affected N negative flag set if bit 7 of Y becomes set Addressing mode bytesused cycles implied 1 2 Example Increment Y register INY Y Y 1 71 JMP Jump to new location PC new address This instruction is the machine code equivalent of aGOTO statement in BASIC An indirect addressing mode is available where the address for the JMP is contained in memory specified by the address in the operand field see examples below Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused cycles absolute 3 3 indirect 3 5 Examples A direct jump JMP entry goto entry An indirect jump a contrived example LDA amp 00 A 0 STA amp 2800 amp 2800 A address low byte LDA amp 40 A 840 STA amp 2801 amp 2801 A address high byte JMP amp 2800 jump to amp 4000 72 JSR Jump Subroutine Push current PC onto stack PC new address This instruction causes a jump but also saves the current program counter on the stack The subroutine which is called returns to the part of the program that called it by pulling the saved address and jumping back to it A subroutine must always be terminated by an RTS instruction which performs the return to the lo
86. passed through this vector WRCHYV Write character vector All writes to the screen and printer etc are passed through this vector RDCHV Read character vector All reads from the currently selected input stream are passed through this vector FILEV Read write a whole file All file loads and saves are passed through this vector ARGSV Read write file arguments All calls of OSARGS pass through this vector BGETV Read one byte from a file Used for BASIC BGET and EXEC commands BPUTV Put one byte to a file Used for BASIC BPUT and SPOOL commands GBPBV Get put a block of bytes from to a file FINDV Open close a file for BPUT BGET GBPB ARGS calls 255 amp 21E F amp 220 1 amp 2223 amp 224 5 amp 226 7 amp 228 9 amp 22A B amp 22C D amp 22E F amp 232 3 amp 234 5 FSCV Various filing system control functions EVENTYV Pointer to the event handling routine UPTV Pointer to user print routine NETV Used by Econet to take control of the computer VDUV Used by the VDU driver to direct unrecognised VDU 23 and PLOT commands KEYV Used by the operating system for all keyboard access Enables the use of external keyboards INSV Insert into buffer vector REMV Remove from buffer vector CNPV Count purge buffer vector amp 230 1 IND1V Spare vector IND2V Spare vector IND3V Spare vector These vectors are now described in more detail 10 1 The user vecto
87. per line control colour mode selection cursor control flash control palette register read registers teletext selection write control register write palette register Volume control VPOS 510 261 VV 253 Wait until vertical sync 257 Windows 264 storage in page 3 263 Wrap around for screen 263 Write a new line OSNEWL 265 Write byte to an open file 326 OSBPUT 281 OSGBPB 262 Write character OS WRCH 260 Write file attributes 264 OSARGS 13 16 254 OSFILE 258 Write I O processor memory 254 Write user flag 261 Z 343 Z80 second processor 343 Zero flag 324 Zero flag see VIA OS usage user 15 Zero page addressing 116 Zero page X or Y addressing 291 135 404 400 403 408 425 408 399 408 417 425 157 156 see 6845 and Video ULA 377 378 378 379 378 379 193 378 173 174 420 158 135 275 419 103 339 339 101 335 249 117 435 41 267 30 36
88. read the interval timer Used for events see chapter 12 The five byte clock value is written to the address contained in the X and Y registers 9 6 OSWORD call with A amp 4 Write interval timer This routine may be used to set the interval timer to a five byte value contained in memory at the address in the X and Y registers 9 7 OSWORD call with A amp 5 Read I O processor memory A byte of I O processor memory may be read across the Tube using this call A 32 bit address should be contained in memory at the address contained in the X and Y registers XY 0 LSB of address to be read I 2 3 MSB of address to be read If the I O processor uses 16 bit memory addressing only least significant two bytes need to be specified On exit The byte read will be contained in location XY 4 9 8 OSWORD call with A amp 6 Write I O processor memory This call permits I O processor memory to be written across the Tube A 32 bit address is contained in the parameter block addressed by the X and Y registers and the byte to be written should be placed in XY 4 249 9 9 OSVVORD call with A amp 7 SOUND command This routine takes an 8 byte parameter block addressed by the X and Y registers The 8 bytes of the parameter block may be considered as the four parameters used for the SOUND command in BASIC e g To perform a SOUND 1 15 200 20 XY 0 Channel LSB 1 amp 01 1 MSB amp 00 2 Amplitude LSB 15 amp F1 3 MSB amp FF 4 Pitch LSB 200
89. respective parameters After call A and C are preserved 168 OSBYTE 8 91 145 Get character from buffer Entry parameters X contains buffer number see OSBYTE with A amp 15 FX 21 for buffer numbers On exit Y contains the extracted character If the buffer was empty then C 1 otherwise C 0 After call A is preserved 169 OSBYTEs 8292 to 8 97 146 to 151 FX 146 to 151 Read or Write to mapped I O Entry parameters X contains offset within page Y contains byte to be written if write OSBYTE call Memory addressed Name read write amp 92 146 amp 93 147 amp FC00 to amp FCFF FRED amp 94 148 amp 95 148 amp FD00 to amp FDFF JIM amp 96 150 amp 97 151 amp FE00 to amp FEFF SHEILA Refer to the hardware section for details about these 1 MHz buses On exit Read operations return with the value read in the Y register After call A is preserved C is undefined 170 OSBYTE amp 98 152 Examine Buffer status Entry parameters X contains buffer number For buffer numbers see OSBY TE call with A amp 15 FX 21 On exit If the buffer is not empty Y pointer to next character to be read from the buffer indexed from zero page locations amp FA and amp FB C 0 If the buffer is empty Y is preserved C 1 After using this call to examine the next character to be read from a non empty buffer the instructions LDA amp FA Y will be required Interrupts should be disabled while the OSBYTE c
90. routines are also indirected through vectors This enables the user to alter the function of the operating system in certain areas For example the buffer management routines can be modified by intercepting INSV and REMV which indirect the buffer insertion and removal routines Vectors also provide a way of allowing the user to enter routines for which there is no direct entry point such as the Filing System Control vector When providing new routines for vectors the programmer should note that there are two basic strategies for returning from vectored code these are 253 a The routine provided completely replaces the standard code In this case the routine should exit with an RTS instruction or an RTI if it is one of the interrupt or break vectors b The routine provided does not completely replace the standard code This is the more usual case and routines of this sort should exit with a JMP oldvec where oldvec is the old vector contents Routines passing control on to further code should exit with the registers preserved from the entry to the routine c A combination of the above This can occur with vectors that provide more than one function this is most of them and some of the functions are to be completely replaced and others passed on to the standard routine In this case both of the above exit strategies should be adopted The use of strategy b allows a whole series of routines to be daisy chained toget
91. should reflect the response time at the transmission end and the time taken for the operating system to act upon the buffer full situation 208 OSBYTE amp CC 204 FX 204 Read write RS423 input suppression flag lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If this location contains 0 then RS423 input is accepted otherwise RS423 input is ignored RS423 receive errors will still cause an event 209 OSBYTE amp CD 205 FX 205 Read write cassette RS423 selection flag lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If this location contains 0 then RS423 data is channelled to the RS423 hardware If this location contains amp 40 then RS423 data is channelled to the cassette hardware This location is only checked and so any change will only come into effect when a baud rate selection is made using FX 7 or 8 210 OSBYTE amp CE 206 FX 206 Read write Econet OS call interception status lt NEW VALUE gt lt OLD VALUES AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If bit 7 of this location is set then all OSBYTE and OSWORD calls except those sent to paged ROMs are indirected through the Econet vector amp 224 to the Econet Bits 0 to 6 are ignored OSBYTE amp CF 207
92. stack B break command bit 4 from stack V overflow flag bit6from stack N negative flag bit 7 from stack Addressing Mode bytesused cycles implied 1 4 Example See the example for PLA above 83 ROL Rotate one bit left M M 2 M0 C C M7 A or M This instruction causes a shift left one bit The bit shifted out of the byte bit 7 is placed in the carry flag The contents of the carry flag are placed in bit 0 lt 76543210 lt C lt ___ Processor Status after use C carry flag set to old value of bit 7 Z zero flag set if result 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of the result is set Addressing Mode bytesused cycles accumulator 1 2 zero page 2 5 zero page X 2 6 absolute 3 6 absolute X 3 7 Example Rotate accumulator contents one bit left ROL A A A rotated left N B The carry flag state should be known before this operation is performed 84 ROR Rotate one bit right M M 2 M7 C C M0 A or M This instruction causes a shift right one bit The bit shifted out of the location bit 0 is placed in the carry flag The contents of the carry flag are placed in bit 7 gt 76543210 gt C m Processor Status after use C carry flag set to old value of bit 0 Z zero flag set if result 0 I interrupt disable not affected D decim
93. terminated by a carriage return character ASCII amp 0D 13 After an OSCLI call A X Y C N V and Z are undefined Interrupt status is preserved but interrupts may be enabled during a call 107 108 8 FX and OSBYTE calls The OSBYTE call to the operating system is a powerful and flexible way of invoking many of the available operating system facilities The FX command can be used to make OSBYTE calls from BASIC programs or directly from the keyboard see section 2 8 OSBYTE OS call specified by the contents of A taking parameters in X and Y Call address amp FFF4 Indirected through amp 20A One entry A selects an OSBYTE routine X contains an OSBYTE parameter Y contains an OSBYTE parameter Any OSBYTE calls which are not recognised by the operating system will be offered to paged ROMs see section 15 1 1 service call 4 If the unrecognised OSBYTE is not claimed by a paged ROM then a Bad command error will be issued error number 254 All the OSBYTE calls recognised by the operating system are described in detail in the following pages Each OSBYTE call or group of related OSBY TE calls is assigned a separate page The description for each call includes details of the entry parameters required and the state of the registers on exit All OSBYTE calls may be made using the FX command but it is not always appropriate to do so i e those calls returning values in the X and Y registers Where it is appropri
94. the operating system to store its internal variables These values are never written by the operating system except at BREAK and are not read by it either After call A is preserved X amp 90 and Y amp 01 C is undefined 180 OSBYTEs amp A8 168 and amp A9 169 Read address of ROM pointer table lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This table of extended vectors consists of 3 byte vectors in the form Location 2 bytes ROM no 1 byte See Paged ROM section 15 1 3 for a complete description of extended vectors On exit X amp 9F low byte Y amp 0D high byte i e address returned is amp 0D9F for OS 1 2 After call A is preserved C is undefined 181 OSBYTEs 8 AA 170 and amp AB 171 Read address of ROM information table lt NEW VALUE gt lt OLD VALUES AND Y EOR X The old value is returned in X the contents of the next location are returned in Y This call returns the origin of a 16 byte table containing one byte per paged ROM This byte contains the ROM type byte contained in location amp 8006 of the ROM or contains 0 if a valid ROM is not present See Paged ROMs chapter 15 On exit X amp A1 Y amp 02 i e origin address amp 02A1 for OS 1 20 After call A is preserved C is undefined 182 OSBYTEs amp AC 172 and amp AD 173 Read address of keyboard translation table l
95. the Y register by one Y Y 1 This instruction decrements the contents of the Y register by one Processor Status after use C carry flag not affected Z zero flag set if Y becomes 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of Y becomes set Addressing mode bytesused_ cycles implied 1 2 Example Decrement Y register DEY Y Y 1 67 EOR Exclusive OR memory with accumulator A A EOR M This instruction performs a bit by bit Exclusive OR of the specified memory location contents with the contents of the accumulator leaving the result in the accumulator The truth table for the logical EOR operation is Acc Mem Result bit bit bit 0 0 0 0 1 1 1 0 1 1 1 0 Processor Status after use C carry flag not affected Z zero flag set if A becomes 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of A becomes set Addressing mode bytesused cycles immediate 2 2 Zero page 2 3 Zero page X 2 4 absolute 3 4 absolute X 3 4 1 if page crossed absolute Y 3 4 1 if page crossed indirect X 2 6 indirect Y 2 5 1 if page crossed Example EOR contents of memory with amp FF LDA amp FF load accumulator with amp FF EOR temp A A EON temp
96. the keyboard is scanned normally otherwise lock keyboard all keys ignored except BREAK This call is used by the REMOTE Econet facility 206 OSBYTE amp CA 202 FX 202 Read write keyboard status byte lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y bit 3 1 if SHIFT is pressed bit 4 0 if CAPS LOCK is engaged bit 5 0 if SHIFT LOCK is engaged bit 6 1 if CTRL is pressed bit 7 1 SHIFT enabled if a LOCK key is engaged then SHIFT reverses the LOCK SHIFT enable bit 7 may be set by holding SHIFT down as the CAPS LOCK key is engaged which enables lower case letters to be typed when capitals are selected by pressing the required key plus SHIFT The only way to set SHIFT enable for the SHIFT LOCK key is to use FX202 144 or OSBYTE amp CA See also OSBYTE with A amp 76 118 207 OSBYTE amp CB 203 FX 203 Read write RS423 handshake extent lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location determines the space remaining in the RS423 input buffer when RS423 input is halted by the operating system entering a buffer full state which sets the RTS line high The default value is 9 The free space remaining in the buffer allows some manipulation of the buffer contents to be carried out before being passed on The value selected
97. the next location are returned in Y This counter is decremented once every vertical sync pulse 50 times per second which is also used for OSBYTE amp 13 FX 19 The timeout counter is used to time interblock gaps and leader tones 185 OSBYTE amp B1 177 FX 177 Read write input source equivalent to OSBYTE with A2 lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location should contain 0 for keyboard input and 1 for RS423 input i e contains buffer no 186 OSBYTE amp B2 178 FX 178 Read write keyboard semaphore lt NEW VALUES lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If this location contains 0 then keyboard interrupts are ignored Keyboard interrupts are enabled if it contains amp FF 187 OSBYTE amp B3 179 FX 179 Read write primary OSHWM for imploded font lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains the OSHWM page value for an imploded font even when character definition RAM explosion has been selected See OSBYTE amp B4 180 and OSBYTE amp 14 20 188 OSBYTE amp B4 180 FX 180 Read write OSHWM equivalent to OSBYTE amp 83 131 on read lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old
98. this pin on the component side of the board between IC 27 and IC 89 Reconnect the cut IC leg to the east pad of link S9 with a short length of insulated wire Ensure that the following link selections have been made S18 NORTH S19 EAST 520 NORTH S21 2 x EAST WEST S22 NORTH S32 WEST S33 WEST On issue 4 boards onwards remove the connector from S9 480 For details of the povver connections see the diagrams belovv Figure H 1 shows the power supply connector on the BBC microcomputer Figure H 2 shows a standard disc drive connector TOP QV OV O 5 Volts 0 12 Volts 1 25 Amps 1 25 Amps O NO CONNECTION 5 Volts BOTTOM TSmA Figure H 1 Auxiliary power socket on BBC Micro PRINTED CIRCUIT BOARD 5V GV OV 412V Figure H 2 Typical disc drive power connector 481 Appendix I Link options 482 ISSUE 3 ONLY There are several options which can be selected using the links on the main circuit board These generally take one of three forms a printed circuit track a soldered wire link or a plug in jumper In the list which follows the link positions are described in points of the compass On this basis NORTH is the direction looking from the keyboard towards the back of the BBC microcomputer All of the link positions are illustrated in figure 1 1 NORTH SOUTH TYPE Note OPEN CLOSED TYPE Note EAST WEST TYPE NORTH SOUTH Note NORTH SOUTH Note WEST
99. tone were turned off immediately after sending the last character it is possible that the last few characters remain in the buffer and be corrupted when sent to the tape 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 312 Fudge factor put 2 dummy bytes in RS423 input buffer to allow control of RTS flag by use of buffer tolerance OSBYTE amp CB 203 138 1 1 138 1 1 Enable receive interrupts to allow control of RTS XD 2 Indicate that RS423 is cassette Hi i El tl a PR PR D D DDD J Uy I IJ x d HH X 205 64 EM Select baud rates EX 7 4 FX 8 4 EM Reset 6850 EX 156737292 FX 156 2 252 EM Turn tone on FX 203 9 EM Turn motor on OTOR 1 EM Inform user RINT Press record and return UMMY GET Select output route RE 240 FX 3 1 250 REM Send ULA synchronisation 260 VDU amp AA 270 REM Wait header tone 280 TIME 290 REPEAT UNTIL TIME 500 300 REM Send data with a tape synchronisation 310 PRINT HELLO THERE 320 REM Wait until buffer empty 330 REPEAT UNTIL ADVAL 3 gt amp BE 340 REM Pause for a short period of tone 350 TIME 0 360 REPEAT UNTIL TIME 50 370 REM Disconnect RS423 output 380 FX 3 0 390 REM Turn off tone 401 FX 203 255 410 REM Wait for an interblock gap of silence 420 TIME 0 431 REPEAT UNTIL TIME 150 440 REM Turn m
100. up one line Clear text area Carriage return Paged mode on Paged mode off Clear graphics area Define text colour Define graphics colour Define logical colour Restore default logical colours Disable VDU drivers or delete current line Select screen MODE Re program display character Define graphics window PLOT K X Y Restore default windows ESCAPE value Define text window Define graphics origin Home text cursor to top left of window Move text cursor to X Y Backspace and delete 459 Appendix E Plot Number Summary Move relative to last point Dravv relative to last point in current foreground colour Dravv relative to last point in logical inverse colour Draw relative to last point in current background colour Move absolute Draw absolute in current foreground colour Draw absolute in logical inverse colour Draw absolute in current background colour ND OP WNF Higher PLOT numbers have other effects which are related to the effects given by the values above 8 15 Last point in line omitted when inverted plotting used 16 23 Using a dotted line 24 31 Dotted line omitting last point 32 63 Reserved for Graphics Extension ROM 64 71 Single point plotting 72 79 Horizontal line filling 80 87 Plot and fill triangle 88 95 Horizontal line blanking right only 96 255 Reserved for future expansions Horizontal line filling These PLOT numbers start from the specified X Y co ordinates
101. user at all 285 8 C4B6 amp C4B9 are a teletext conversion table The teletext character generator uses slightly different character codes to the rest of the BBC microcomputer This table contains four bytes an ASCII value followed by the byte to be stored in screen memory for three characters amp 23 is translated to amp 5F amp 5F _ is translated to amp 60 amp 60 is translated to amp 23 286 12 Events and Event Handling The concept of events and event handling provides the user with an easy to use pre packaged interrupt A routine may be written to perform a second function while another program is running Because the microprocessor can only do one thing at a time the second function will be executed by interrupting the first program and then returning control to allow it to continue from where it was interrupted The interrupt facility is provided by the designers of the microprocessor and is a fairly primitive level of control The operating system uses interrupts extensively and also provides the user with the ability to trap interrupts see Interrupts chapter 13 The operating system interrupts whatever is going on in the foreground e g the user program every 10 milliseconds and performs any tasks which are required This background work includes controlling the sound chip storing key strokes in the input buffer and keeping up with ADC conversions In the process of performing the background
102. values These are in the order channel 1 2 3 and 4 amp 2BA amp 2BD are the high bytes of the most recent analogue converter values amp 2BE is the analogue system flag This contains the number of the last channel to finish conversion or zero if no channels have finished since this value was last read This byte is read by OSBYTE amp 80 amp 2BF amp 2C8 are the event enable flags If zero the event is disabled otherwise enabled See the chapter on events number 12 amp 2C9 is the soft key expansion pointer The next byte to be expanded in a soft key is to be found at amp B01 amp C9 amp 2CA is the first auto repeat count This is the next value to go into the auto repeat counter at amp E7 This location can be considered a one byte queue for the counter amp 2CB amp 2CD are used as workspace for two key rollover processing 273 amp 2CE is the sound semaphore If it is zero it means that an envelope interrupt is being processed so another must be ignored If it is amp FF it means that the envelope software is free amp 2CF amp 2D7 are buffer busy flags Bit 7 of these bytes is set if the matching buffer is empty For a list of buffer numbers see OSBYTE amp 15 21 amp 2D8 amp 2E0 are the buffer start indices They contain the offset of the next byte to be removed from each buffer The offsets are adjusted so that the highest location in the buffer has the offset amp FF for all buffers irrespective of
103. 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag cleared Addressing mode bytesused cycles accumulator zero page zero page X absolute absolute X WWNN FR 2 5 6 6 7 1 if page crossed Example Shift accumulator contents right one bit LSR A C bit 0 A A 2 77 NOP No operation This is a dummy instruction which has no effect on any memory or register contents except to increment the program counter by one Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing Mode bytes used cycles implied 1 2 Example A NOP instruction NOP this instruction does nothing 78 ORA OR memory with accumulator A A ORM This instruction performs a bit by bit logical OR operation between the contents of the accumulator and the contents of the specified memory and places the result in the accumulator The truth table for logical OR is Acc Mem Result bit bit bit 0 0 0 0 1 1 1 0 1 1 1 1 Processor Status after use C carry flag not affected Z zero flag set if A 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow fl
104. 0 0 1 14 0 0 1 0 13 0 0 1 1 12 0 1 0 0 11 0 1 0 1 10 0 1 1 0 9 0 1 1 1 8 1 0 0 0 7 1 0 0 1 6 1 0 1 0 5 1 0 1 1 4 1 1 0 0 3 1 1 0 1 2 1 1 1 0 1 1 1 1 1 0 OFF 420 23 3 2 Noise generator The noise generator comprises a noise source and volume control The noise generator parameters are defined by three bits FB this bit when set to 0 causes PERIODIC NOISE to be generated When set to 1 it causes WHITE NOISE to be generated Noise frequency control the noise base frequency can be defined in 4 possible states by bits NF1 and NFO NF1 NFO FREQUENCY 0 0 low 0 1 medium 1 0 high 1 1 tone generator 1 frequency 23 3 3 Sound chip register address field R2 R1 RO Description 0 0 0 Tone 3 frequency 0 0 1 Tone 3 volume 0 1 0 Tone 2 frequency 0 1 1 Tone 2 volume 1 0 0 Tone 1 frequency 1 0 1 Tone 1 volume 1 1 0 Noise control 1 1 1 Noise volume 421 23 4 PROGRAMMING BYTE FORMATS The sound generator is programmed by sending it bytes in the following format 23 4 1 Frequency First byte Register Address Bit 7 6 5 4 1 R2 R1 RO 23 4 2 Frequency Second byte Bit 7 6 5 4 0 X F9 F8 F3 3 F7 Data F2 F1 FO Data 2 1 0 F6 F5 F4 Note that the second low order frequency byte may be continually updated without rewriting the first byte 23 4 3 Noise source byte Register Address Bit 7 6 5 4 1 R2 R1 RO 23 4 4 Update volume level Register Address Bit 7 6 5 4 1 R2 R1 R
105. 0 for modes 0 1 and 2 amp 4000 for mode 3 amp 5800 for modes 4 and 5 amp 6000 for mode 6 and 8 7C00 for mode 7 teletext 281 Memory is thus allocated OSHVVM HIMEM User programs HIMEM amp 7FFF Screen memory 11 13 The ROMs The upper 32K of address space is allocated to the various ROMs on the BBC microcomputer amp 8000 amp BFFF is allocated to the paged ROMs including the BASIC and Disc Filing System ROMs amp C000 amp FFFF is allocated to the operating system ROM Locations amp FC00 amp FEFF are taken out to provide space for the memory mapped peripherals There exist near the start of the operating system ROM some tables which are used by the operating system to assist in high resolution graphics processing The user may be interested in the existence of these tables when disassembling the operating system While it is possible to use these tables to speed up graphics routines it should be noted that these tables are not Acorn supported so a copy of the tables should be taken rather than use them directly These tables have not moved between operating systems 1 00 and 1 20 but there is no guarantee that these locations will not move in a future operating system version amp C31F amp C32E is a lookup table of byte masks for four colour modes The table is normally used for writing characters to the screen The table is used thus Prepare a four bit binary number with a bit set fo
106. 1 Latch Registers 22 2 4 Timer 1 one shot mode This mode allows a single interrupt to be generated for each timer load operation The delay between writing T1C H and generation of the interrupt to the 6502 is a direct function of the data loaded into the counter T1 can be programmed to produce a single negative pulse on the PB7 peripheral pin as well as generating a single interrupt With output enabled ACR 1 writing T1C H will cause PB7 to go low PB7 will go high again when T1 times out The overall result of this is a programmable width pulse on PB7 405 Writing into the high order latch has no effect on the operation of T1 in the one shot mode It is however necessary to ensure that the low order latch contains the correct data before initiating the countdown by writing T1C H When the 6502 writes into the high order counter the T1 interrupt flag is cleared the contents of the low order latch are transferred into the low order counter and the timer begins to decrement at 1MHz If PB7 output is enabled then it will go low after the write operation Upon reaching zero the T1 interrupt flag is set an interrupt is generated if enabled and PB7 goes high The counter continues to decrement at the system clock rate The 6502 is then able to read the contents of the counter to determine the time since the interrupt occurred The T1 interrupt must be cleared before it can be set again 22 2 5 Timer 1 free run mode The advantage
107. 1 Low Order Latches T1 High Order Latches S AIJovj ol C R R R oj ej oje T2 Low Order Latches T2 Low Order Counter 9 T2 High Order Counter 11 1 0 1 1 ACR Auxiliary Control Register 12 Peripheral Control Register 13 14 1 1 1 0 IER Interrupt Enable Register 15 Same as Reg 1 Except No Handshake Figure 22 1 6522 Internal Register Summary Q 99 22 2 1 Operation of port A and port B There are two data direction registers DDRA and DDRB which specify whether the peripheral pins are to operate as inputs or outputs Placing a 0 in a bit of a DDR will cause the corresponding bit of that port to be defined as an input A 1 will cause it to be defined as an output Each of the port s I O pins is controlled by a bit in an output register ORA or ORB and an input register IRA or IRB When programmed as an output a port line will be controlled by the corresponding bit in the output register If the line is defined as an input then writing data into its output register will have no effect Reading from a peripheral port will read the value of the input register IRA or IRB With input latching disabled IRA will contain the value present at PAO PA7 when the read is performed If input latching is enabled then IRA will contain the value present at PAO PA7 when the latching occurred via CA1 The IRB register is similar to the IRA register but there is a difference for pins progr
108. 2 SRMODE 5 The shift rate is controlled by T2 as in mode 4 if the SR flag in the IFR is set then the shifting operation is triggered by the read or write of the SR Alternatively the first shift will occur at the next timeout of T2 after a read or write of the SR With each write or read of the SR the SR counter is reset and 8 bits are shifted onto CB2 Eight shift pulses appear on the CB1 output to facilitate the control of shifting into external devices When the 8 shift pulses have occurred shifting is disabled the SR interrupt flag is set and CB2 remains fixed at the last data bit level a TuUUUUITI UT U UI manana WRITE SR OPERATION N 2 CYCLES N 2 CYCLES i 4 CBI OUTPUT 1 SHIFT CLOCK a ora I SIN FH a CNN lt Figure 22 16 Output Mode 5 Timing Shift out under control of the system clock SRMODE 6 In this mode the shift rate is controlled directly by the IMHz system clock WAITE SR OPERATION CB1 OUTPUT A M M SHIFT CLOCK 1 3 4 7 6 am SMX XX SD S SEE TS Figure 22 17 Output Mode 6 Timing 412 Shift out under control of external CB1 clock SRMODE 7 Shifting is controlled by pulses applied to the CB1 pin by an external device in this mode The SR interrupt flag is set each time that the SR counter counts 8 pulses but the shifting function is not disabled The SR interrupt flag is reset and the SR counter is initialised to begin counting the next 8 shift pulses on CB1 each time that the
109. 2C H must be rewritten to re enable the interrupt flag The interrupt flag is cleared by reading T2C L or by writing T2C H 407 22 2 8 Timer 2 pulse counting mode In this mode T2 counts a predetermined number of negative going pulses applied to PB6 This can be accomplished by first of all loading a number into T2 Writing into T2C H will clear the interrupt flag and allow the counter to decrement every time that a pulse is applied to PB6 The interrupt flag is set when T2 counts down past zero The timer continues to decrement with each pulse applied to PB6 T2C H must be rewritten to allow the interrupt flag to set on subsequent down counts REG 11 AUXILIARY CONTROL REGISTER Pa LATCH ENABL E DES ABL E Pa 0 DISABLE 1 ENABLE LATCHING 7 6 OPERATION TIMED INTERRUPT EACH TIME T1 IS LOADED DISABLED la INTERRUPTS TIMED INTERRUPT EACH TIME T115 OUTPUT LOADED CONTINUOUS SQUARE INTERRUPTS WAVE OUTPUT T2 TIMER CONTROL SHIFT REGISTER CONTROL DOC III Sd lototo DISABLED fo 1 1 SHIFT IN UNDER CONTROL OF EXT CER 1 1 o o SHIFT OUT FREERUNNING ATT RATE DOL sHiFT OUT UNDER CONTROL OF T2 7 Tio Swift OUT UNDER CONTROL OF Of 1113 sFr our UNDER CONTROL OF EXT cik s oPeRATiON __ O TIMED INTERRUPT COUNT DOWN WITH PULSES ON PB6 Figure 22 10 Auxiliary Control Register 22 2 9 Shift register operation The shift register SR enables serial data to be transferred into and
110. 3 FX3 Select output stream Entry parameters X determines output device s Y 0 BIT 0 Enables RS423 driver BIT 1 Disables VDU driver BIT 2 Disables printer driver BIT 3 Enables printer independent of CTRL B or C BIT 4 Disables spooled output BIT 5 Not used BIT 6 Disables printer driver unless the character is preceded by a VDU 1 or equivalent BIT 7 Not used FX 3 0 selects the default output options which are RS423 disabled VDU enabled Printer enabled if selected by VDU 2 Spooled output enabled if selected by SPOOL This OSBYTE call uses OSBYTE call with A amp EC 236 It is thus possible to set Y as a bit mask and only change those bits which are required After call A is preserved X contains the old FX 3 status Y and C are undefined 119 OSBYTE 8 04 4 FX 4 Enable disable cursor editing Entry parameters X determines editing keys status Y 0 FX 4 0 X 0 Enable cursor editing default setting FX 4 1 X 1 Disable cursor editing The cursor control keys will return the following codes COPY LEFT RIGHT DOWN UP amp 87 135 amp 88 136 amp 89 137 amp 8A 138 amp 8B 139 TEX 2 X Disable cursor editing and make the keys act as soft keys with the following soft key association numbers COPY LEFT RIGHT DOWN UP after call A is preserved X contains the previous FX 4 setting Y and C are undefined 120 11 12 13 14 15 OSBYTE 8 05 5 FX 5
111. 35 23 36 24 37 25 amp 38 26 39 27 40 28 41 29 42 2A 43 2B 44 2C 103 99 102 66 45 2D 24 E8 23 17 46 2E 104 98 103 67 47 2E 121 87 120 68 0 48 30 40 D8 39 27 1 49 31 49 CF 48 30 2 50 32 50 CE 49 31 3 51 33 18 EE 17 11 4 52 34 19 ED 18 12 5 53 35 20 EC 19 13 6 54 36 53 CB 52 34 7 55 37 37 DB 36 24 8 56 38 22 EA 21 15 9 57 39 39 D9 37 25 58 3A 73 B7 72 48 59 3B 88 A8 87 57 lt 60 3C 61 3D gt 62 3E 63 3F 64 40 72 B8 71 47 A 65 41 66 BE 65 41 B 66 42 101 9B 100 64 C 67 43 83 AD 82 52 D 68 44 51 CD 50 32 E 69 45 35 DD 34 22 F 70 46 68 BC 67 43 G 71 47 84 AC 83 53 H 72 48 85 AB 84 54 I 73 49 38 DA 38 26 J 74 4A 70 BA 69 45 K 75 4B 71 B9 70 46 L 76 4C 87 A9 86 56 M 77 4D 102 9A 101 65 N 78 4E 86 AA 85 55 O 79 4E 55 C9 54 36 P 80 50 56 C8 55 37 Q 81 51 17 FE 16 10 456 Rey ASCII INREY Internal key No character dec hex dec hex dec hex R 82 52 52 CC 51 33 S 83 53 82 AF 81 51 T 84 54 36 DC 35 23 U 85 55 54 CA 53 35 V 86 56 100 9C 99 63 W 87 57 34 DE 33 21 X 88 58 67 BD 66 42 Y 89 59 69 BB 68 44 Z 90 5A 98 9E 97 61 91 5B 57 C7 56 38 92 5C 121 87 120 78 93 5D 89 A7 88 58 A 94 5E 25 E7 24 18 _ 95 5F 41 D7 40 28 96 60 a 97 61 b 98 62 c 99 63 d 100 64 e 101 65 f 102 66 g 103 67 h 104 68 i 105 69 j 106 6A k 107 6B I 108 6C m 109 6D n 110 6E o 111 6F p 112 70 q 13 1 r 114 72 s
112. 4 1 if page crossed absolute Y 3 4 1 if page crossed indirect X 2 6 indirect Y 2 5 1 if page crossed Example Clear the top 4 bits of location amp 80 LDA amp 80 load value to be ANDed into A AND amp F0O perform AND mask 11110000 STA amp 80 load memory with the modified value 45 ASL Arithmetic Shift Left M M 2 C M7 or accumulator This operation shifts all the bits of the accumulator or memory contents one bit left Bit 0 is set to 0 and bit 7 is placed in the carry flag The effect of this operation is to multiply the memory contents by 2 ignoring 2 s complement considerations setting the carry if the result will not fit in 8 bits C lt 7654321 lt 0 Processor Status after use C carry flag set to old contents of bit 7 Z zero flag set if result 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of the result is set Addressing mode bytes used cycles accumulator 1 2 zero page 2 5 Zero page X 2 6 absolute 3 6 absolute X 3 7 Example Rapid multiplication of memory contents by 4 ASL data data data 2 ASL data data data 2 gross effect 4 46 BCC Branch on Carry Clear Branch if C 0 This instruction causes a relative jump if the carry flag is clear The address to which the branch is directed must be within relative addressing range other
113. 4 5 Cursor delay bits 6 7 Scan lines per character Cursor start blink type Cursor start bits 0 4 Cursor blink bit 6 Cursor type bit 5 Cursor end Screen start address Cursor position Light pen position amp 3F 63 amp 28 40 amp 31 49 amp 04 4 amp 02 2 amp 1E 30 amp 02 2 amp 19 25 amp 1B 27 amp 01 1 amp 00 0 amp 00 0 amp 09 9 amp 67 103 amp 07 7 amp 01 1 amp 01 1 amp 09 9 Variable Variable Variable MODE 6 DE aa ee dE a ETS Ep is 86138 86006 a EE pe eee EE BLANK ENA E EEN ees ge ee ene eee in o artoc LD a7ED 87E0C amp 7E0D 87E0E amp 7E0F BLANK BLANK b a ee 8 PIXELS 1 BIT PIXEL N B The screen layout is only as shown after a CLS it will change when the screen is hard scrolled 84000 should be subtracted for each location on a model A 475 MODE 7 Screen layout Graphics Colours Text Registers RO R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 476 TELETEXT graphics only TELETEXT 40 x 25 Horizontal total Characters per line Horizontal sync position Horizontal syne width Vertical sync time Vertical total Vertical total adjust Vertical displayed characters Vertical sync position Interlace mode bits 0 1 Display dela
114. 40 JSR OSWRCH perform carriage return no LF 350 JSR OSRDCH A GET 360 CMP amp 0D was it RETURN 370 BNE start if not back to the start 380 JSR OSNEWL carriage return and line feed 390 RTS back to BASIC 33 100 110 NEXT 120 amp 80 0 INT Binary Coded Decimal 140 PRINT press key to add 1 150 PRINT press RETURN to exit 160 CALL start Bod os SO BS LS ia Ww Oo tu pe This program could be altered to subtract 1 each time a Rey is pressed by changing line 100 to SEC and changing the ADC instructions in lines 120 and 150 to SBC instructions The decimal flag must alvvays be cleared before using operating system routines There is no standard representation of negative numbers using BCD In order to implement more complex arithmetic including floating point applications the programmer must define his own conventions and number formats 34 5 Addressing Modes When an assembly language instruction needs some data or an address to work on this must be provided in the operand field of the assembler statement Although there are a limited number of different machine code instructions which can be used with the 6502 the power of the instruction set is enhanced by a number of different addressing modes by which the data or addresses used by each instruction may be provided The addressing mode used by the assembler depends on the syntax of the assembly language statement The following
115. 45 14 141 203 149 344 324 15 15 111 109 15 15 323 15 18 16 16 256 18 18 18 161 393 18 18 163 343 349 19 206 19 165 192 19 324 330 349 19 13 344 19 19 20 SPOOL file closing file handle OSFSC paged ROM service call T TAPE TAPE12 TAPE3 TV OS variable locations 16032 second processor 1MHz bus 6502 break flag bug carry flag decimal mode flag instruction instruction set interrupt disable mnemonics negative flag overflow flag program counter second processor stack pointer status register unused flag zero flag 6522 versatile interface adapter 6845 address register characters per line cursor blanking cursor positions cursor shape display blanking fast animation horizontal sync horizontal timing interlace control introduction lightpen software lightpens number of character rows programming register summary scan lines per character screen display start address scrolling scrolling example vertical sync vertical timing wrap around see One megahertz bus 20 141 204 344 324 20 20 164 192 20 20 20 168 272 12 435 42 37 41 42 43 41 41 43 42 42 42 434 42 41 42 41 395 360 361 366 367 366 365 373 362 361 364 359 369 367 363 360 375 366 370 371 374 362 363 362 373 499 6850 IRQ bit mask 6850 ACIA read modify control register 76489 sound chip A Absolute addressing Absolute X or Y addressing Accumul
116. 5 134 222 280 151 419 138 442 213 250 274 264 151 280 138 231 418 423 177 212 178 119 STA Store A in memory 92 Stack 497 position in memory 272 Stack pointer 4 Start up message 218 Start up options 246 String processing 105 STX Store X in memory 93 STY Store Y in memory 94 Subroutine jump to 73 return from 87 Syntax in assembler programs 35 System 6522 see system VIA System cloch page tvvo address 272 read OSVVORD 248 write OSWORD 249 System VIA hardware 417 interrupt processing 302 IRQ bit mask 229 T TAB key definition 222 Tape filing system selection of 164 Tape format CFS 347 TAX Transfer A to X 95 TAY Transfer A to Y 96 Terminals RS423 310 Text colour bytes zero page locations 268 Text cursor read character from 159 Text cursor position 158 Timer see interval timer Timer switch 237 Tone generators 420 Transducer 497 Tri state 497 TSX Transfer S to X 97 Tube 16032 second processor 435 6502 second processor 434 fast BPUT 176 filing systems 345 introduction 433 paged ROM service calls 325 presence flag 230 Read I O processor memory 249 ULA 433 Write I O processor memory 249 Z80 second processor 435 Two key roll over zero page locations Two s complement TXA Transfer X to A TXS Transfer X to S TYA Transfer Y to A U ULA video ncommitted logic array pgrading to discs PTV S MOS difference from UK User vector zero page User 6522 IR
117. 5 B4 add 0 1 2 20K amp 3000 12K 1 1 3 16K amp 4000 16K 0 0 4 5 10K amp 5800 or amp 1800 22K 1 0 6 8K amp 6000 or amp 2000 24K 0 1 B6 Operates the CAPS lock LED B7 Operates the SHIFT lock LED 23 3 The 76489 sound chip The sound chip on the BBC microcomputer is in itself a very simple chip There are three channels for which the frequency and volume of output can be defined There is also a fourth white noise generator The output from all of these channels is automatically mixed on chip The complex sound commands available from BASIC are very powerful but require a large amount of time to process especially if complex envelopes are defined In fast machine code programs it may sometimes be advantageous to write directly to the sound chip The example program shows how this can 419 be done The data to be written into the sound chip is first of all put onto the slow databus Note that interrupts are disabled before this is started The sound generator write enable line is then pulled low for at least 8 uS then pulled high again 23 3 1 Tone generators There are 3 tone generators The frequency of each channel is determined by 10 bits of data F9 is the most significant bit The frequency of each channel can be calculated as frequency 4000000 32 x 10 bit binary number The volume level for each channel is variable to 16 different levels these are Bit A3 Bit A2 Bit A1 Bit AO VOLUME 0 0 0 0 15 MAX 0
118. 6 Cursor type bit 5 Cursor end Screen start address Cursor position Light pen position amp 50 amp 26 amp 00 amp 00 amp amp O m Gy amp 08 8 Variable Variable Variable MODE 1 amp 3008 EC CEE A ny EE EE EE INT ESO ECO ape et LE ee Lee er TO Sa a ya EE ESE a a es EE EE EE EE DES REIS EE Es Can EE EE ECO SE DES EPA oe RRI 83280 DE SE EE ge T 83281 DRET EES A ee os SES Datos esa ed Duc tes i retir Par ae DE bre Pes dia es ee ESE CED Oe DE EES CES EE RE EC EE RE EN OE El Geena ape EES EE are aaa 4 PIXELS 2 BITS PIXEL N B The screen layout is oniy as shown after a clear screen it will change when the screen is hard scrolled 465 MODE 2 Screen layout Graphics Colours Text Registers RO R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 466 160 x 256 16 20 x 32 Horizontal total Characters per line Horizontal sync position Horizontal syne width Vertical sync time Vertical total Vertical total adjust Vertical displayed characters Vertical sync position Interlace mode bits 0 1 Display delay bits 4 5 Cursor delay bits 6 7 Scan lines per character Cursor start blink type Cursor start bits 0 4 Cursor blink bit 6 Cursor type bit 5 Cursor end Screen start address Cursor position Light pen position amp 50 amp 26 amp 00 amp 00
119. 7 437 438 440 447 495 495 338 339 337 339 342 495 109 116 101 105 106 101 103 102 104 107 107 103 102 106 101 101 104 505 Operating system high water mark 136 155 OPT Options on files Options at start up ORA OR A with memory OS command paged ROM activation unrecognised command OS command unrecognised OSFSC OS commands paged ROMs zero page text pointer zero page work space OS ROM tables OS variables read start address of OS version number OSARGS CFS OSASCI OSBGET CFS OSBPUT CFS ROM filing system OSBYTE paged ROM service call summary table zero page register storage OSBYTES OSCLI OSEVEN OSFILE CFS filing system OSFIND CFS ROM filing system OSFSC CFS OSGBPB CFS OSHWM read write OSNEWL OSRDCH OSRDRM OSWORD envelope command paged ROM service call parameter block read character definition read graphics cursor positions read I O processor memory read interval timer 506 24 163 246 79 321 321 344 11 271 268 282 180 15 337 346 104 338 346 339 346 349 322 111 271 109 107 107 335 346 349 346 349 343 346 339 346 136 154 188 189 189 103 102 106 247 250 322 247 251 252 249 249 read line read palette read pixel value read system clock sound command summary unrecognised user write I O processor memory write interval timer write palette write system clock zero page register storage OSWRCH Outpu
120. All other possible combinations of bits 6 and 4 must also be programmed The following table shows how to program logical colours 0 3 For example to program logical colour 0 it is necessary to program four separate locations in the palette 380 Logical colour 0 bit 7 AMAnAnAaAnAcGcCC CC CO OC DO PBPROOHFRHFROORrFROOHRHOSD bit 6 bit 5 MSRP rPrRoOoQqoorrRrRrROGCO O POrROrROrFROrFROR OR ORO bit 4 19 2 2 General Summary for logical colour programming Mode 2 colour 4 colour 16 colour Bit7 Logical colour Bit 0 Logical colour Bitl Logical colour Bit3 Bit6 x Logical colour Bit 2 Bit5 x Logical colour Bit0 Logical colour Bit 1 Bit4 x Logical colour Bit 0 381 19 2 3 Physical colour field The physical colours are amp 00 0 black amp 01 1 red amp 02 2 green amp 03 3 yellow green red amp 04 4 blue amp 05 5 magenta red blue amp 06 6 cyan green blue amp 07 7 white amp 08 8 flashing black white amp 09 9 flashing red cyan amp 0A 10 flashing green magenta amp 0B 11 flashing yellow blue amp 0C 12 flashing blue yellow amp 0D 13 flashing magenta green amp 0E 14 flashing cyan red amp 0OF 15 flashing white black It is these colour numbers which should be used with FX155 Note however that the actual number sent to the Palette is the above number EOR amp 07 ie with the three colour bits inverte
121. BELL envelope number read write BELL frequency read write BELL duration read write critical region read write message BOOT opts read write BREAK intercept code read write last BREAK type read write start up options dec hex 136 88 137 89 139 8B 141 8D 140 8C 144 90 16 10 17 11 128 80 188 BC 189 BD 190 BE 211 D3 212 D4 213 D5 214 D6 200 C8 215 D7 247 F7 248 F8 249 F9 253 FD 255 FF 449 buffer bus cursor Econet ESCAPE events explode 450 brief description flush selected class flush particular buffer get buffer status insert value into buffer get character from buffer examine buffer status insert char into I P buffer read from FRED write to FRED read from JIM write to JIM read from SHEILA write to SHEILA enable cursor editing read text cursor position read character at cursor read write cursor editing state read write net keyboard disable read write OS call intercept read write OSRDCH intercept read write OSWRCH intercept clear ESCAPE condition set ESCAPE condition acknowledge ESCAPE read write BREAK ESCAPE effects read write ESCAPE key value read write ESCAPE key status read write ESCAPE effects flag disable event enable event character definition RAM read write explosion state dec hex 15 F 21 15 128 80 138 8A 145 91 152 98 153 99 146 92 147 93 148 94 149 95 150 96 151 97 4 4 134 86 135 87 237 ED 201 C9 206 CE 207 CF 208 DO
122. C microcomputer has been designed to be used in a variety of different configurations and the operating system software provides extensive information about the current hardware and software status The operating system makes most of the allowances required for the different configurations automatically but only when the legal techniques are adhered to so please use them The final paragraph in this introduction must be a word of apology to those programmers engaged in the task of software protection Many of the details contained in this book will give those intent on pirating software inspiration to circumvent their protection techniques On the other hand these same details may also give the software protectors inspiration In the end no software protection is complete Any protection technique relies on the fact that the person trying to break the protection has a threshold at which he decides that the effort and resources required are greater than the reward For some this threshold is higher than others but for these people the reward is often the victory in the intellectual battle with the programmer of the protection method For those intent on denying the software producer his income one hopes that this threshold is somewhat lower 1 Introduction for those new to machine code There comes a time in every programmer s life well most programmers anyway when the constraints of a high level language e g BASIC prevents him from
123. CPU Upon receiving an interrupt the 6502 will normally run a special interrupt routine program before continuing with the task in hand before it was interrupted Latch a latch is used to retain information applied to it after the data has been removed It is rather like a memory location except that the outputs from the bits within the latch are connected to some hardware LED Light emitting diode acts like a diode by only allowing current to pass in one direction Light is emitted whilst current is passed Low sometimes used to designate logic 0 Machine code the programs produced by the 6502 BASIC Assembler are machine code A machine code program consists of a series of bytes in memory which the 6502 can execute directly Mnemonic the name given to the text string which defines a particular 6502 operation in the BASIC assembler LDA is a mnemonic which means load accumulator Opcode the name given to the binary code of a 6502 instruction For example amp AD is the opcode which means load accumulator Open Collector this is a characteristic of a transistor output line It simply means that the collector pin of the transistor is not driving a resistor load ie it is open Operand a piece of data on which some operation is performed Usually the operand will be a byte in the accumulator of the 6502 or a byte in some memory location 495 Page a page of memory in the 6502 memory map is amp
124. CTER 80 FX 154 224 90 amp 360 amp F REM COLOUR MASK 100 amp 361 1 REM PIXELS PER BYTE I 110 amp 34F amp 20 REM BYTES PER CHARACTER 4 wide x 8 high 120 amp 363 amp 55 RE GRAPHICS RIGHT MASK 130 amp 362 amp AA RE GRAPHICS LEFT MASK 140 amp 30A 9 REM NO OF CHARS PER LINE 150 VDU 20 160 REM DEMO 170 MOVE 0 0 DRA 640 512 DRAW 1279 0 180 PRINT TAB 1 2 190 AS HelloThere 200 COLOUR 129 210 FOR A 1 TO 16 220 IF A 9 THEN PRINT TAB 1 8 230 COLOUR A 1 240 PRINT MIDS AS A 1 250 NEXT A 260 PRINTI 383 384 20 The serial system Sheila address amp 08 amp 1F The serial system on the BBC microcomputer deals with the transmission and reception of asynchronous serial data Serial data is used in conjunction with the cassette and RS423 interfaces where it is impractical to use the 8 databus lines The heart of the system is the 6850 Asynchronous Communications Interface Adapter ACIA This chip interfaces serial data to the 8 bit parallel processor bus The input and output devices ie cassette or RS423 and baud rates are all defined by the serial ULA As with all the other hardware interfaces in this guide the official operating system commands should be used to communicate with the chip registers This will allow programs written in this way to operate over the Tube 20 1 General description of a serial interface This general description aims to introduce the terms which are used
125. DIM MC 100 20 OSASCI amp FFE3 0 3 USERV amp 200 40 FOR opt 0 TO 3 STEP3 50 PS MC 60 70 OPT opts 80 write 90 CMP 0 is this CODE 100 BEQ code if CODE call act upon it 110 BRK anything else print error message 120 130 PS 255 P P 1 REM error number 140 SPS CODE only please 150 P P LEN P 160 170 OPT opts reset OPT 180 BRK op code value 0 190 code TXA transfer contents of X req to Acc 200 JSR OSASCI print ASCII character 210 RTS return to BASIC 220 230 NEXT 240 2 USERV write MOD 256 250 USERV 1 write DIV 256 This example prints out the ASCII character corresponding to the value of the first parameter given to the CODE command After this program has been run typing in CODE 65 or FX 136 65 will cause a letter A to be printed The second parameter stored in Y if included is ignored See also LINE 2 7 EXEC lt filename gt E Text files from the currently selected filing system can be used as if they were keyboard input using this command A typical application might involve the setting up of a user s favourite soft key definitions which are EXECed in at the beginning of a programming session See also SPOOL and OSBYTE amp C6 198 14 2 8 FXa x y F OSBYTE calls may be performed directly from the keyboard using this command A X and Y are loaded by the operating system
126. E NR EY cas Figure 22 5 Write Handshake Timing Selection of operating modes for CA1 CA2 CB1 and CB2 is controlled by the Peripheral Control Register see figure 22 6 403 REG 12 PERIPHERAL CONTROL REGISTER CB2CONTROL T 716 STOPERATION 0J010JINPUT NEGATIVE ACTIVE EDGE CA1 INTERRUPT CONTROL 0 NEGATIVE ACTIVE EDGE 1 POSITIVE ACTIVE EDGE GES INDEPENDENT INTERRUPT INPUT NEG EDGE CAP CONTROL 0 1J0JINPUT POSITIVE ACTIVE EDGE 312111 OPERATION RRE INDEPENDENT INTERRUPT INPUT POS EDGE ifofo hanosmare OUTPUT non 0 0 0 INPUT NEGATIVE ACTIVE EDGE pE INDEPENDENT INTERRUPT INPUT NEG EDGE O INPUT POSITIVE ACTIVE EDGE 8 INDEPENDENT INTERRUPT g 0 1 INPUT POS EDGE HANDSHAKE OUTPUT CB1 INTERRUPT CONTROL 0 NEGATIVE ACTIVE EDGE 1 POSITIVE ACTIVE EDGE Figure 22 6 CA1 CA2 CB1 CB2 Control 22 2 3 Timer operation The interval timer referred to from now on as T1 consists of two 8 bit latches and a 16 bit counter After it has been loaded the counter decrements at the system clock rate 1 MHz until it reaches zero When it reaches zero an interrupt flag will be set and an interrupt will be requested of the 6502 if enabled The timer then disables any further interrupts or automatically transfers the contents of the latches into the counter and continues to decrement The timer may also be programmed to invert the output level
127. F For more information about interrupts see chapter 13 229 OSBYTE amp EA 234 Read flag indicating Tube presence lt NEW VALUE gt lt OLD VALUES AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains 0 if a Tube system is not present and amp FF if Tube chips and software are installed No other values are meaningful or valid 230 OSBYTE amp EB 235 Read flag indicating speech processor presence lt NEW VALUE gt lt OLD VALUES AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains 0 if the speech processor is not present and amp FF if it is 231 OSBYTE amp EC 236 FX 236 Read write write character destination status lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This call is used by OSBYTE amp 3 FX 3 232 OSBYTE amp ED 237 FX 237 Read write cursor editing status lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This call is used by OSBYTE amp 4 FX 4 233 OSBYTEs A amp EE 238 8 FF 239 and amp FO 240 Read write location amp 27E amp 27F and amp 280 lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next locatio
128. FX 207 Read write Econet read character interception status lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If bit 7 of this location is set then input is pulled from the Econet vector OSBYTE amp DO0 208 FX 208 Read write Econet write character interception status lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If bit 7 of this location is set then output is directed to the Econet Output may go through the normal write character on return from the Econet code See expansion vectors section 10 7 211 OSBYTE amp D1 209 FX 209 Read write speech suppression status lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains the value sent to the speech processor when speech is output A value of amp 50 represents the SPEAK op code and is the default value speech enabled Writing amp 20 NOP to this location will disable speech 212 OSBYTE amp D2 210 FX 210 Read write sound suppression status lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If this location contains any value other than 0 then sound output is disabled 213 OSBYTE
129. INOD aSa AddOld 1228 OYVOUAIH UOLVEIN3O ONNOS SWOY TWHHAS 6892 TOHINOS UCOSS1904Ud H533dS ozzs TOHINOD HaLYSANOD t5889 Te LIDId LAINO34 Ol 30990 1VNY 2002 aNSOTWNY 13NO04 354 Refer to figure 17 1 whilst reading the following outline of the hardware At the centre of the system is the 6502A central processing unit CPU This is the chip which executes all of the programs including BASIC It is connected to the rest of the system via three buses These are the data bus the address bus and the control bus For clarity on the diagram these buses are all compressed into one which is represented by the double lines terminated with arrows at each major block A bus is simply a number of electrical links connected in parallel to several devices Normally one of these devices is talking to another device on the bus The communication protocols which enable this transfer of data to take place are set up by the control address and data buses In the case of the address bus there are 16 separate lines which allow 65536 different combinations of l s and 0 s The maximum amount of directly addressable memory on a 6502 is therefore 65536 bytes The data bus consists of 8 lines one for each bit of a byte Any number between 0 and amp FF 255 can be transferred across the data bus Communication between the peripherals memory and the CPU occurs over the data bus The CPU can either send out a byte or receive a
130. L function key status soft key or code Input buffer characters amp AO to amp AF OSBYTE amp E4 228 FX 228 Read write CTRL SHIFT function key Status soft key or code Input buffer characters amp B0 to amp BF lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y These locations determine the action taken by the operating system when a function key is pressed value 0 totally ignore key value 1 expand as normal soft key value 2 to amp FF add n base to soft key number to provide ASCII code 225 The default settings are fn keys alone amp 01 fn keys SHIFT amp 80 fn ReystCTRL amp 90 fn keys SHIFT CTRL amp AO expand using soft key strings code amp 80 soft key number code amp 90 soft key number code amp A0 soft key number When the BREAK key is pressed a character of value amp CA is entered into the input buffer The effect of this character may be set independently of the other soft keys using OSBYTE amp DD 221 One of the other effects of pressing the BREAK key is to reset this OSBYTE call and so the usefulness of this facility is limited 226 OSBYTE amp E5 229 FX 229 Read write status of ESCAPE key escape action or ASCII code lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If this location c
131. MP 5 A 5 comparison BCC noerr if A lt 5 then branch round error BRK cause error OSEE nee a x rest of program or error message 54 BVC Branch if overflow clear Branch if V 0 A relative branch is made if the overflow flag is clear The relative address calculation is performed by the assembler which will flag an error if given an address out of relative addressing range Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused_ cycles relative 2 2 1 if branch succeeds 2 if to new page Example Branching on overflow when carry is deliberately set ADC amp 80 A A amp 80 4 C SEC set carry flag BVC somewhere goto somewhere if no overflow 55 BVS Branch if overflow set Branch if V 1 Branch to a relative address if the overflow flag is set Overflow is generally set when the carry flag is set except when a subtraction is performed In this case overflow is set when the carry flag is cleared The address specified in the operand field of the assembler statement must be within the relative addressing range otherwise an assembly error will be flagged Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not af
132. O 422 A3 FB NF1 NFO Data A2 Al AO 23 5 Example program for direct control of the sound generator 10 REM Demonstration of direct poke to Sound chip 20 PROCINIT 30 REPEA 40 INPUT Byte to send to sound chip AS 50 A EVAL AS 60 CALL DIRECT 70 UNTIL FALSE 80 DEF PROCINIT 90 DIM Q 40 100 OSBYTE amp FFF4 110 FOR C 0 TO 3 STEP 3 120 P Q 130 OPT C 140 DIRECT SEI Disable interrupts 150 PHA 160 LDA amp 97 170 LDX amp 43 Data direction register A 380 LDY amp FF Set all B bits as output 190 JSR OSBYTE Write to SHEILA OSBYTE CALL 200 LDX amp 41 Output register A 210 PLA 220 TAY Y holds byte to sound chip 230 LDA amp 97 Write to SHETLA OSBYTE CALL 240 JSR OSBYTE Output to slow data bus 250 LDX amp 40 Output register B 260 LDY amp 00 Set sound chip write pin low 270 JSR OSBYTE 280 LDY amp 08 Set sound chip write pin high 290 JSR OSBYTE 300 CLI Enable interrupts 310 RIS 4 320 EXT 330 ENDPROC Run the example program and enter amp 80 amp 20 and amp 90 to generate a frequency at maximum volume on channel 3 23 6 The speech chip The Speech processor can be added as an optional upgrade It can be programmed through OSBYTE CALLS amp 9E amp 9F and SOUND amp FFxx The speech data is held in a special serial speech ROM The standard one provided with th
133. PUT DATA Figure 22 13 Output Mode 2 Timing 410 Shift in under control of external CB1 clock SRMODE 3 CB1 is a clock input in mode 3 so that external devices can load the shift register at their own pace The shift register counter will generate an interrupt each time that 8 bits have been shifted in The SR counter does NOT stop the shifting operation it simply operates as a pulse counter Reading from or writing to the shift register resets the interrupt flag and initialises the SR counter to count another 8 pulses Note that data is shifted in on the first system clock cycle following the positive going edge of the CB1 shift pulse Data must therefore be held stable during the first full system clock cycle after CB1 has gone high IMH2E Cet INPUT SHIFT CLOCK cara WLLL LL ND LM ID XII a oe Figure 22 14 Output Mode 3 Timing Shift out free running at T2 clock rate SRMODE 4 In this mode the shift rate is controlled by timer 2 T2 Unlike mode 5 the SR counter will not stop the shifting operation Shift register bit 7 is recirculated back into bit 0 so the 8 bits loaded into the shift register will be clocked onto CB2 repetitively The shift register counter is disabled in this mode TITAN II i I I WRITE SR OPERATION NiZ CYCLES N2 CYCLES C81 OUTPUT 1 2 3 4 8 9 SHIFT CLOCK TS MM ED GED GEIS AD GED GEE Figure 22 15 Output Mode 4 Timing 411 Shift out under control of T
134. Q bit mask ser defined characters ser defined keys KEY ser event ser flag ser port ser print routine ser print routines ser print vector ser vector indirect via USERV ser VIA interrupt processing SERV CODE LINE use through OSWORDS USR from BASIC U U U U GEG eee Gre Cc El V VDU page three work space read VDU variable value unrecognised codes VDU character output entry point VDU codes summary VDU driver zero page work space VDU extension vector VDU queue storage in page 3 VDU status zero page location VDU variables read origin of table 270 31 98 99 100 497 377 497 480 258 478 13 16 30 229 136 15 293 117 235 425 121 258 258 256 160 304 160 256 13 16 247 256 28 274 179 261 104 459 268 261 221 276 139 268 184 509 VDUV Vector break BRK buffer count purge buffer insert buffer remove default table extended extended vector storage keyboard control network spare user user printer user supplied routines VDU extension Vectors filing system control OSFSC paged ROM service call Versatile interface adapter Version operating system Version number Vertical sync event wait VIA counter data direction registers handshaking interrupts printer pulse counting register summary shift register system user Video RAM start address for any screen mode for currently selected mode Video subsystem Video ULA characters
135. R X The old value is returned in X The contents of the next location are returned in Y This call is used by OSBYTE amp 0C FX 12 202 OSBYTE amp C6 198 FX 198 Read write EXEC file handle lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains zero if no file handle has been allocated by the operating system 203 OSBYTE amp C7 199 FX 199 Read write SPOOL file handle lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains the file handle of the current SPOOL file or zero if not currently spooling 204 OSBYTE amp C8 200 FX 200 Read write ESCAPE BREAK effect lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y bit 0 0 Normal ESCAPE action bit 0 1 ESCAPE disabled unless caused by OSBYTE amp 7D 125 bit 1 0 Normal BREAK action bit 1 1 Memory cleared on BREAK e g A value 0000001x binary will cause memory to be cleared on BREAK Note memory location used is amp 258 205 OSBYTE amp C9 201 FX 201 Read write keyboard disable lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If this location contains 0 then
136. RDR If no parity is selected then both transmitter parity output generation and receiver parity input checks are inhibited 20 7 8 Interrupt Request IRQ Bit 7 This indicates the state of the IRQ output Whenever the IRQ output is low the IRQ bit is high IRQ is cleared by a read operation to the Receive Data Register or a write operation the Transmit Data Register Note that OSBYTE amp E8 is used to mask the 6850 ACIA LRQ See chapter 13 on interrupts for further details about using interrupts 390 20 8 Bit mM ND OP WN 6850 ACIA Summary table Control Register WRITE ONLY Counter Divide select 1 CRO Counter Divide select 2 CR1 Word select I CR2 Word select 2 CR3 Word select 3 CR4 Transmit control 1 CR5 Transmit control 2 CR6 Receiver interrupt enable CR7 Status Register READ ONLY Receive Data Register Full RDRF Transmit Data Register Empty TDRE Data Carrier Detect Clear To Send Framing Error FE Receiver overrun Parity error PE Interrupt request IRQ 391 20 9 THE SERIAL ULA SHEILA amp 10 The serial ULA performs several functions related to the cassette and RS423 interfaces It allows the input to the 6850 to be switched between cassette and RS423 It produces the transmit and receive data clocks for the 6850 thereby defining the baud rate This ULA synthesises the data carrier signal for the cassette recording and has a data separator and run in detector wh
137. ROM whose contents are not directly accessible by the service routine OSBYTE amp BA will give the ROM number of the ROM which was active when the BRK occurred Unrecognised OSBYTE call The user has issued an OSBYTE call not known to the operating system The A X and Y registers on entry to the OSBYTE are stored at amp EF amp F0 and amp F1 respectively The OSBYTE call should be recognised on the contents of amp EF Unrecognised OSWORD call The user has issued an OSWORD call not known to the operating system The A X and Y registers on entry to OSWORD are stored at amp EF amp F0 and amp F1 respectively The OSWORD call should be recognised on the contents of amp EF Note that it is not worth trapping OSWORD calls with numbers greater than amp EO as these are all sent to the user vector at amp 200 Note also that OSWORD number 7 make a sound will cause this service call if an unrecognised channel number is used in the range amp 2000 to amp FFFF allowing for future sound channel expansion over the 1MHz bus 09 0A 0B 0C HELP instruction expansion This service call is made whenever the user issues a HELP command ROMs should allow all resident ROMs to respond to it at once so that the user can find out about all the resident ROMs at once On entry the rest of the command after the HELP is pointed to by the contents of locations amp F2 and amp F3 plus the Y register ROMs should recognise keywords on tha
138. ROM workspace table at DFO to amp DFF for ROMs 0 to amp F respectively and add to the Y register the length in 256 byte pages of the private workspace required Because the absolute workspace is shared between all the paged ROMs each ROM that uses it should keep a flag within its private workspace which indicates whether or not it currently has control of the absolute workspace This enables the ROM to claim the workspace when it needs it and to respond to such a claim from other ROMs Auto boot Each service ROM is given the opportunity to initialise itself on break This is primarily used for filing systems to set up their vectors on break rather than having to be reselected every time with an operating system command To allow lower priority ROMs a look in each ROM should examine the keyboard before initialisation and initialise only if no key is pressed or a key exclusive to that ROM is pressed eg the discs are selected by D break If the ROM initialises it should look for and RUN EXEC or LOAD a boot file typically called IBOOT if on entry the Y register is Zero Unrecognised command When the user issues an OS command that is not recognised by the central operating system the command is first offered to the paged ROMs and then to the currently active filing system ROMs containing general utilities should use this call to activate them Filing system and language ROMs should trap this to catch their selection co
139. Read write first byte of BREAK intercept code Read write second byte of BREAK intercept code Read write third byte of BREAK intercept code Read write location amp 28A not used by OS1 20 Read write location amp 28B not used by OS1 20 Read write current language ROM number Read write last BREAK type Read write available RAM Read write start up options 115 OSBYTE 2 00 0 FX 0 Identify Operating System version Entry parameters X 0 Execute BRK with a message giving the O S type X lt gt 0 RTS with O S type returned in X On exit X 0 OS 1 00 X 1 OS 1 20 A and Y are preserved C is undefined 116 OSBYTE 8 01 1 FX 1 Read write the user flag Entry parameters The user flag is replaced by lt old value gt AND Y EOR X i e Y 0 for write Y amp FF for read On exit X old value This call uses OSBYTE call with A amp F1 241 This OSBYTE call is left free for user applications and is not used by the operating system The user flag is stored in location amp 281 and its default value is 0 117 OSBYTE 8 02 2 FX 2 Select input stream Entry parameters X determines input device s FX 2 0 X 0 keyboard selected R5423 disabled FX 2 1 X 1 RS423 selected and enabled FX 2 2 X 2 keyboard selected RS423 enabled Default FX 2 0 On exit X 0 if previous input was from the keyboard X 1 if previous input was from RS423 After call A is preserved Y and C are undefined 118 OSBYTE amp 03
140. Reading from or writing to the SR will trigger a shifting operation if the SR flag in the IFR is set If it isn t set then the first shift will occur when T2 next times out after a read or 409 write SR Data is shifted first into the low order bit of the SR then into the next higher order bit and so on the negative edge of each shift clock pulse The input data should then change before the next positive going edge of CB1 Data is shifted into the shift register on the positive going edge of the CB1 pulse After 8 CB1 clock pulses the shift register interrupt flag will be set and an interrupt will be requested of the 6502 IMHzE ATTA WAITE OR READ SHIFT REG Ne CYCLES Nea CYCLES l l 2 j l 3 j 7 Cal OUTPUT SHIFT CLOCK enn TTX NOCITIIX SLE NC la Figure 22 12 Input Mode 2 Timing Shift in under control of system clock SRMODE 2 In mode 2 the shift rate is a direct function of the 1MHz system clock Pulses for controlling external devices are generated on the CB1 output Timer 2 has no effect on the SR and acts as an independent interval timer The shifting operation is triggered by reading or writing the SR Data is first shifted into bit 0 and then into successively higher order bits on the trailing edges of system clock pulses After 8 clock pulses the shift register interrupt flag will be set and output clock pulses from CB1 will cease 1MHZE READ SA OPERATION Cal OUTPUT SHIFT CLOCK CB2 IN
141. The Advanced User Guide for the BBC Microcomputer Andrew C Bray St Catherines College Dept of Computer Science Cambridge University Adrian C Dickens Churchill College Dept of Engineering Cambridge University Mark A Holmes BA Fitzwilliam College Dept of Clinical Veterinary Medicine Cambridge University Contents Introduction 1 Introduction for those new to machine code Operating System Commands 2 Operating System commands Assembly Language Programming The BASIC assembler Machine code arithmetic Addressing modes The assembler mnemonics OH JIA W Operating System interfaces 7 Operating system calls 8 FX OSBYTE calls 9 OSWORD calls 10 Vectors 11 Memory usage 12 Events 13 Interrupts 14 RS432 15 Paged ROMs 16 Filing systems 11 21 31 41 101 109 247 253 267 287 295 309 317 333 Hardware 17 18 19 20 21 22 23 24 25 26 27 28 An introduction to the hardware The video circuit 6845 The video ULA The serial interface The paged ROM select register Programming the 6522 VIA The system 6522 including sound and speech The user 6522 Disc and Econet interfaces The analogue to digital converter The Tube The 1 MHz bus Appendices A B C D E F G H I J K FX OSBYTE call index Operating System calls summary Table of key numbers VDU codes PLOT number summary Screen mode layouts US MOS differences Disc upgrade Circuit board link
142. W VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This value contains the duration parameter as for SOUND command used for the BELL sound Default value 7 217 OSBYTE amp D7 215 FX 215 Read write start up message suppression and BOOT option status lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y bit 7 If clear then ignore OS startup message If set then print up OS startup message as normal bitO If set then if an error occurs in a IBOOT file in ROM carry on but if an error is encountered from a disc IBOOT file because no language has been initialised the machine locks up If clear then the opposite will occur i e locks up if there is an error in ROM This can only be over ridden by a paged ROM on initialisation or by intercepting BREAK see OSBYTE calls amp F7 to amp F9 218 OSBYTE amp D8 216 FX 216 Read write length of soft key string lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains the number of characters yet to be read from the soft key buffer of the current soft key It may be useful to set this value to 0 to cancel a soft key expansion without clearing the input buffer To clear input buffer use FX 15 OSBYTE amp OF
143. WRCH amp FFEE 50 EVNTV 6220 60 DIM MC 100 10 DIM clocx parc 5 80 DIM count 1 90 FOR I 0 TO 2 STEP 2 100 PS MC 110 120 entry OPT I 130 PHP 140 PHA 150 TXA 160 PHA 130 TYA 180 PHA save registers 190 LDX clock_pars AND 255 200 LDY clock_pars DIV 256 210 LDA 4 220 JSR OSWORD write to interval timer 230 LDA amp 86 240 JSR OSBYTE read current text cursor position 250 TYA and save it on the stack 260 PHA 270 TXA 280 PHA 290 LDA 831 reposition text cursor 300 JSR OSWRCH with VDU 31 38 1 310 LDA 38 320 JSR OSWRCH 330 LDA 1 340 JSR OSWRCH 350 LDA count put count in A 360 JSR OSWRCH write it not 370 CMP ASC 9 has count reached 9 380 BNE over jump next bit if it hasn t 390 LDA ASC 400 STA count put 1 in count 410 over INC count count count 1 420 LDA 31 restore old cursor position 430 JSR OSWRCH using VDU 31 291 440 PLA 450 JSR OSWRCN 460 PLA 470 JSR OSWRCH 480 PLA 490 TAY 500 PLA 510 TAX 520 PLA 530 PLP restore registers 540 RTS return from event handler 5590 J 560 EXT I 570 PEVNTV MC S AND 6FF 580 EVNTV 1 MC DIV amp 100 590 clockpars amp FFFFEF9C 600 clock_pars 4 amp FF REM clock valu 100 centiseconds 610 FX 14 5 620 REM interval timer event 630 count ASC 0 REM initialise count 640 CALL entry REM initialise clock Th
144. YTE 2 08 8 FX 8 Set RS423 baud rate for data transmission Entry parameters X determines transmission rate FX 8 1 X 1 75 baud transmit FX 8 2 X 2 150 baud transmit FX 8 3 X 3 300 baud transmit FX 8 4 X 4 1200 baud transmit FX 8 5 X 5 2400 baud transmit FX 8 6 X 6 4800 baud transmit FX 8 7 X 7 9600 baud transmit FX 8 8 X 8 19200 baud transmit After call A is preserved X and Y contain the old serial ULA register contents C is undefined 124 OSBYTE 8 09 9 FX 9 Set duration of the mark state of flashing colours Duration of first named colour Entry parameters X determines length of duration Y 0 FX 9 0 X 0 Sets mark duration to infinity Forces mark state if space is set to 0 FX On X n Sets mark duration ton centiseconds n 25 is the default setting After call A and X are preserved Y contains the old mark duration C is undefined 125 OSBYTE amp 0A 10 FX 10 Set duration of the space state of flashing colours Duration of second named colour Entry parameters X determines length of duration Y 0 FX 10 0 X 0 Sets space duration to infinity Forces space state if mark is set to 0 FX 10 n X n Sets space duration to n centiseconds n 25 is the default setting After call A and X are preserved Y contains the old space duration C is undefined 126 OSBYTE amp 0B 11 FX 11 Set keyboard auto repeat delay Entry parameters X determines delay before repeating starts Y 0
145. able Others 5 Not Writable Others 336 6 Not Executable Others 7 Not Deletable Others In this instance you means the user reading the attributes and others means other users of say the Econet filing system File types returned in the accumulator are 0 Nothing found 1 File found 2 Directory found On exit X and Y are preserved The accumulator contains the file type C N V and Z are undefined Interrupt status is preserved but may be enabled during a call 16 3 OSARGS Read or write an open file s arguments Call address amp FFDA Indirected through amp 214 This routine reads or writes an open file s attributes On entry X points to a four byte zero page control block Y contains the file handle as provided by OSFIND or Zero The accumulator contains a number specifying the action required If Y is zero A 0 Returns the current filing system in A 0 No filing system currently selected 1 1200 baud cassette 2 300 baud cassette 3 ROM filing system 337 4 Disc filing system 5 Econet filing system 6 Telesoftvvare system A 1 Returns the address of the rest of the command line in the base page control block This gives access to the parameters passed with RUN or command A amp FF Update all files onto the media ie ensure that the latest copy of the memory buffer is saved If Y is not zero A 0 Read sequential pointer of file BASIC PTR A 1 Write sequential pointer of file
146. ad write ESCAPE condition acknowledge ESCAPE condition clear ESCAPE condition set ESCAPE flag zero page location Event ESCAPE keyboard Event disable ADC conversion complete character entering buffer ESCAPE pressed input buffer full interval counter crossing 0 network error output buffer empty RS423 error start of vertical sync user Event enable ADC conversion complete character entering buffer ESCAPE pressed input buffer full interval counter crossing 0 network error output buffer empty RS423 error start of vertical sync user Event vector Events ADC conversion complete character entering input buffer disabling Econet error enabling ESCAPE condition detected generator of using OSEVEN handling routines input buffer full interval timer crossing zero 28 257 54 205 172 292 147 227 228 227 16 223 149 147 148 272 172 172 129 129 129 129 129 129 129 129 129 129 130 130 130 130 130 130 130 130 130 130 258 288 287 290 290 129 293 130 292 107 288 289 291 output buffer empty page two flags RS423 error user vertical sync EVNTV Example hardware scrolling MODE 8 implementation Exploring soft character RAM Extended messages Extended vector space Extended vectors F Field File attributes OSFILE File handle for EXEC file File handle for SPOOL file File options Files Files opening closing OSFIND FILEV OSFILE Filing system messages Filing systems co
147. address 120 STA amp 206 IRQ2V low 130 LDA tint DIV 256 High byte of address 140 STA amp 207 IRQ2V high 150 Chit 160 RTS Exit 170 int LDA amp FC Do save 180 PHA 190 TXA 200 PHA 210 YA 220 PHA 230 LDA amp FE4D Get system VIA interrupt status 240 AND amp 81 Mask Out bits out interested in 250 CMP amp 81 Is it a keyboard interrupt 260 BNE exit No exit 270 STA amp FE4D Clear interrupt 280 LDA 7 BELL character 290 JSR amp FFEE Make a tone 300 exit PLA Restore registers 310 TAY 320 PLA 330 TAX 340 PLA 350 STA amp FC Just in case its changed 360 JMP oldv Continue interrupt chain 370 oldv EQUB 0 BASIC II reserve space 380 390 NEXT opt 400 REM grab the vector 410 CALL init 420 REM grab keyboard interrupts 430 REN Using mask 11111110 254 440 FX 233 254 450 REM Demonstrate machine not crashed by 460 X 0 470 REPEA 480 PRINI X 490 VDU 13 500 XS XS41 510 UNTIL FALSE 306 This example introduces some interesting points When a key is held down for some time the machine seizes up until the key is released This is because keyboard interrupts occur continuously so that the operating system has no time for anything other than interrupt processing The operating system on receiving an keyboard interrupt immediately disables it and polls the keyboard The keyboard interrupts are only re enabled when all keys are released Note the cleari
148. ag not affected N negative flag set if bit 7 of A set Addressing mode bytesused cycles immediate 2 2 Zero page Z 3 absolute 2 4 absolute X 3 4 1 if page crossed absolute Y 3 4 1 if page crossed indirect X 2 6 indirect Y 2 5 1 if page crossed Example Set the top 4 bits of the accumulator ORA amp F0 mask is 1111000 1 OR anything 1 79 PHA Push accumulator onto stack Push A This instruction places the value held in the accumulator onto the stack This value is accessible using the instruction PLA pull A from stack Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing Mode bytesused cycles implied 1 3 Example Save registers at the beginning of a routine entry PHP save status register s below PHA save accumulator contents TXA A X PHA save X register contents TYA A Y PHA save Y register contents ei rest of program 80 PHP Push Status register onto stack Push P This instruction places the value held in the status register onto the stack This value is accessible using the instruction PLP pull P from stack Processor Status after use C carry flag not affected Z zero flag not affected J interrupt disable not affected D deci
149. ains the ASCII character and key which will generate an ESCAPE e g FX 220 32 will make the SPACE bar the ESCAPE key Default value amp 1B 27 223 OSBYTEs amp DD 221 to amp EO 224 FX 221 to 224 Read write I P buffer code interpretation status lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y These locations determine the effect of the character values amp C0 192 to amp FF 255 when placed in the input buffer See OSBYTEs amp E1 225 to amp E4 228 for details about the different effects which may be selected Note that these values cannot be inserted into the input buffer from the keyboard RS423 input or a user keyboard handling routine may place these values into the input buffer OSBYTE amp DD affects interpretation of values amp C0 to amp BF OSBYTE amp DE affects interpretation of values amp D0 to amp CF OSBYTE amp DF affects interpretation of values amp E0 to amp EF OSBYTE amp EO affects interpretation of values amp FO to amp FF Default values amp 01 amp D0 amp E0 and amp FO respectively 224 OSBYTE amp E1 225 FX 225 Read write function key status soft keys or codes Input buffer characters amp 80 to amp 8F OSBYTE amp E2 226 FX 226 Read write SHIFT function key status soft key or code Input buffer characters amp 90 to amp 9F OSBYTE amp E3 227 FX 227 Read write CTR
150. al mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of the result is set Addressing Mode ytes used cycles accumulator 1 2 zero page 2 5 zero page X 2 6 absolute 3 6 absolute X 3 7 Example Reverse the order of bits in a byte start STA amp 80 store byte in amp 80 LDX 8 set loop count to 8 loop ROL amp 80 bit 7 of amp 80 to carry ROR A carry to bit 8 of A DEX decrement loop count BNE loop if not 0 goto loop RTS exit with A reversed 85 RTI Return from Interrupt Status register and PC pulled from stack This instruction is used to return from an interrupt handling routine When an interrupt occurs the current program counter and status register are pushed onto the stack These are restored by the RTT instruction Processor Status after use C carry flag bit 0 from stack Z zero Hag bit 1 from stack I interrupt disable bit 2 from stack D decimal mode flag bit 3 from stack B break command bit 4 from stack V overflow flag bit 6 from stack N negative flag bit 7 from stack Addressing Mode Bytes used cycles implied 1 6 Example Instruction at the end of an interrupt handling routine sper code dealing with the interrupt RTI back to what we were doing before 86 RTS Return from subroutine Pull PC from stack The RTS instruction is used to terminate the execution of a sub
151. all is made and the buffer examined to prevent any interrupt changing the buffer After call A and X are preserved This appears to be at variance with an ACORN press release which said that Y was returned containing the value of the character itself 171 OSBYTE 8 99 153 FX 153 Insert character into input buffer checking for ESCAPE Entry parameters X contains buffer number 0 or 1 and Y contains the character value X 0 keyboard buffer X 1 R5423 input If RS423 input is enabled and X 1 then RS423 ESCAPEs are suppressed this is the default state plus OSBYTE call with A amp B5 and X 1 FX181 1 this is identical to OSBYTE call with A amp 8A FX 138 Otherwise if the character to be inserted is not the ESCAPE character set by OSBYTE amp DC FX 220 or if ESCAPE characters are to be treated as normal characters following OSBYTE with A amp E5 FX 229 then an input event even if input is from RS423 is caused and the character is inserted into the buffer If the character is an ESCAPE character and ESCAPEs are not protected using OSBYTE amp C8 FX 200 then an ESCAPE event is generated instead of the keyboard event after call A is preserved X Y and C are undefined 172 OSBYTE amp 9A 154 YFX 154 Write to video ULA control register and OS copy Entry parameters X contains value to be written This call writes to register 0 of the video ULA and also writes the value in location amp 248 of
152. ammed as outputs When reading IRA it is the voltage level on PAO PA7 which determines the level read back When reading IRB it is always the bit in the output register which is read back This means that with loads which pull an output 1 low or an output 0 high reading IRA may indicate a different logic level to that written to the output Reading IRB will however always read back the value programmed no matter what loading is applied to the pin 400 REG O ORB IRB ECEGEESO i i PBO ie PBI id 3 PB2 is OUTPUT REGISTER B ORB 4 PB3 d OR S P l 84 INPUT REGISTER B ORB PBS iz d PB6 E i PA Pin I Data Direction WRITE Selection DORB 1 OUTPUT MPU writes Output Level MPU reads output register bit ORB in ORB Pin tevel has no affect DDRB 0 INPUT MPU writes into ORB but MPU reads input level on PB input latching disabled no effect on pin level until pin DORB changed DORB 0 INPUT MPU reads IRB bit which is Input latching enabled the level of the PB pin at the time of the last CB1 active transition Figure 22 2 Port B Input Output 401 REG 1 ORA IRA uoggoggg pad PAI PA2 OUTPUT REGISTER A ORA PA3 OR PAU iNPUT REGISTER A NRA PAS PAG PA Pin Data Direction WRITE Selection DORA 1 OUTPUT MPU writes Output Level IORA input latching disabled DDRA 1 OUTPUT MPU reads IRA bit which is input l
153. amp 8F inclusive These are the only zero page locations that a user program resident in RAM should use if the program is to be made commercially available or run on a variety of other BBC Microcomputers The locations from amp 0 to amp 6F which are part of BASIC s zero page workspace are available to the machine code program if BASIC is not required while the code is running Depending on the nature of the machine code program other zero page locations may be available See chapter 11 memory usage for more details 30 4 Machine Code Arithmetic 4 1 2 s Complement The 6502 microprocessor normally performs all arithmetic using the 2 s complement method of representing numbers In 2 s complement representation the most significant bit of the value is a sign bit If the most significant bit is clear then the number is positive The remaining bits represent the binary value of the positive number Negative values are represented by the complement of the positive value plus 1 The complement of any binary value is made by flipping each bit i e changing each 1 to a0 and each 0 to a 1 When negative values are represented by the complement of the positive value this is called l s complement The disadvantage with l s complement is that there are two ways of representing 0 a positive 0 all bits clear and a negative 0 all bits set By adding one to the complemented value 2 s complement there is only one way of re
154. amp C8 5 MSB amp 00 6 Duration LSB 20 amp 14 7 MSB amp 00 This call has exactly the same effect as the SOUND command 9 10 OSWORD call with A amp 8 Define an ENVELOPE The ENVELOPE parameter block should contain 14 bytes of data which correspond to the 14 parameters described in the ENVELOPE command This call should be entered with the parameter block address contained in the X and Y registers 9 11 OSVVORD call with A amp 9 Read pixel value This routine returns the status of a screen pixel at a given pair of X and Y co ordinates A four byte parameter block is required and result is contained in a fifth byte XY LSB of the X co ordinate MSB of the X co ordinate LSB of the Y co ordinate MSB of the Y co ordinate OU NBeO 250 On exit XY 4 contains the logical colour at the point or amp FF if the point specified was off screen 9 12 OSWORD call with A amp A Read character definition The 8 bytes which define the 8 by 8 matrix of each character which can be displayed on the screen may be read using this call The ASCII value of the character definition to be read should be placed in memory at the address stored in the X and Y registers After the call the 8 byte definition is contained in the following 8 bytes XY 0 Character required 1 Top row of character definition 2 Second row of character definition 8 Bottom row of character definition 9 13 OSVVORD call with A amp B Read palette The physical colour associ
155. and not affected V overflow flag set to bit 6 of memory N negative flag set to bit 7 of memory Addressing mode bytes used cycles Zero page 2 3 absolute 3 4 Example Test bit 7 of location amp 8F LDA amp 02 load mask into accumulator 00000010 BIT flags A AND flags if bit 1 1 then Z 0 BNE flagset action to be performed if bit 1 set 50 BMI Branch if negative flag set Branch if N 1 This relative branch is performed if the result of a previous operation was negative Relative branch calculations are made by the assembler which will flag an error if an address is given outside the relative addressing range Branch occurs after a result which sets bit 7 of the accumulator All 8 bit 2 s complement negative numbers have this bit set Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused cycles relative 2 2 1 if branch succeeds 2 if to new page Example Branching if a byte of memory contains a negative number LDA amp 3010 load accumulator from memory N set if Ve BMI negative branch if amp 3010 is negative 51 BNE Branch on result not zero Branch if Z 0 This instruction causes a relative branch if the zero flag is clear whe
156. ansferring some 296 data to or from the interrupting unit and clearing the interrupt condition The interrupt condition must be cleared because most devices that use interrupts continue to signal an interrupt until they have been serviced The completion of servicing often has to be signalled by the processor writing to a special register in the device Interrupts must not have any effect on the interrupted program The interrupted program will expect the processor registers and flags to be exactly the same after return from an interrupt routine as they were before the interrupt occurred Thus an interrupt routine must either not alter any registers which is difficult or restore all register contents to their original values before returning Interrupt routines are entered with interrupts disabled so a second interrupt cannot occur whilst an interrupt routine is still processing unless interrupts are deliberately enabled because the interrupt servicing is likely to take an appreciable time When this is done the interrupt routine must be written with care because the interrupt service routine can then itself be interrupted For this reason it is usual to save register contents onto the processor stack rather than to fixed memory locations which may get overwritten in a subsequent incarnation of the interrupt routine 13 2 Interrupts on the BBC microcomputer Interrupts are required on the BBC microcomputer to process all of the ba
157. aracter is inserted into the keyboard buffer The soft key characters may be calculated by adding the soft key number to amp 80 128 A soft break actually places a character of value amp CA in the input buffer but this behaves identically to character amp 8A See OSBYTEs amp DD 221 to amp E4 228 for more information about the function keys Control codes may be introduced into the string by using an escape character This acts in a similar way to the CTRL key and so G gives a bell sound as the CTRL and G keys would if pressed simultaneously G actually places a character of value 7 into the soft key buffer The character may be inserted using the sequence and a quotation mark may be represented by preceding the quotation mark with the escape character The delete character ASCII amp 7F 127 may be introduced using the escape character followed by a question mark Characters of value greater than 127 may be inserted by using the escape sequence this sequence will add 128 to the value of the next character in the string eg IA 128 65 193 JA 128415129 This method of including non printable characters into a string may be used in any string which is processed using the operating system routines GSINIT and GSREAD see section 7 9 and is not restricted to use of the KEY command 2 11 LINE lt text gt LL This command executes machine code at the
158. atching enabled the level of the PA pin at the time of the last CAT active ransition MPU reads level on PA pin MPU reads IRA bit which is the devel of the PA pin at the time of the last CA1 active transition DDRA 0 INPUT Input latching d sabled MPU writes into ORA but no effect on pin level until DORA changed DDRA 0 INPUT Input latehing enabled Figure 22 3 Port A Input Output REG 2 DDRB AND REG 3 DDRA pels Le le Le Le fo i PBO PAG PB1 PA1 PB2 PA2 PB3 PA 3 DATA DIRECTION REGISTER pgapaa B OR A DORB DDRA PBS PAS PB6 PAG PBT PA7 TOT ASSOCIATED PB PA PIN IS AN INPUT HIGH IMPEDANCE ASSOCIATED PB PA PIN IS AN OUTPUT WHOSE LEVEL IS DETERMINED BY ORB ORA REGISTER BIT z Figure 22 4 Data Direction Registers 4UZ 22 2 2 Write handshaking data transfer Handshaking allows data transfers between two asynchronous devices Write handshaking operates with data ready and data taken signals The 6522 provides the data ready CA2 or CB2 signal and accepts the data taken CA1 or CB1 signal from the peripheral device This data taken signal sets the interrupt flag and clears the data ready output See the timing diagram figure 22 5 WRITE ORA OAB OPERATION DATA READY HANDSHAKE MODE ICA2 C92 DATA READY l PULSE MODE ICAJ CBZ 3 DATA TAKEN ICA1 C81 a aes E
159. ate to use a FX command this has been indicated Preceding the full OS BYTE descriptions is a complete summary of the OSBYTE calls ina list 109 OSBYTE calls amp A6 166 to amp FF 255 can be used to read or write operating system status flags or variables In OS 1 20 these memory locations extend from amp 236 to amp 28F The action of these calls is to replace the contents of the specified location with lt old value gt AND Y EOR X To read a location set X 0 Y amp FF To write a location set X value Y 0 On exit X old value Y value of next location Many of these calls repeat the function of lower value OSBYTEs N B These equivalent calls are not guaranteed to have an identical effect when used to set flags or OS variables and other calls are of no practical use these are included for completeness 110 OSBYTE FX Call Summary dec function Print operating system version User OSBYTE call read write location amp 281 Select input stream Select output stream Enable disable cursor editing Select printer destination Set character ignored by printer Set RS423 baud rate for receiving data Set RS423 baud rate for data transmission Set flashing colour mark state duration Set flashing colour space state duration Set keyboard auto repeat delay interval Set keyboard auto repeat rate Disable events Enable events Flush selected buffer class Select ADC channels to be sampled Force an ADC conversion Res
160. ated with each logical colour may be read using this routine On entry the logical colour is placed in the location at XY and the call returns with 4 bytes stored in the following four locations corresponding to a VDU 19 statement e g Assuming that a VDU 19 1 3 0 0 0 had previously been issued then OSWORD amp B with 1 at XY would yield XY 0 1 logical colour 3 physical colour 0 padding for future expansion 0 0 ES GN 251 9 14 OSWORD call with A amp C Write palette This call performs the same task as a VDU 19 command which can be used from machine code using OSWRCH The advantage of using this OSWORD call rather than the conventional VDU route is that there is a significant saving in time Another advantage is that OSWORD calls can be used in interrupt routines while VDU routines cannot This call works in the same way as OSWORD amp B see above a parameter block should be set up with the logical colour being defined at XY the physical colour being assigned to it in XY 1 and XY 2 to XY 4 containing padding Os 9 15 OSWORD call with A amp D Read last two graphics cursor positions The operating system keeps a record of the last two graphics cursor positions in order to perform triangle filling if requested These cursor positions may be read using this call X and Y should provide the address of 8 bytes of memory into which the data may be written XY previous X co ordinate low byte previous X co ordinate high
161. ator Accumulator addressing Active low Actual colours ADC conversion complete event page two locations ADC Add with Carry ADC channel read current channel read maximum channel number ADC channel read ADC conversion forcing ADC conversion type ADC end of conversion ADC number of channel select Address bus Addressable latch Addressing absolute absolute X or Y accumulator immediate implicit indexed indirect modes post indexed indirect pre indexed indirect relative zero page zero page X or Y ADVAL American MOS differences from UK Analogue to digital converter joystick connection AND Logical AND Animation fast hardware ARGSV OSARGS ASL Arithmetic Shift Left Assembler addressing modes conditional assembly delimiters EQU 500 229 175 419 36 37 41 35 493 382 493 290 273 44 196 197 151 133 198 418 132 493 419 36 37 35 36 35 37 37 35 39 38 40 36 38 151 478 429 493 432 45 373 337 46 35 29 22 26 EQUB EQUW EQUD EQUS errors forward referencing from BASIC label listing location counter macros mnemonics operand OPT options syntax two pass Asynchronous Auto repeat countdown timer in zero page countdown timer queue Auto repeat delay keyboard Auto repeat rate Available RAM B Backslash in assembler programs BASIC level 1 level 2 new ROM socket with BASIC Baud rate Baud rate selection for RS423 BCC Branch on Carry
162. aud is equal to I bit of data transferred per second The standard cassette baud rate of 1200 baud is therefore equal to 1200 bits per second Bidirectional a communication line is bidirectional if data can be sent and received over it The data bus lines are bidirectional Bit of memory this is the fundamental unit of a computer s memory It may only be in one of two possible states usually represented by a0 or 1 Buffer there are two types of buffer in the BBC microcomputer A software buffer is an area of memory set aside for data in the process of being transferred from one device or piece of software to another A hardware buffer is put into a signal line to increase the line s drive capability For example the printer port has buffered outputs which are capable of supplying several milliamperes The inputs to the buffer could only supply microamperes 493 Byte of memory 8 bits of memory Data is normally transferred between devices one byte at a time over the data bus Chip derived from the small piece of silicon wafer or chip which has all of the computer logic circuits etched into it A chip is normally packaged in a black plastic case with small metal leads to connect it to the outside world Clock since there are so many devices in the BBC microcomputer it is necessary to provide some master timing reference to which all data transfers are tied The clock provides this synchronisation A 2MHz clock is applied to the
163. ber amplitude Read write BELL frequency Read write BELL duration Read write startup message and IBOOT options Read write length of soft key string Read write number of lines printed since last page Read write number of items in VDU queue Read write TAB character value Read write ESCAPE character value Read write character amp CO to amp CF status Read write character amp DO to amp DF status Read write character amp EO to amp EF status Read write character amp FO to amp FF status Read write function key status Read write SHIFT function key status Read write CTRL function key status Read write CTRL SHIFT function key status Read write ESCAPE key status Read write flags determining ESCAPE effects Read write JRQ bit mask for user 6522 Read write IRQ bit mask for 6850 Read write IRQ bit mask for system 6522 Read flag indicating Tube presence Read flag indicating speech processor presence Read write write character destination status Read write cursor editing status Read write location amp 27E not used by 05 1 20 Read write location amp 27F not used by 05 1 20 Read write location amp 280 not used by 05 1 20 Read write location amp 281 used by FX 1 Read RAM copy of serial processor ULA Read write timer switch state Read write soft key consistency flag Read write printer destination flag 246 247 248 249 250 251 252 253 254 255 F6 F7 F8 F9 FA FB FC FD FE FF Read write character ignored by printer
164. ber of characters per line the clock rate sent to the 6845 the width in bytes of each character and the master cursor size FX154 should be used to write data into this register NUMBER OF TELETEXT FLASH CHARACTERS PER NORMAL COLOUR LINE SELECT SELECT Fig 19 1 The Video Control Register 377 19 1 1 Selected flash colour bit 0 This bit selects which colour of the two flashing colours is actually displayed at any particular time It is continually changed by the operating system to generate the flashing colours FX9 and FX10 control how long each colour is on the screen and can be defined down to one fiftieth of a second By varying the flash rates of the colours it is possible to generate new colours This is because a flash rate of one fiftieth of a second is fast enough to fool the eye into seeing a single colour rather than two rapidly flashing colours 0 first colour selected 1 second colour selected 19 1 2 Teletext output select bit 1 This bit selects whether RGB input comes from the video serialiser in the ULA or from the teletext chip 0 on chip serialiser used 1 teletext input selected 19 1 3 Number of characters per line bits 2 3 These two bits determine the actual number of displayed characters per line It is by varying this number that the new 10 character per line mode can be generated Bit 3 Bit 2 Number of characters per line 1 1 80 1 0 40 0 1 20 0 0 10 19 1 4 6845 Vid
165. bers 15 and 16 The majority of this information is specific to OS 1 2 although most of it is correct on other series 1 operating systems and the general overview is true for operating system 0 1 It should be noted that all locations described here are highly unofficial and are not documented by Acorn For compatibility with future operating systems users should not use any of these locations directly unless it is totally unavoidable Access to these locations via OSBYTE calls should remain fairly operating system independent Acorn have not documented OSBYTE amp AO so the VDU variable locations should therefore not be relied upon to remain constant The locations listed here should prove of great use to those disassembling the operating system ROM 11 1 Zero page The zero page on the 6502 is very valuable as many instructions and addressing modes need to work through page zero For this reason areas of zero page are allocated to each of the main memory contenders Zero page is allocated thus amp 00 amp 8F are allocated to the current language BASIC reserves locations amp 70 amp 8F for the user amp 90 amp 9F are allocated to the Econet system 267 amp A0 amp A7 are allocated to the current NMI owner see section in paged ROMs number 15 3 2 This area is not used on basic cassette machines It is used extensively by the disc and network filing systems amp A8 amp AF are allocated for use by operating sy
166. bler up to the next colon or end of line N B In level 1 BASIC colons cannot be included in expressions Missing out a colon in a multi statement line will result in the statement after the intended colon being ignored by the assembler This error is often difficult to spot in a program which assembles without error but then fails to function as the programmer had anticipated During assembly of the example program above the following printout is produced with PAGE amp 1900 gt RUN 1BEA OPT opt 1BEA A2 00 entry LDX 0 set index count in X reg to 0 1BEC 00 5A 1C Loop LDA data X load next VDU parameter 1BEF 20 E3 FF JSR OSASCI perform VDU command 1BF2 E8 INX increment index count 1BF3 EQ 20 CPX 8620 has count reached 32 8620 9 1BF5 00 F5 BNE loop if not then go round again 1BP7 60 RIS back to BASIC location label mnemonic address op code data comment 23 3 2 OPT assembler option selection OPT is an assembler directive or non assembling statement which can be included within an assembly program to select a number of different assembler options The OPT command should be followed by a number to make the option selection The assembler options are selected on the state of the least significant 2 or 3 bits of the OPT parameter bit 0 if set assembly listing enabled bit 1 if set assembler errors enabled bit 2 if set assembled c
167. bust enough so that multibit errors are unlikely to cancel out Parity checking is used in some data transfer situations where the consistency of the data is being checked Parity checking involves adding a bit to each byte of output This bit is calculated so that the total number of set bits including the parity bit is consistently either odd or even Whether odd or even is not important as long as it is consistent CRCs are usually used rather than byte by byte parity checking because if two bits are in error in a single byte parity may not pick it up though a CRC will CRCs may be calculated by using the following code which calculates the CRC for a block of bytes of length IbIR and starting at data On exit H contains the CRC high byte and L contains the CRC low byte LDA 0 STA H Initialise the CRC to zero STA L TAY Initialise the data pointer nbyt LDA H Work data into CRC EOR data Y STA H LDX 8 Perform polynomial recycling loop LDA Loop is performed 8 times once for bit ROL A Test if a bit is being cycled out BCC biz LDA H Yes add it back in 8 5 EOR 8 STA LDA L EOR amp 10 STA L b7z ROL E Always rotate whole CRC left one bit ROL H DEX BNE loop Do once for each bit INY Point to next data byte CPY lblk All done yet BNE nbyt RTS All done H CRC Hi L CRC Lo 348 16 11 ROM filing system The ROM filing system is standard on all BBC microcomputers
168. byte The data bus is therefore called a bidirectional bus because data flows in any one of two directions The address bus is unidirectional since the 6502 provides but cannot receive addresses Note that some address buffers are included in the video circuit to allow either the 6845 5050 or 6502 to provide addresses for system random access memory RAM In order to control the direction of data flow on the data bus a read or write signal is provided by the control bus Hardware connected to the system can thereby determine whether it is being sent data or is meant to send data back to the CPU The other major control bus functions are those of providing a clock interrupts and resets The clock signal keeps all of the chips running together at the same rate The RESET line allows all hardware to be initialised to some predefined state after a reset An interrupt is a signal sent from a peripheral to the 6502 requesting the 6502 to look at that peripheral Two forms of interrupt are provided One of these is the interrupt 355 request IRQ vvhich the 6502 can ignore under softvvare control The other in the non maskable interrupt NMI which can never be ignored Refer to chapter 13 on interrupts for more information When power is first applied to the system a reset is generated to ensure that all devices start up in their reset states The 6502 then starts to get instructions from the MOS ROM These instructions tell the 6502 what it
169. byte previous Y co ordinate low byte previous Y co ordinate high byte current X co ordinate low byte current X co ordinate high byte current Y co ordinate low byte current Y co ordinate high byte ND OF B WN AH 252 10 Vectors One of the features of the BBC microcomputer that greatly enhances its power is the extensive use of VECTORS A vector is a word in memory containing the address of a service routine Many of the more important operating system routines are indirected through vectors The write character routine OSWRCH uses a vector called WRCHV Each time OSWRCH is called it jumps to the routine whose address is contained in WRCHV All of the vectors used for operating system routines are initialised on reset by the operating system Normally each vector contains the address of the relevant routine in the operating system The advantage of using vectors is that standard operating system routines can be intercepted by changing the address contained in the vector The user can replace the address in the vector with the address of his own routine This routine may totally replace the operating system routine or it may perform some function and then pass control to the routine whose address was previously contained in the vector when changing the operating system vector the user would have to save the old contents of the vector and store them in a vector of his own in his routine Some of the internal operating system
170. c or to write data to a disc Many other tasks have to be performed to ensure that this one basic task is carried out properly For example the 8271 will detect read errors refuse to write to a protected disc automatically position the read write head on the disc drive plus much more It is an exceptionally complex chip but luckily there are several very powerful filing system and OSBYTE options available to communicate with the 8271 at a higher level than sending bits to control registers and examining results bit by bit Refer to the Disc System User Guide and chapter 16 on filing systems for more information about using discs This list of 8271 register addresses is included for reference Sheila address Read function Write function amp 80 Status register Command register amp 81 Result register Parameter register amp 82 Reset register amp 83 Not used Not used amp 84 Read data Write data DMA Ack set DMA Ack set 427 25 2 The 68B54 Advanced Data Link Controller Sheila amp A0 Ec BF The 68B54 ADLC is the central component in the Econet Interface circuit In an Econet system up to 255 BBC microcomputers can be connected together The advantage of doing this is that they may all share expensive peripheral devices such as discs and printers This is of immense use in an educational environment where a large number of users can have access to expensive peripherals without purchasing them for each user Refer to the
171. cation from which the subroutine was called Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing Mode bytesused cycles absolute 3 6 Examples Using an OS call LDA ASC X JSR OSWRCH print X on screen 73 LDA Load accumulator from memory A M This instruction is used to set the contents of the accumulator to that contained in a specified byte of memory Processor Status after use C carry flag not affected Z zero flag set if A 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of A set Addressing mode bytesused cycles immediate zero page zero page X absolute absolute X absolute Y indirect X indirect Y 1 if page crossed 2 3 4 4 4 4 1 if page crossed 6 5 NUNWWWNN N 1 if page crossed Example Load accumulator with ASCII value for A LDA ASC A A 65 74 LDX Load X register from memory X M This instruction is used to set the contents of the X register to that contained in a specified byte of memory Processor Status after use C carry flag not affected Z zero flag set if X 0 I interrupt disable not affe
172. ch the clock the countdown timer INKEY the keyboard colour flash and analogue converters will all cease to work It is therefore inadvisable to disable interrupts for more than about 2ms at any time 13 10 User VIA interrupt processing Port B of the user VIA is reserved for user applications and port A is used as a parallel printer interface refer to chapter 22 on VIAs in general and chapter 24 on the user VIA 304 The standard interrupt routine only uses the CA1 interrupt all others are passed over to the user The CA1 interrupt is used to signify that the parallel printer is ready to accept a new character A new character is sent to the printer if the printer output buffer is not empty OSBYTE call amp E7 can be used to mask out the CA1 interrupt and cause it to be passed on to the user in the same way as interrupts are masked out of the system VIA 13 11 Intercepting interrupts If the user intercepts either IRQ1V or IRQ2V by changing the vector value at amp 204 5 or amp 206 7 the following conditions apply on entry to the user s interrupt routine The original processor status byte and return address are already stacked ready for an RTI instruction The original X and Y states are still in their registers The original A register contents are in location amp FC Note that the interrupt routine should not call any operating system routines if at all possible This is because it is possible that the foreground pr
173. chapter on interrupts has been read and thoroughly understood Both Disc and Econet systems rely heavily upon NMIs for their operation so take care Note that NMIs are triggered on negative going edges of NMI signals NIRQ pin 8 NPGFC pin 10 NPGFD pin 12 NRST pin 14 Not Interrupt Request This is connected directly to the 6502 IRQ input Any devices connected to this input should have open collector outputs The line is pulled up to 5 volts with a 3K3 resistor Interrupts from the 1MHz bus must not occur until the software on the main system is able to cope with them All interrupts must therefore be disabled after a reset Note that the main system software may operate very slowly if considerable use is made of interrupts Certain functions such as the real time clock which is incremented every 10 mS will be affected if interrupts are masked for more than this period Refer to the chapter on interrupts section 13 1 for more information Not page amp FC This signal is derived from the 6502 address bus It goes low whenever page amp FC is written to or read from FRED is the name given to this page in memory and it is described in more detail in section 28 2 Not page amp FD This signal is derived from the 6502 address bus It goes low whenever page amp FD is accessed JIM is the name given to this page in memory and it is in section 28 3 Not RESET This is an active low output from the system reset line
174. ckground operating system tasks These tasks include incrementing the clock processing envelopes or transferring keys pressed to the input buffer All of these tasks must continue whilst the user is typing in or running his program The use of interrupts can give the impression that there is more than one processor one for the user one updating the clock one processing envelopes etc As was mentioned in the introduction normal maskable interrupts may be disabled Care should be taken to ensure that interrupts are not disabled for a long time If they are then the operating system will cease to function properly Interrupts should only be disabled for such critical things as 297 changing the two bytes of a vector writing to the system VIA see the system VIA chapter 23 or handling an interrupt or event Interrupts should not be disabled for long because whilst interrupts are disabled the clock stops and all other interrupt activity ceases Interrupts are disabled by the SEI assembler instruction and re enabled with CLI Most devices that generate interrupts will continue to signal an interrupt until it is serviced and so will wait through the period of interrupts being disabled For this reason most short periods of interrupts disabled are safe it is only if a second interrupt occurs from a device before the first is serviced that problems can occur 13 3 Using Non Maskable Interrupts Generally NMIs are reserved for
175. command MOTOR FX137 or OSBYTE 8289 is available to do this 20 10 The BUG fix required when using the cassette system On early versions of the operating system there was a bug which led to erroneous recording of programs on cassettes This has been corrected on later versions of the operating system 1 0 onwards but it is necessary to know how to fix the bug if the cassette hardware is being used directly from a user program The problem occurred because it was possible for the serial ULA to get out of sync for a few bits when the 6850 divide bits were changed This tended to corrupt the first character of the first block in a SAVE or the first character of any block during sequential access since the 6850 is reset for each block during putbytes The cure is to write a dummy byte to tape at the start of a SAVE and the start of every block during putbytes If the leader of a prerecorded program is played back a run in tone followed by a blip the dummy byte followed by more run in tone will be heard It is necessary to have a run in period of high tone after the dummy byte Preferably this should be done by polling the 6850 to check if the TDR is empty since it is difficult to accomplish if the 6850 is continually interrupting The 6850 can then be turned on to interrupt just before starting the block write operation 393 394 21 Paged ROM select register Sheila address amp 30 This 4 bit write only register determin
176. condition No entry parameters This call clears any ESCAPE condition vvithout any further action The Tube is informed if active The ESCAPE flag is stored as the top bit of location amp FF and should never be interfered with directly After call A X and Y are preserved C is undefined 147 OSBYTE amp 7D 125 FX 125 Set Escape condition No entry parameters This call partially simulates the ESCAPE key being pressed The Tube is informed if active An ESCAPE event is not generated After call A X and Y are preserved C is undefined 148 OSBYTE amp 7E 126 FX 126 Acknowledge detection of an ESCAPE condition No entry parameters This call attempts to clear the ESCAPE condition All active buffers will be flushed and any open EXEC files closed On exit X amp FF if the ESCAPE condition cleared X 0 if the ESCAPE condition not cleared After call A is preserved Y and C are undefined 149 OSBYTE amp 7F 127 Check for end of file on an opened file Entry parameters X contains file handle On exit X lt gt 0 If end of file has been reached X 0 If end of file has not been reached See filing system section 16 8 After call A and Y are preserved C is undefined 150 OSBYTE 2 80 128 Read ADC channel ADVAL or get buffer status Entry parameters X determines action and buffer or channel On entry On exit X 0 Y contains channel number range 1 to 4 showing which channel was last used
177. ct the language on errors and soft breaks b Displays the ROM name c Leaves the error message pointer pointing to the copyright message or the version string if present d If a second processor is active copies the language across the Tube to the second processor and executes it there If there is a relocation address it is taken account of during the transfer e Enters the language at its language entry point When entered at the language entry point a language should always set up the BRK vector to its error handler Even the simplest language requires error handling since just writing a character to the screen can cause an error when output is spooled The error handler should output the error message and continue execution within the language ROM at a suitable well defined point such as the keyboard input prompt 327 The BRR vector does not need to be an extended vector as the operating system automatically svvitches to the current language ROM on a BRK When a BRK occurs the currently active paged ROM number is recorded and can be read with OSBYTE amp BA Service paged ROMs normally copy their error messages into RAM so that the language does not have to read the error message from another ROM Languages must also enable interrupts otherwise the operating system will fail to work Languages have the following workspace available amp 0000 amp 008F Page zero amp 0400 amp 07FF Main language workspace OSHWM
178. cted D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of X set Addressing mode bytesused cycles immediate 2 2 Zero page 2 3 zero page X 2 4 absolute 3 4 absolute Y 3 4 1 if page crossed Example Load X register with contents of location amp 80 LDX amp 80 X amp 80 75 LDY Load Y register from memory Y M This instruction is used to set the contents of the Y register to that contained in a specified byte of memory Processor Status after use C carry flag not affected Z zero flag set if Y 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of Y set Addressing mode bytesused cycles immediate 2 2 Zero page 2 3 zero page X 2 4 absolute 3 4 absolute X 3 4 1 if page crossed Example Load Y register with contents of location labelled data with an offset in X LDY data X Y data X 76 LSR Logical Shift Right by one bit M M 2 or A This instruction causes each bit in the memory location or accumulator to shift one bit left Bit 7 is set to 0 and the carry flag will be set to the old contents of bit 0 The arithmetic effect of this is to divide the value by 2 0 gt 76543210 gt C Processor Status after use C carry flag set to bit of operand Z zero flag set if result
179. d and the flash bit as above 19 2 4 Some interesting effects using the palette Because of the necessity to program 4 different palette locations for each colour in a four colour mode some nasty effects can be produced on the screen if all four locations are not programmed with the same colour To illustrate this point try displaying four colours on the screen at once then run this line of BASIC A 155 REPEAT X RND 255 CALL amp FFF4 UNTILO 382 19 3 MODE 8 Implementation example This example program will set up a brand new mode which has been nominated as MODE 8 This is a full 16 colour mode and can be implemented on a Model A as well as a Model B since only 10K of RAM is used There will only be 10 characters across the screen but printing and plotting will operate properly One word of warning however Do not try to redefine the text window when this mode is in use because it will not work All error checking on window bounds will be ineffective in this mode since the operating system does not expect mode 8 to exist 10 RE CREATE MODE 87 20 REM NOTE NO WINDOWING ALLOWED 30 RE 40 REM NOTE POKING VDU VARIABLES 50 RE IS GENERALLY ILL ADVISED 60 MODE 5 REM BASIC MODE 70 RE CONFIGURE VIDEO ULA FOR 10 COLUMN 16 CHARA
180. d C are undefined 137 OSBYTE 8 15 21 FX 21 Flush specific buffer Entry parameters X determines the buffer to be cleared FX 21 0 X 0 Keyboard buffer emptied FX 21 1 X 1 RS423 input buffer emptied PX 21 2 X 2 RS423 output buffer emptied PX 21 3 X 3 Printer buffer emptied PX 21 4 X 4 Sound channel 0 buffer emptied PX 21 5 X 5 Sound channel 1 buffer emptied FX 21 6 X 6 Sound channel 2 buffer emptied FX 21 7 X 7 Sound channel 3 buffer emptied FX 21 8 X 8 Speech buffer emptied See also OSBYTE calls with A amp 0F FX15 and A amp 80 128 After call A and X are preserved Y and C are undefined 138 OSBYTE amp 75 117 Read V DU status No entry parameters On exit the X register contains the VDU status Information is conveyed in the following bits Bit 0 Printer output enabled by a VDU 2 Bit 1 Scrolling disabled Bit 2 Paged scrolling selected Bit 3 Software scrolling selected i e text window Bit 4 not used Bit 5 Printing at the graphics cursor enabled by VDU 5 Bit 6 Set when input and output cursors are separated i e cursor editing mode Bit 7 Set if VDU is disabled by a VDU 21 After call A and Y are preserved C is undefined 139 OSBYTE 2 76 118 Reflect keyboard status in keyboard LEDs This call reflects the keyboard status in the state of the keyboard LEDs and is normally used after the status has been changed by OSBYTE amp CA 202 On exit A is preserved X has bit 7 is s
181. d be provided with the facility for adding optional termination The recommended way of terminating lines is to connect each one to 5V with a 2K2 resistor and to OV with a 2K2 resistor 28 6 5 Timing requirements The 1MHz bus timing requirements are illustrated in the timing diagram figure 28 5 It should be noted that these timings are based on the assumption of only one peripheral being attached to the bus Heavier loading may extend the rise and fall times of IMHZzE with possible adverse effects on timings 447 i i 1 MHzE l I I DATA READ CYCLE I Figure 28 5 1MHz bus timing The timing requirements are Description Address and Read Write Set up time Address and Read Write Hold time NPGFC amp NPGFD Set up time NPGFC amp NPGFD Hold time Write data set up time Write data hold time Read data set up time Read data hold time 448 Symbol tas tah t pgs t pgh t dsw t dhw t dsr t dhr Min 300nS 30nS 250nS 30nS 50nS 200nS 30nS Max 1000nS 1000nS 150nS Appendix A FX OSBYTE call index CODE MOTOR OPT ROM TAPE TV ADC BELL ctrl G BREAR reset brief description perform CODE perform MOTOR perform OPT perform ROM perform TAPE perform TV select channels sampled force ADC conversion read ADC channel read current ADC channel read write max channel number read ADC conversion type read write BELL channel number read write
182. d input stream and returns the character read in the accumulator After an OSRDCH call C 0 indicates that a valid character has been read C 1 flags an error condition A contains an error number If an escape condition occurs then A amp 1B 27 and C 1 if detected an escape condition must be acknowledged using an OSBYTE call with A amp 7E 126 X and Y are preserved N V and Z are undefined The interrupt status is preserved though interrupts may be enabled during a call 102 7 4 Non vectored OSRDCH Call address amp FFC8 This routine is normally used by OSRDCH and the call address is placed in the OSRDCH vector on reset The non vectored OSRDCH is believed to be used by the Tube system This call has not been documented by Acorn and should be used with caution 7 5 OSNEWL Write a newline to selected output stream Call address amp FFE7 Not indirected This routine uses OSWRCH to write a linefeed ASCII amp 0A 10 followed by a carriage return ASCII amp 0D 13 to the currently selected output stream s After an OSNEWL call A amp 0D 13 X and Y are preserved C N V and Z are undefined Interrupt status is preserved though it may be enabled during a call 103 7 6 OSASCI Write character routine where OSNEVVL is called when A amp 0D 13 Call address amp FFE3 Not indirected This routine performs an OSWRCH call with the accumulator contents unless called with accumulator contents of amp
183. d on the rear of all model Bs This is the analogue port connector The pin designations and layout are illustrated in figure 26 1 Connections required for the various joysticks are also illustrated Note the two fire buttons are there as well Their states can be determined using OSBYTE amp 80 Apart from giving the information needed to construct joysticks this diagram should be helpful to anyone wishing to be a bit more adventurous with their hardware One use for the full 10 bits available would be communication with a graphics tablet By using high quality variable resistors and a series of pulleys or lever arms the position of a pointer can be determined This would allow high resolution graphic drawing to be entered into the BBC microcomputer for display on the high resolution screen Some other possibilities include measuring temperature light level pH hydrogen ion concentration current voltage resistance or pressure 432 27 The Tube Sheila addresses amp E0 amp FF 27 1 General introduction to the Tube The BBC microcomputer is provided with three basic methods of expansion The user port and the one megahertz bus are covered in other chapters This chapter covers the third expansion route the Tube The Tube itself is just a fast parallel communication link between two computers First of all the basic fundamentals required of any Tube system are explained followed by some specific examples of its use with dif
184. d value is returned in X The contents of the next location are returned in Y This location contains the number of the ADC channel currently being converted This call should not be used to force ADC conversions use OSBYTE amp 11 FX 17 196 OSBYTF amp BD 189 Read maximum ADC channel number lt NEW VALUE gt lt OLD VALUES AND Y EOR X The old value is returned in X The contents of the next location are returned in Y The maximum channel number to be used for ADC conversions in the range 0 to 4 Set by OSBYTE amp 16 FX 10 197 OSBYTE amp BE 190 Read ADC conversion type 12 or 8 bits lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y Set to amp 00 default 12 bit Set to amp 08 8 bit conversion Set to amp 0C 12 bit conversion Other values have undefined effects 8 bit conversion creates values in the same range 0 to amp FFFF but with less precision and two to three times as fast 198 OSBYTE amp BF 191 FX 191 Read write RS423 use flag lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y bit 7 set RS423 free bit 7 clear RS423 busy bits 0 to 6 undefined 199 OSBYTE amp C0 192 Read RS423 control flag lt NEW VALUE gt lt OLD VALUES AND Y EOR X The old value is returned in X The contents of the
185. d was issued within a graphics mode the co ordinates are converted to internal co ordinates with relative plots and the graphics origin taken into account See the memory usage section number 11 4 for more information on internal co ordinates and VDU variables space layout 10 9 The keyboard control vector REYV amp 228 9 This vector is used whenever the keyboard is accessed It has the following entry conditions C 0 V 0 Test the SHIFT and CTRL keys exit with the N minus flag set if the CTRL key is pressed and the V flag overflow is set if the SHIFT key is pressed C 1 V 0 Scan the keyboard Exactly as OSBYTE call amp 79 On exit the accumulator is equal to the X register on exit C 0 V 1 Rey pressed interrupt entry Each time a key is pressed the system VIA generates an interrupt The operating system uses this interrupt to provide the type ahead facility C 1 V 1 Timer interrupt entry This entry is used for most of the keyboard processing Keyboard auto repeat timing is performed during this call as is the two key rollover processing This vector can usefully be used as an entry point into the operating system as well as replacing the normal routine provided This entry can be used for example to test the control and shift keys 262 10 10 The buffer insert vector INSV amp 22A B The routine indirected through this vector is used by the operating system to enter a character into a buffer On
186. d write BREAK intercept code lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y The contents of these locations must be JMP instruction for BREAKs to be intercepted the operating system identifies the presence of an intercept by testing location amp 287 contents equal to amp 4C JMP This code is entered twice during each break On the first occasion C 0 and is performed before the reset message is printed or the Tube initialised The second call is made with C 1 after the reset message has been printed and the Tube initialised Note memory locations used in OS1 02 Jmp lobyte hibyte amp 287 amp 288 amp 289 241 OSBYTEs amp FA 250 and amp FB 251 FX 250 to 251 Read write locations amp 28A and amp 28B lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y Not used by the operating system Default values 0 242 OSBYTE amp FC 252 FX 252 Read write current language ROM number lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location is set after use of OSBYTE amp 8E FX 126 This ROM is entered following a soft BREAK or a BRK error 243 OSBYTE amp FD 253 Read hard soft BREAR lt NEW VALUE gt lt OLD VALUES AND Y EOR X The old
187. driven by IC76 model B do NOT make this link with IC76 fitted enables JIM disables ROM output from page amp FD disables JIM enables ROM output from page amp FD link 16 must be CLOSED if link 14 is OPEN 15 16 17 18 19 20 21 OPEN CLOSED Note OPEN CLOSED Note OPEN CLOSED Note NORTH SOUTH WEST EAST Note NORTH SOUTH NORTH SOUTH x2 EAST WEST x2 enables fast access to page amp FD disables fast access to page amp FD via IC23 link 15 must be CLOSED when link 17 is OPEN enable fast access to page amp FC via IC23 disable fast access to page amp FC link 16 must be CLOSED if link 14 is OPEN disable FRED enable ROM output from page amp FC enable FRED disable ROM output from page amp FC link 15 must be CLOSED if link 17 is OPEN fast access to IC100 memory chip slow access to IC100 memory chip slow access to memory chips IC52 IC88 and IC101 fast access to memory chips IC52 IC88 and IC101 diodes D10 D11 and D12 can be removed to speed up ROMs IC101 IC88 and IC52 respectively when link 19 is in the WEST position ROMSEL provides HIGH ROM select bit to IC20 A13 provides HIGH ROM select bit to IC20 1C51 blocks 8 08 amp 0B IC52 IC88 IC100 amp IC101 blocks amp 0C amp 0F IC5I blocks amp 0C amp 0F IC52 IC88 IC100 amp IC101 blocks amp 08 amp 0B 485 22 23 24 25 26 27 28 29
188. e available on other CPM machines Some of this software will operate on the BBC microcomputer with a Z80 second processor provided that the disc format is correct 27 2 3 The 16032 Second processor As with the 6502 and Z80 Tubes the old BBC micro 6502 still does all input output and the 16032 runs languages Unlike the 6502 and Z80 which are both relatively old 8 bit processors the 16032 is one of a new generation of 16 bit processors Its internal structure operates on 32 bits and it has a very nicely organised instruction set which is very powerful With one of these sitting on the end of the Tube and a Hard Disc Drive connected to the host processor the computing power available will be equal that available on many mainframe computers One standard operating system provided on 16032 Tube machines will be UNIX 435 436 28 The One Megahertz bus 28 1 Introduction to the 1MHz bus There are basically two routes which a user can take towards adding his own hardware One of these is the 6522 USER port The problem with the USER port is that there are only 8 I O lines and a couple of control lines For more complex peripherals direct access to the 6502 address and data buses are required This interface is provided by the one megahertz bus Physically the one megahertz bus interface is a 34 pin connector mounted at the front edge of the main BBC microcomputer circuit board It is accessed from underneath the keyboard A buffe
189. e machine code arithmetic chapter 4 Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag cleared B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused cycles implied 1 2 Example Turn decimal mode off CLD No more BCD arithmetic 58 CLI Clear interrupt disable flag I 0 This instruction is used to re enable interrupts after they have been disabled by setting the interrupt flag In a machine where the operating system relies heavily on interrupts it is unwise to play around with the interrupt flag without good reason For information about interrupts see chapter 13 Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable cleared D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused_ cycles implied 1 2 Example Re enabled interrupts CLI interrupts responded to now 59 CLV Clear the overflow flag V 0 This instruction forces the overflow flag to be cleared Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag cleared N negat
190. e or unmade connection The function of each link bit is described under OSBYTE amp FF The diagram below illustrates the layout of the links If a user wishes to vary the settings fairly often it is a good idea to buy a standard 8 way SPST switch in a 16 pin DIL package and solder it onto the keyboard ooo 00000 CORNER OF KEYBOARD BIT SWITCH NOT NOT DISC DRIVE DESCRIPTION SED USED SPEED SELECT Figure J Illustration of the keyboard links PIERS ed Lee ee PEPON DE SCREEN MODE I AT POWER UP Se AND AFTER RESET 489 SNOULdO df LUYIS ZMS 4 t O Fi Ad0I 1 Lo z m iga y E 2 Cis Ivan 3901818V2 r ES N Id fs EE 1 a El it Diagram ircui Figure J 2 Complete Keyboard C 490 Bibliography Acorn User Magazine published monthly Addison Wesley 6502 Assembly Language Programming L A Leventhal OSBORNE Mc Graw Hill Berkeley California BBC Microcomputer Application Note No 1 1MHz Bus Kim Spence Jones Acorn Computers Ltd 1982 The BBC Microcomputer User Guide John Coil British Broadcasting Microcomputer London 1982 Beebug Magazine published every five weeks BEEBUG PO Box 109 High Wycombe Bucks HD6845 Cathode Ray Tube Controller Data Sheet Hitachi Disc System User Guide Brian Ward British Broadcasting Corporation London 1982 Econet System User Guide Britis
191. e phases of the system clock The 6845 is responsible for producing the correct format on the display device positioning the cursor performing interlace if it is required and monitoring the light pen input Other video processing functions involving colour and teletext are dealt with in conjunction with other sections in Sheila Inside the 6845 is a very powerful and complex VLSI chip From a user s point of view it is useful to know how to define a specialised screen layout and how the screen layouts modes have been defined by Acorn A generalised overview of the 6845 is therefore given first followed by the values in each of the registers in the various modes This chapter ends with a general summary table which describes the functions of the various registers Appendix F contains diagrams illustrating all of the screen modes in a very concise and easily referred to format 359 18 2 Programming the 6845 General illustration of a CRT Format Total number of horizontal characters Nht 1 Number of displayed horizontal characters Nhd HORIZONTAL RETRACE PERIOD DISPLAY PERIOD VERTICAL RETRACE PERIOD Total scan line adjust Nadj Number of vertical displayed characters Nvd Maximum scan lines Nr 1 Total number of vertical characters Nvt 1 oo Gal Figure 18 1 Illustration of a general screen format The 6845 possesses 18 internal registers 14 of which are write only RO R13 2 of wh
192. e Acorn speech upgrade kit has a selection of words spoken by the newsreader Kenneth Kendall It is also possible to purchase serial ROMs for the speech system which contain games These plug into the slot on the left hand side of the keyboard Again system software is available to read data from these ROMs using OSBYTE calls amp 9E amp 9F and ROM For more information about the speech system refer to the Speech System User Guide 423 424 24 The User Printer VIA Sheila addresses amp 60 amp 6F Full programming details for the 6522 VIA are contained in chapter 22 This brief section is designed to help anyone who specifically wishes to use the USER VIA 24 1 PORT A The printer port All of the port A lines PAO PA7 are buffered before being connected to the printer connector This means that they can only be operated as output lines but they do have a much larger drive capacity than do unbuffered lines CA1 can be used directly as described in the general section on 6522s but note that it is connected to 5 volts via a 4K7 resistor CA1 normally acts as an acknowledge line when a printer is used CA2 is buffered so that it has become an open collector output only It usually acts as the printer STROBE line Note that CA2 can be connected directly to the edge connector using link option 1 see Appendix I 24 2 PORT B The user port All of port B lines ie PBO PB7 and CB1 CB2 are available directly on the user port
193. e address of the first character on each screen row in the teletext mode amp C3E7 amp C3EE contain one byte per display mode giving the number of character rows displayed minus one amp C3EF amp C3F6 contain one byte per display mode giving the number of character columns displayed minus one amp C3F7 amp C3FE contain one byte per display mode giving the value stored in the video ULA control register for that mode amp C3FF amp C406 contain one byte per display mode giving the number of bytes storage taken per character This is 1 for teletext 8 for two colour modes 16 for 4 colour modes and 32 for 16 colour modes amp C407 amp C408 contain the mask table for sixteen colour modes There are two entries one per pixel in the byte Each mask contains the value that would be stored in screen memory if the appropriate pixel were set to colour 15 and the other to zero The left pixel is stored in the first byte of the table and the right in the second 283 amp C409 amp C40C contain the mask table for four colour modes There are four entries one per pixel in the byte Each mask contains the value that would be stored in screen memory if the appropriate pixel were set to colour 3 and all the others to zero The leftmost pixel is stored first byte in the table and the rightmost in the last amp C40D amp C414 contain the mask table for two colour modes There are eight entries one per pi
194. e buttons PB6 and PB7 inputs from the speech processor PB6 is the speech processor ready output and PB7 is from the speech processor interrupt output CB1 input The CB1 input is the end of conversion EOC signal from the 7002 analogue to digital converter It can be used to interrupt the 6502 whenever a conversion is complete See chapter 26 on the Analogue to Digital Converter CB2 input This is the light pen strobe signal LPSTB from the light pen It also connects to the 6845 video processor see section 18 9 CB2 can be programmed to interrupt the processor whenever a light pen strobe occurs See the light pen example in the interrupts chapter 13 418 23 2 The addressable latch This 8 bit addressable latch is operated from port B lines 0 3 inclusive PBO PB2 are set to the required address of the output bit to be set PB3 is set to the value which should be programmed at that bit An example illustrating how to use this latch from BASIC is described in conjunction with the sound generator see section 23 5 The functions of the 8 output bits from this latch are BO Write Enable to the sound generator IC B1 READ select on the speech processor B2 WRITE select on the speech processor B3 Keyboard write enable see Appendix J B4 B5 these two outputs define the number to be added to the start of screen address in hardware to control hardware scrolling Mode Size Start of screen Numberto B
195. e contents of the accumulator to the X register Processor Status after use C carry flag not affected Z zero flag set if X becomes 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of X is set Addressing mode Bytes used Cycles implied 1 2 Example Transfer contents of A to X TAX X A 95 TAY Transfer A to Y Y A This instruction is used to copy the contents of the accumulator to the Y register Processor Status after use C carry flag not affected Z zero flag set if Y becomes 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of Y is set Addressing mode Bytes used Cycles implied 2 Example Transfer contents of A to Y TAY Y A 96 TSX Transfer S to X X S This instruction is used to copy the contents of the stack pointer to the X register Processor Status after use C carry flag not affected Z zero flag set if X becomes 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of X is set Addressing mode Bytes used Cycles implied 1 2 Example Transfer contents of S to X TSX X S 97 TXA Transfer X to A
196. e how the carry flag may be used The overflow flag is set when the sign of the result is incorrect following an arithmetic operation During additions overflow will occur in two situations a When a carry occurs from bit 6 into bit 7 without the generation of an external carry b When an external carry is generated without a carry occurring from bit 6 into bit 7 During subtractions the carry flag is used as a borrow source The overflow flag will be set in the analogous situations where borrows occur rather than carries When the overflow flag is set it indicates that the 2 s complement 8 bit result of an arithmetic operation is incorrect It is often more convenient to think of bytes as always containing positive values The eight bits of the byte can represent a maximum binary value of 255 amp FF This is no problem because the microprocessor performs exactly the same arithmetic operations regardless of the sign of the values involved When the result of any arithmetic operation has bit 7 set then a negative flag is set in the status register The programmer can test this flag if the program must react to negative values The overflow and carry flags will also be set as described above 32 4 2 Binary Coded Decimal A binary coded decimal arithmetic mode may be selected by setting the decimal flag in the status register The binary coded decimal form of representing numbers uses each byte to store a two digit decimal value
197. e of OSBY TE amp E8 The 6850 status register is ANDed with the OSBYTE value Any bit cleared by this is ignored and passed over to the user interrupt vector The user is then responsible for clearing the interrupt condition This is done by either reading the receive data register or writing to the transmit data register of the 6850 see serial hardware chapter 20 13 9 System VIA interrupt processing The system VIA controls and monitors many of the BBC microcomputer s internal hardware devices An interrupt generated by the system VIA may have many different interpretations The reader is referred to the VIAs in general chapter 22 and the system VIA in particular chapter 23 sections in the hardware section When it generates an interrupt the system VIA s status byte indicates the type of interrupt bit0 Set if a key has been pressed bit1 Setif vertical synchronisation has occurred on the video system a 50Hz time signal bit 2 Setif the system VIA shift register times out This should not normally occur since the shift register is not used on the system VIA bit3 Setif a lightpen strobe off the screen has occurred bit4 Set if the analogue converter has finished a conversion bit5 Setif timer 2 has timed out Used for the speech system bit6 Set if timer 1 has timed out This timer provides the 100Hz signal for running the internal clocks bit 7 Setif the system VIA was the source of the interrupt 302 The sta
198. e operating system OSBYTE amp 9C 156 is a powerful command to do this The X and Y registers must be set the old contents of the control register are then AND Y EOR X Y therefore masks bits and X 387 toggles bits Setting Y amp FF and X 0 will generate no change at all The routine returns with the old value of the control register in the X register This register is normally set to amp 56 when using a cassette based system which isn t in the process of transmitting or receiving anything 20 6 1 Counter Divide Select Bits CRO and CR1 These bits determine the divide ratios used in both the transmitter and receiver sections of the ACIA Additionally these bits provide a master reset which initialises the transmitter receiver and status register The clock division rate is normally set to 64 whilst the RS423 system is in operation The cassette system selects between 300 and 1200 baud using this division ratio The serial ULA is always set to 300 baud for cassette so division by 64 actually generates 300 baud Division by 16 makes it 4 times faster so 1200 baud is generated Division by 1 would make it a further 16 times faster ie 19200 baud but the cassette will not operate at this speed CR1 CRO Function 0 0 divide by 1 0 1 divide by 16 1 0 divide by 64 1 1 Master reset 20 6 2 Word Select Bits CR2 CR3 and CR4 Select bits are used to select word length parity and the number of stop bits Any changes become effect
199. e quantity such as temperature humidity or gas concentration into another quantity usually voltage or current which can be measured by a computer Tristate in the BBC microcomputer it is often necessary to connect the outputs of several chips together At any one time only one of the chips should have priority If the other chips were allowed to be in the opposite logic state the power supplies would effectively be shorted through the chips A special tristate level is therefore provided on some chips in which the output doesn t care if it is high or low ULA Uncommitted logic array these are special chips which contain a large number of logic gates The connection between the gates is defined when the chip is manufactured Acorn have produced two special ULAs on containing most of the serial circuitry the other containing some of the video circuitry 497 498 Index IBOOT OSFSC B BASIC CAT OSFSC ROM filing system CO CODE user vector B ENABLE OSFSC EXEC file closing file handle files closed at ESCAPE OSFSC paged ROM service call F FX summary table FX calls H HELP paged ROM service call KEY L LL LINE user vector LOAD M MOTOR O OPT OSFSC ROM filing system R REMOTE RO ROM filing system paged ROM service calls paged ROM softvvare ROM filing system RUN address OSFSC S SAVE SP 218 12 12 343 12 12 12 344 349 13 13 256 14 14 3
200. e screen but it doesn t ever need to do any language processing Programs will generally run at almost twice their original speed If the above process sounds rather complex then don t worry BBC BASIC will appear to operate almost exactly as normal over the Tube The major difference will be the extra memory available Since all of the screen memory resides in the host processor the parasite has all of its memory available for 434 program and data storage Naturally 16K of the parasite s RAM will be required for BASIC but most of the remaining 48K is available to the programmer compared with a maximum of 27K on an ordinary BBC microcomputer The 16K operating system ROM stays in the host processor s memory map and is not transferred across the Tube 27 2 2 The Z80 Second processor Like the 6502 the Z80 is a microprocessor but with a different instruction set It can also operate over the Tube The set up is still very similar to that for a 6502 since the Z80 does all language processing and the BBC microcomputer 6502 does all of the I O processing Machine code programs written to run on the BBC micro will not operate with a Z80 Tube However the vast amount of Z80 software which is available will operate on a Z80 Tube machine The operating system called CP M is supplied as standard with all Z80 second processors There is a large quantity of CP M software business packages most languages games almost everything els
201. ecialised hardware is passed on through the second interrupt vector Within this interrupt routine the devices are serviced in the following order of priority see the hardware section chapters 17 et seq highest first The 6850 serial chip The system 6522 VIA The user 6522 VIA 13 6 Interrupt Request Vector 2 IRQ2V Any interrupts which cannot be dealt with by the operating system are passed on through this lower priority vector This vector is reserved for user supplied interrupt routines The user should intercept the interrupts at this point rather than at IRQ1V whenever possible It should only be necessary to intercept IRQ1V when a very high priority is required by the user routine Note that the user supplied routine must return control to the operating system routine to ensure clean handling of interrupts It is therefore advisable to store the original contents of the indirection vector in memory somewhere This will enable the user routine to jump to the correct operating system routine Also by using this method of jumping to the old contents of the vector several user routines can all intercept it correctly 13 7 Operating system interrupt processing The following sections describe how the operating system deals with interrupts indirected via IRQ1V In very specialised circumstances the programmer may wish to process these interrupts himself in some special way 299 13 8 Serial interrupt processing The 6850
202. ed The connections for such a pen are illustrated in figure 18 4 A special photosensor called a Sweet spot is available from RS Components or most of their distributors and is supplied as part number RS 303 292 368 ANALOGUE PORT CONNECTOR O7 O6 QS 04 03 Ow OQi3 O12 O11 O10 LPSTB UNDERSIDE VIEW OF PHOTOSENSOR 1 5V 2 OUTPUT 3 0V PHOTOSENSOR see text PEN SHELL VIEW INTO ANALOGUE PORT CONNECTOR SHOWING CONNECTIONS FOR A LIGHT PEN Figure 18 4 Light pen circuit 18 9 4 Light pen software In order to take account of the different screen start addresses for the various modes a further correction factor must be subtracted from the contents of the light pen register These correction factors are lt e Qu O Correction factor amp 0606 1542 amp 0606 1542 amp 0606 1542 amp 0806 2054 amp 0B04 2820 amp 0B04 2820 amp 0C04 3076 amp 2808 10248 ND OP WNF The light pen position in terms of x y co ordinates is given by y L p register correction DIV number of characters per line x L p register correction MOD number of characters per line 369 This x value will be in terms of 6845 characters and will have to be modified by multiplying by Number of characters per line on screen Number of characters as seen by 6845 e g for mode 2 20 80 1 4 The resolutions are therefore Modes single displayed character 0 3 4 6 7 Mode
203. ed for the networked printer If the user printer driver recognises a call to itself it should respond to the following codes in the accumulator A 0 The operating system enters the user print routine on interrupt once every 10 milliseconds if it is not dormant The user print routine can poll the printer in response to this call This should eliminate the need for the user printer hardware to generate interrupts 1 The printer is to be activated because at least one character is in the print buffer This entry is only made if the driver had previously marked itself as dormant with OSBYTE amp 7F The driver should take one character from the designated buffer and send it to the printer It should exit with the carry flag clear if the printer is going active When the printer is active it is no longer warned of characters entering the buffer but is expected to use the 10ms A 0 entry to continually poll the printer hardware and the designated buffer until the last character in the buffer has been printed A 2 Warning that a VDU 2 has been received Note that characters can be printed without a VDU 2 occurring if some options of FX 3 are used 259 A 3 Warning that a VDU 3 has been received Printer type change The X register contains the new printer type being selected This call is made every time an OSBYTE 5 is made irrespective of the currently selected printer type or that being selected Printer drivers should declare
204. elected after a soft reset If set this indicates that a second processor relocation address is provided and that the code in this ROM excepting that for the service entry has been assembled from that address This is only relevant if there is a language resident in the ROM service routines are never copied across the Tube This bit controls the Electron soft key expansions and is not relevant on the BBC microcomputer Must be set d The copyright offset pointer is the offset from the beginning of the ROM to the zero byte preceding the copyright message 318 e The binary version number is ignored by the operating system It should be used to indicate the version number to anyone examining the ROM f The title string is printed out on selection of the ROM as a language ROM Apart from this its only use is to identify the ROM to those examining the ROM g The version string is optional and if it exists it must be preceded by a zero byte It is only really of relevance to languages because on entry to a language the error pointer amp FD and amp FE will point to this or if not present the copyright message REPORT in BASIC demonstrates that BASIC has no version string h The copyright string is essential This is because the ROM is recognised by this string and if it does not exist the ROM will be ignored The format must always be a zero byte followed by 4 CY I i The tube relocation address if pre
205. en playing back cassette tapes It produces the data carrier present signal from the cassette whenever a pre recorded program is played back The Serial ULA is operated from a single 8 bit control register The various bits operate as follows 20 9 1 Serial ULA bits 0 2 These define the transmit baud rate so that 000 generates 19200 baud and 111 generates 75 baud Note that this relies upon the 6850 control register being set to divide the incoming clock signal by 64 FX8 is used to select the transmit baud rate on an RS423 input 20 9 2 Serial ULA bits 3 5 These operate in a similar way to bits 0 2 except that they define the receiver baud rate FX7 is used to select the receiving baud rate for the RS423 interface Bit 5 Bit 4 Bit3 Bit 2 Bit 1 Bit 0 Baud rate 0 0 0 0 0 0 19200 1 0 0 1 0 0 9600 0 1 0 0 1 0 4800 1 1 0 1 1 0 2400 0 0 1 0 0 1 1200 1 0 1 1 0 1 300 0 1 1 0 1 1 150 1 1 1 1 1 1 75 392 20 9 3 Serial ULA bit 6 This selects between the cassette or RS423 system If it is set to 0 then the cassette system is selected If it is set to 1 the RS423 is selected Normally the cassette will only be selected when input from or output to cassette is in progress under the cassette filing system OSBYTE amp CD is provided to select between the RS423 and cassette serial systems 20 9 4 Serial ULA bit 7 The cassette motor relay and LED can be turned on by setting this bit to 1 or off by setting it to 0 Note that the
206. en the new routine has finished Using this method allows more than one event handling routine to be used at one time An RTS instruction may be used to exit the routine if no other event handling is to be allowed Event Descriptions 12 5 Output buffer empty 0 This event enters the event handling routine with the buffer number see OSBYTE amp 15 FX21 in X It is generated when a buffer becomes empty i e just after the last character is removed 12 6 Input buffer full 1 This event enters the event handling routine with the buffer number see OSBYTE amp 15 FX 21 in X It is generated when the operating system fails to enter a character into a buffer because it is full Y contains the character value which could not be inserted 289 12 7 Character entering input buffer 2 This event is normally generated by a Rey press and the ASCII value of the key is placed in Y It is generated independently of the input stream selected For example 10 OSWORD FFF1 20 EVNTV 6 amp 220 30 DIM MC 100 40 DIM sound_pars 8 50 FOR I 0 TO 3 STEP3 60 PS MC 70 80 OPT T 90 POP 100 PHA 110 TXA 120 PHA 130 TYA 40 PHA save registers 150 STY sound_pars 4 SOUND pitch key ASCII value 160 LDX sound_pars AND 255 170 LDY tsound pars DIV 256 180 LDA 7 190 JSR OSWORD perform SOUND command 200 PLA 210 TAY 220 PLA 230 TAX 240 PLA 250 PLP restore registers 260 RTS return fro
207. enteen bytes define the start and end locations of the sixteen soft keys The rest of the page is allocated to the keys themselves The start offset of soft key string n is held at location amp B00 n The address of the first character of the string is amp B01 amp B00 n The address of the last character of the string is amp B00 amp BO1 n 11 10 Page twelve amp C00 amp CFF This page contains the font for characters 224 255 Each character requires eight sequential bytes The first byte corresponds to the top line of the character the second for the line below etc 11 11 Page thirteen amp D00 amp DFF This page is used for three functions it contains the NMI processing routine the expanded vector set and the paged ROM private workspace table amp D00 amp D9IE NMI routine amp D9F amp DEF Expanded vector set See vectored entry to paged ROMs section 15 1 3 for details amp DFO amp DFF Paged ROM workspace storage locations There is one byte per ROM containing the upper byte of the address of the first location of its private workspace See paged ROMs chapter section 15 1 1 for more details on private workspace 11 12 The remainder of RAM amp E00 amp 7FFF Location amp E00 is nominally the start of user workspace OSHWM but OSHWM may be raised by font explosion or by paged ROMs taking workspace HIMEM is the location of the start of the screen memory it has the value amp 300
208. entry A character to be inserted X buffer number No range checking is done on this number On exit A X preserved Y is undefined C is set if the insertion failed due to the buffer being full It is the responsibility of the calling routine to attempt retries or abandon the insertion if the insertion failed 10 11 The buffer remove vector REMV amp 22C D The routine indirected through this vector is used by the operating system to remove a character from a buffer or to examine the buffer only On entry X buffer number No range checking is done on this number The overflow flag is set if only an examination is needed If the buffer is only examined the next character to be withdrawn from the buffer is returned but not removed hence no buffer empty event can be caused If the last character is removed a buffer empty event will be caused On exit A is the next character to be removed for the examine option undefined otherwise X is preserved Y is the character removed for the remove option C is set if the buffer was empty on entry 263 10 12The buffer count purge vector CNPV amp 22E F The routine indirected through this vector is used by the operating system to count the entries in a buffer or to purge the contents of a buffer On entry X buffer number No range checking is done on this number The overflow flag is set if the buffer is to be purged The overflow flag is clear if the buffer is to be
209. eo Controller Chip Clock Rate Select Bit 4 The clock frequency sent to the 6845 can be varied using this bit 0 low frequency clock modes 4 7 1 high frequency clock modes 0 3 378 19 1 5 Width of cursor in bytes bits 5 6 These two bits determine the number of bytes of memory required to generate a cursor width Bit 6 Bit 5 Number of bytes per cursor 0 0 1 modes 0 3 4 6 0 1 not defined 1 0 2 modes 15 7 1 1 4 mode 2 19 1 6 Master cursor width bit 7 If set this bit will cause a large cursor to be generated If reset it will cause a small cursor to be generated NOTE setting bits 5 6 and 7 to 0 will cause the cursor to vanish from the screen under ALL conditions 19 1 7 General summary of the video control register Cursor Bytes per Clock Number of chrs Flash size cursor speed perline select Mode Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0 Hex 0 1 0 0 1 1 1 0 x amp 9C 1 1 1 0 1 1 0 0 x amp D8 2 1 1 1 1 0 1 0 x amp F4 3 1 0 0 1 1 1 0 x amp 9C 4 1 0 0 0 1 0 0 x amp 88 5 1 1 0 0 0 1 0 x amp C4 6 1 0 0 0 1 0 0 x amp 88 7 0 1 0 0 1 0 1 1 amp 4B x signifies that the flash bit is changed regularly 19 2 THE PALETTE SHEILA amp 21 WRITE ONLY The Palette is a 64 bit RAM in the video ULA which defines the relationship between the logical and actual colours displayed on the screen If you don t understand the difference between logical and actual colours yet then refer to the COLOUR section
210. ers long The media title is the disc title and is up to tvvelve characters long 16 13 Econet filing system The net filing system is not provided as standard vvith the BBC microcomputer Extra hardware and software are required The networked filing system has the following characteristics File attributes are complete The second and third bytes of the attribute block are used to store the date when the file was created The device identity is not applicable and its length is zero Directory names are one to ten ASCII characters long File names are one to ten ASCII characters long The media title is the disc title and is up to sixteen characters long 351 352 17An Introduction to Hardvvare Most users of the BBC microcomputer will be familiar with BASIC programs but from BASIC the hardware is virtually invisible Commands are provided to deal with output to the screen input from the keyboard and analogue to digital converter plus all of the other hardware The same applies to machine code to a large extent through the use of OSBYTES OSWORDS and other operating system commands However a much more detailed understanding of the hardware and how it can be controlled from machine code programs is very useful and allows certain features to be implemented which would have been impossible in BASIC The hardware section of this book satisfies the requirements of two types of people those who wish to use the hardware feature
211. es and its vertical position in a character slot can be defined as well 18 8 1 The cursor start register R10 This 7 bit write only register controls the cursor format see figure 18 3 Bit 7 is not used Bit 6 enables or disables the blink feature Bit 5 is the blink timing control bit When bit 5 0 blink frequency 1 16th of field rate When bit 5 1 blink frequency 1 32nd of the field rate The cursor start line is set by the lower five bits 366 o o y PF Ww a DO Cursor Start Address 9 Cursor Start Address 9 Cursor Start Address 2 Cursor End Address 9 Cursor End Address 10 Cursor End Address 6 Figure 18 3 Cursor layout examples 18 8 2 The cursor end register R11 This 5 bit write only register sets the cursor end scan line see diagram Mode 0 1 2 3 4 5 6 7 Cursor end 8 8 8 9 8 8 9 19 18 8 3 Cursor position register R14 and R15 This 14 bit read write register stores the current cursor location It consists of 8 low order R15 and six high order R14 bits 18 9 LIGHT PENS 18 9 1 Light pens in general A typical light pen consists of a small light sensitive device fixed to the end of a pen shaped holder The sensor picks up the light given out from the monitor screen and sends an electronic signal into the micro This LPSTB light pen strobe signal can be decoded because the screen is scanned on a raster basis and the position of the pen head determined 367 Light pens can be used
212. es which paged ROM is switched into the memory map eg BASIC FORTH or LISP etc Up to 16 paged ROMS are therefore catered for 4 of which are on the main circuit board The operating system keeps track of which paged ROM is being used at any one time so it will change the value in this register quite often It is not advisable to POKE directly to this register especially from BASIC since it is likely to crash the machine The ROM sockets on the main BBC microcomputer circuit board hold paged ROM numbers 12 13 14 and 15 These correspond to IC52 IC88 IC100 and IC101 respectively ROM number 15 is the highest priority ROM There is an official way to read a byte from any paged ROM This is by CALLing the routine OSRDRM at amp FFB9 with the relevant paged ROM number in the Y register and the address in the paged ROM in locations amp 00F6 and amp 00F7 The value in the byte at this location will be returned in the A register For more details about paged ROMs refer to the paged ROM chapter 15 395 396 22 The 6522 Versatile Interface Adapters Sheila addresses amp 40 amp 7F There are two 6522 VIAs Versatile Interface Adapters inside the BBC Micro One of these is dedicated to the MOS and controls the keyboard sound speech joystick fire buttons etc The other drives the parallel printer port and the user port The devices connected to each VIA are therefore completely different The 6522 by itself will be considered firs
213. et if CTRL is pressed Y is undefined 140 OSBYTE 8 77 119 FX 119 Close any SPOOL or EXEC files This call closes any open files being used as SPOOLed output or EXEC d input to be closed This call also performs a paged ROM call with A amp 10 16 See paged ROM section 15 1 1 On exit A is preserved X Y and C are undefined 141 OSBYTE amp 78 120 FX 120 Write current keys pressed information The operating system operates a two key roll over for keyboard input recognising a second key press even when the first key is still pressed There are two zero page locations which contain the values of the two key presses which may be recognised at any one time If no keys are pressed location amp EC contains 0 and location amp ED contains 0 If one key is pressed location amp EC contains the internal key number 128 see table below for internal key numbers and location amp ED contains 0 If a second key is pressed while the original key held down location amp EC contains the internal key number 128 of the most recent key pressed and location amp ED contains the internal key number 128 of the first key pressed Internal Key Numbers hex dec key hex dec key amp 00 0 SHIFT amp 40 64 CAPS LOCK amp 01 1 CTRL amp 41 65 A amp 02 2 bit7 amp 42 66 X amp 03 3 bit6 amp 43 67 F amp 04 4 bit5 amp 44 68 Y amp 05 5 bit4 amp 45 69 J amp 06 6 bit3 amp 46 70 K amp 07 7 bit2 amp 47 71 amp 08
214. et soft keys Wait for vertical sync Explode soft character RAM allocation 21 15 Flush specific buffer OSBYTE FX calls 22 amp 15 to 116 amp 74 are not used by OS1 20 117 75 Read VDU status 118 76 Reflect keyboard status in LEDs 119 77 Close any SPOOL or EXEC files 120 78 Write current keys pressed information 121 79 Perform keyboard scan 122 7A Perform keyboard scan from 16 amp 10 123 7B Inform OS printer driver going dormant 124 7C Clear ESCAPE condition 125 7D Set ESCAPE condition 126 7E Acknowledge detection of ESCAPE condition 127 7F Check for EOF on an open file CON DOF WNF CO RS ES a EES a ee oo SOmNaATRONFS a os et et dr 111 128 129 130 131 182 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 80 Read ADC channel or get buffer status 81 Read Rey vvith time limit 82 Read machine high order address 83 Read top of OS RAM address OSHWM 84 Read bottom of display RAM address HIMEM 85 Read bottom of display address for a given MODE 86 Read text cursor position POS and VPOS 87 Read character at cursor position 88 Perform CODE 89 Perform MOTOR 8A Insert value into buffer 8B Perform OPT 8C Perform TAPE 8D Perform ROM 8E Enter language ROM 8F Issue paged ROM service request 90 Perform TV 91 Get character from buffer 92 Read from FRED 1 MHz bus 93 Write to FRED 1 MHz bus 94 Read fro
215. ette filing system number 16 10 for details of header block layout 278 amp 39D contains the offset of the next byte to be output into the BPUT buffer amp 39E contains the offset of the next byte to be read from the BGET buffer amp 39F amp 3A6 are not used in OS 1 2 amp 3A7 amp 3B1 contain the filename of the file being BGETed amp 3B2 amp 3D0 contains the block header of the most recent block read amp 3B2 amp 3BD Filename terminated by zero amp 3BE amp 3C1 Load address of the file amp 3C2 amp 3C5 Execution address of the file amp 3C6 amp 3C7 Block number of the block amp 3C8 amp 3C9 Length of the block amp 3CA Block flag byte amp 3CB amp 3CE Four spare bytes amp 3CF amp 3D0 Checksum bytes amp 3D1 contains the sequential block gap as set by OPT 3 amp 3D2 amp 3DC contain the filename of the file being searched for Terminated by zero amp 3DD amp 3DE contain the number of the next block expected for BGET amp 3DF contains a copy of the block flags of the last block read This is used to control newlines whilst printing file information during file searches amp 3E0 amp 3FF are used as the keyboard input buffer It should be noted that although OSBY TE amp A0 is officially for reading VDU variables it may be used to read any of the values in page three 11 5 Pages four through seven amp 400 amp 7FF This memory is the main workspace for the currently active language such
216. ex for the next character to be read Carry flag is set if the end of string is reached X is preserved This routine has not been documented by Acorn but has been used in applications software 7 10 OSRDRM Read byte in paged ROM Call address amp FFB9 On entry Y ROM number Locations amp F6 and amp F7 should contain the address of the byte to be read On exit A contains the value of the byte read This routine has not been documented by Acorn but has been used in applications software 106 7 11 OSEVEN Generate an event Call address amp FFBF This call generates or causes an event The event number should be placed in the Y register when this routine is called The accumulator contents are transferred to the Y register and the event number is placed in the accumulator when the event handling routine is entered See Events chapter 12 for more information about events This routine has not been documented by Acorn and should be used with caution The Command Line Interpreter For details of which commands are recognised by the command line interpreter see chapter 2 Operating System Commands 7 12 OSCLI Passes line of text to the CLI Call address amp FFF7 Indirected through amp 208 This routine passes a line of text to the command line interpreter which decodes and executes any command recognised On entry X and Y should point to a line of text X low byte Y high byte The line of text should be
217. f the IFR Note however that IFR bit 7 is not a flag as such and will not be cleared by writing a 1 into it It can only be cleared by clearing all the flags in the register or by disabling ALL of the active interrupts The 6502 can set or clear selected bits in the interrupt enable register vvithout affecting the other bits This is accomplished by writing to the IER If bit 7 of the byte written is a 0 then each 1 in bits 0 6 will clear the corresponding bit in the IER For each zero in bits 0 6 the corresponding bit will not be affected Selected bits can be SET in a similar manner In this case bit 7 of the written byte should be set to 1 Each 1 in bits 0 6 will then SET the selected bit A zero will cause the corresponding bit to remain unaffected The contents of the IER can be read by the 6502 Bit 7 is then always read as a logic 1 REG 13 INTERRUPT FLAG REGISTER SET BY CLEARED BY CA2 ACTIVE EDGE READ OR WRITE cazaerive roor REGS onal REG 1 ORA SHIFT REG CB2 ACTIVE EDGE READ OR WRITE ORB CBt ACTIVE EDGE READ OR WRITE ORB TIME OUT OF T2 READ T2 LOW OR WRITE T2 HIGH TIME OUT OF T1 READ T1 LOW OR WRITE T1 HIGH ANY ENABLED CLEAR ALL INTERRUPT INTERRUPTS t IF THE CA2 CB2 CONTROL IN THE PCR tS SELECTED AS INDEPENDENT INTERRUPT INPUT THEN READING OR WRITING THE OUTPUT REGISTER ORA ORB WILL NOT CLEAR THE FLAG BIT INSTEAD THE BIT MUST BE CLEARED BY WRITING INTO THE IFR AS DESCRIBED PREVIOUSLY
218. f the cassette filing system is called whilst doing a BGET for EXEC or a BPUT for SPOOL It is used to ensure that no messages are printed during the access amp EC contains the internal key number of the most recently pressed key or zero if none is currently pressed See the table of internal key numbers in Appendix D 270 amp ED contains the internal key number of the first key pressed of those still pressed or zero if one or no keys are pressed This is used to implement two key rollover amp EE contains the internal key number of the character to be ignored when scanning the keyboard with OSBYTE amp 79 Note that Acorn have allocated this location for a RAM copy of the 1MHz bus paging register see section 28 2 When using the 1MHz memory OSBYTE calls amp 79 and amp 7A should not be used as unpredictable results can occur amp EF contains the accumulator value for the most recent OSBYTE OSWORD amp FO contains the X register value for the most recent OSBYTE OSWORD or the stack pointer value at the last BRK instruction amp F1 contains the Y register value for the most recent OSBYTE OSWORD amp F2 and amp F3 are used as a text pointer for processing operating system commands and filenames amp F4 contains a RAM copy of the number of the currently selected paged ROM This location should always reflect the contents of the paged ROM selection latch at location amp FE30 amp F5 contains the current logical s
219. fected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused_ cycles relative 2 2 1 if branch succeeds 2 if to new page Example Branching if overflow occurs during subtraction SEC set the carry flag LDA 8 load A with the value 8 SBC 686 A A M carry if required BVS help if overflow has occurred goto help STA 686 otherwise put new value in 8486 N B A BCC instruction would have performed the same purpose in this instance 56 CLC Clear carry flag C 0 This instruction clears the carry flag This is often a sensible operation to perform before using an ADC instruction if there is any doubt as to the status of the carry flag Processor Status after use C carry flag cleared Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused cycles implied 1 2 Example Clearing the carry flag before an 8 bit addition CLC clear carry flag ADC increment add second low order to it LDA counter load first low order byte STA counter place new value in counter 57 CLD Clear decimal flag D 0 This flag is used to place the 6502 into decimal mode This instruction returns the processor into non decimal mode Se
220. fer has not been completed the number of bytes or names which have not been transferred are written to the parameter block in the number of bytes to transfer field The address field is always adjusted to point to the next byte to be transferred and the sequential pointer always points to the next entry in the file to be transferred On exit X Y and the accumulator are preserved N V and Z are undefined C is set if the transfer could not be completed Interrupt state is preserved but may be enabled during operation 16 7 OSFIND Open or close a file for byte access Call address amp FFCE Indirected through amp 21C OSFIND is used to open and close files Opening a file declares a file requiring byte access to the filing system Closing a file declares that byte access is complete To use OSARGS OSBGET OSBPUT or OSGBPB with a file it must first be opened On entry The accumulator specifies the operation to be performed If A is zero a file is to be closed Y contains the handle for the file to be closed If Y 0 all open files are to be closed If A is non zero a file is to be opened X and Y point to the file name X lovv byte Y high byte The file name is terminated by carriage return amp 0D 342 The accumulator can take the following values amp 40 a file is to be opened for input only amp 80 a file is to be opened for output only amp C0 a file is to be opened for update random
221. ferent processors The Tube connector on the BBC microcomputer simply consists of the system data bus the lower half of the system address bus and some miscellaneous control lines These connect via a ribbon cable to a second processor card The second processor is connected to the Tube interface by a Tube ULA uncommitted logic array chip which has been designed specifically for this application by Acorn The Tube ULA provides a completely asynchronous parallel interface between the two processor systems The original BBC microcomputer is the HOST system and the new second processor forms the heart of the PARASITE system To each system the Tube resembles a conventional peripheral device which occupies 8 bytes of memory or I O space Four byte wide read only and four byte wide write only latches are provided within this address space together with their associated control registers The byte wide communication paths fall into two distinct categories The first set are simply latches so data written in on one side is read out directly on the other side The second set are FIFO first in first out buffers which store two or more bytes at a time These bytes can then be read out on the other side of the Tube in the same order that they were entered into the buffer 433 Data and messages are passed bach and forth through the various registers according to carefully designed software protocols Proper allocation of the registe
222. filing system options byte as set by the OPT command The byte is organised as two nibbles the top four bits are used for load and save operations and the bottom four bits are used for sequential access The format of each nibble is Bits 0 and 1 the least significant bits of the nibble are used to control what happens after a tape error When accessing the 269 EXEC file the retry and ignore error options are ignored so the EXEC is always aborted These bits have the following meanings note the higher bit is mentioned first 00 Ignore errors 10 Retry after an error 01 Abort after an error Bits 2 and 3 the most significant bits of the nibble are used to control the printing of messages during access These bits have the following meanings note the format given is high bit low bit 00 No messages 10 Short messages 11 Long messages amp E4 amp E6 are used as general operating system workspace amp E7 is the auto repeat countdown timer This is decremented at 100Hz to zero at which point the key is re entered into the buffer amp E8 and amp E9 are a pointer to the input buffer into which data is entered by OSWORD amp 01 amp EA is the RS423 timeout counter which can take the following values 1 The cassette filing system is using 6850 0 The RS423 system holds 6850 but has timed out lt 0 The RS423 system holds 6850 but has not yet timed out amp EB is the cassette critical flag Bit 7 is set i
223. flag not affected N negative flag not affected Addressing mode ytes used cycles zero page 2 3 zero page X 2 4 absolute 3 4 absolute X 3 5 absolute Y 3 5 indirect X 2 6 indirect Y 2 6 Example Store accumulator in location save Y offset STA save y save Y A 92 STX Store X contents in memory M X This instruction is used to copy the contents of the X register into a memory location Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused cycles Zero page 2 3 zero page Y 2 4 absolute 3 4 Example Store X in location amp 80 STX 680 amp 80 X 93 STY Store Y contents in memory M Y This instruction is used to copy the contents of the Y register into a memory location Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode ytes used cycles zero page 2 3 zero page X 2 4 absolute 3 4 Example Store Y in location amp 5FFO STY amp 5FFO amp 5FF0 Y 94 TAX Transfer A to X X A This instruction is used to copy th
224. for ADC conversion Note that OSBYTE calls with A amp 10 16 and A amp 11 17 set this value to 0 A value of 0 indicates that no conversion has been completed Bits 0 and 1 of X indicate the status of the two fire buttons X 1to4 Xand Y contain the 16 bit value X low Y high read from channel specified by X X lt 0 If X contains a negative value in 2 s complement amp FF notation then this call will return information about various buffers X 255 keyboard buffer amp FF X 254 RS423 input buffer amp FE X 253 RS423 output buffer amp FD X 252 printer buffer amp FC X 251 sound channel 0 amp FB X 250 sound channel 1 amp FA X 249 sound channel 2 amp F9 X 248 sound channel 3 amp F8 X 249 speech buffer amp F7 151 For input buffers X contains the number of characters in the buffer and for output buffers the number of spaces remaining After call A is preserved C is undefined 152 OSBYTE 2 81 129 Read key with time limit INREY Entry parameters X and Y specify time limit in centiseconds If a time limit of n centiseconds is required X n AND amp FF LSB Y n DIV amp 100 MSB Maximum time limit is amp 7FFF centiseconds 5 5 minutes approx On exit If a character is detected X ASCII value of key pressed Y 0 and C 0 If a character is not detected within timeout then Y amp FF and C 1 If Escape is pressed then Y amp 1B 27 and C 1 If called with Y
225. for a multitude of tasks such as drawing painting designing layouts playing games etc but their use in many applications is limited by the resolution The reason for this is that a fairly large area of screen ie perhaps 5cm x 5cm is usually required to provide sufficient light to operate the pen The maximum resolution for defining the position of the light pen is therefore a patch on the screen of this size so accurate line drawings are impossible The position of the light pen is stored to the nearest character position so this limits the resolution to a character cell 18 9 2 Light pen position register R16 and R17 This 14 bit read only register is used to store the location of a light pen sensor placed in front of the screen The register is modified whenever the LPSTB signal is pulsed high 18 9 3 Constructing a light pen The light pen hardware must produce a positive going TTL pulse whenever the display scan position is under the sensor The light pen position will then be stored in the light pen register R16 R17 Note that slow light pen response will require a delay factor to be subtracted from R16 R17 to produce the correct light pen position Luckily there are small light sensitive devices available which provide a direct TTL logic level output If one of these is fixed to the end of an empty pen and connected to the light pen input on the rear of the BBC microcomputer an operational light pen can be construct
226. ght Row multiplication table zero page locations RS423 buffer storage controlling the cassette Port DCD empty input buffer empty output buffer error detected event example program handshaking extent input buffer status input select input suppression flag insert character in buffer interrupt processing output buffer status printer output 496 245 359 338 339 106 102 106 337 335 249 248 117 496 496 40 393 263 21 28 134 84 497 142 497 243 195 194 165 349 271 349 271 324 192 182 181 85 269 280 311 313 138 138 292 311 208 151 186 209 172 301 151 121 507 printers read control flag read vvrite mode read write use flag receive baud rate RTS selection for input selection for output terminals transmit baud rate RS423 timeout counter zero page location RS423 cassette selection flag RTI Return from interrupt RTS RTS Return from subroutine S Save file OSFILE SBC Subtract memory from A Screen selection for output vertical position wrap around 309 200 190 199 123 312 118 119 310 124 270 210 393 86 312 87 335 88 119 20 373 Screen display see 6845 and video ULA Screen format Screen mode storage in page 3 Screen mode at power up Screen modes layouts Screen positioning Scrolling disabling example hardware paged scroll selected soft scrolling selected SEC Set Carry flag Second processor 16032 6502 Z80 SED Se
227. h start adr execution adr All numeric values are printed in hexadecimal See also CAT 18 2 15 ROM RO OSBYTE with A amp 8D 141 The ROM filing system is initialised using this command The ROM filing system is able to use paged ROMs or serially accessed ROMs associated with the speech processor These ROMs must contain data in a block format similar to that used in the cassette filing system With the ROM filing system initialised all other filing systems are disabled For further details see section 16 11 in the filing systems chapter 2 16 RUN lt file name gt R This command causes a file to be loaded into memory at its start address and then the microprocessor jumps using JSR to the execution address This is a method of loading and running machine code programs Any text following the file name is available to pass parameters to the program Parameter passing is not implemented for the cassette or ROM filing systems see filing systems chapter 16 2 17 SAVE lt file name gt lt start addr gt lt end addr gt lt exec addr gt lt reload addr gt S The contents of memory may be saved to a file on the currently selected filing system using SAVE Only the start address and the end address are mandatory If omitted the execution address will default to the start address The reload address allows the start address stored with the file to be different to the actual start address used when saving The end
228. h Broadcasting Corporation London 1982 8271 Floppy Disc Controller Data Sheet Intel Microprocessor amp Memory Data Book Thomson EFCIS 1981 NEC 1982 Catalogue uPD7002 NEC Electronics Europe GmbH 1982 Programming the 6502 Rodnay Zaks Sybex 1980 Service Manual for the BBC Microcomputer Acorn Computers Ltd Cambridge 1982 SN76489N Sound Generator Data Sheet Texas Instruments Inc 1978 Speech System User Guide Acorn Computers Ltd Cambridge 1983 491 R6522 Versatile Interface Adapter Data Sheet Rockwell International 1981 TTL Data Book Texas Instruments Inc 1980 TMS 6100 Voice Synthesis Memory Data Manual Texas Instruments Inc 1980 TMS 5220 Voice Synthesis Processor Data Manual Texas Instruments Inc 1981 492 Glossary Address Bus a set of 16 connections each one of which can be set to logic 0 or logic 1 This allows the CPU to address amp FFFF 65536 different memory locations Active low signals which are active low are said to be valid when they are at logic level 0 Analogue to digital converter ADC this is a chip which can accept an analogue voltage at one of its inputs and provide a digital output of that voltage The ADC in the BBC microcomputer is a 7002 Asynchronous two devices which are operating independently of one another are said to be operating asynchronously Baud Rate used to define the speed at which a serial data link transfers data One b
229. he display blanking signal This signal must be enabled for all of the character output period and is used to take account of the time to transfer data from memory to the video output circuitry No delay is required in modes 0 6 but a one character delay is required in mode 7 because the SAA5050 character generator is used Display blanking delay Bit5 Bit4 Description 0 0 No delay 0 1 One character delay 1 0 Two character delay 1 1 Disable video output 365 18 6 3 Cursor blanking delay bits 6 7 Bits 6 and 7 control the cursor blanking signal This signal must be enabled at the exact time when a cursor should appear on the screen No delay is required in modes 0 6 but a two character delay is required in mode 7 Cursor enable signal Bit 7 Bit 6 Description 0 0 No delay 0 1 One character delay 1 0 Two character delay 1 1 Disable cursor output 18 7 Scan lines per character R9 This 5 bit write only register determines the number of scan lines per character row including spacing The programmed value is one less than the total number of output scan lines Mode 0 1 2 3 4 5 6 7 Scans per 7 7 7 9 7 7 9 18 character 18 8 THE CURSOR It is possible to program a cursor to appear at any character position defined by R14 and R15 Its blink rate can be set to 16 or 32 times the field period of 20 ins Optional non blink and non display i e no cursor on the screen modes can also be selected Its height in number of lin
230. he macro e g 10 DIM MC 100 20 FOR opt 0 TO 3 STEP 3 30 PS MC 40 50 OPT opts 60 add CLC clear carry 70 LDA amp 80 A amp 80 80 ADC amp 81 AA amp 81l carry 90 STA amp 81 amp 81 A 100 OPT FNdebug TRUE 110 120 NEXT 130 amp 80 1 29 140 26812 150 CALL add 160 PRINT Result of addition amp 81 170 PRINT A amp amp 70 X amp amp 71 Y amp 7 E amp T72 80 END 190 DEF FNdebug switch 200 IF switch OPT opt STA amp 70 STX amp 71 STY amp 72 save registers 210 OPT opt RTS 220 opts This highly contrived program adds two bytes together It uses a macro within which conditional assembly occurs Hanging a function on the end of an OPT command enables the programmer to call the macro in a tidy manner If FNdebug is called with the value TRUE then some code which saves the registers in zero page is inserted into the program otherwise an RTS instruction is inserted The function returns with the value to which OPT was set in the first place This example indicates how the close inter relation of the mnemonic assembler with BASIC results in a very powerful assembler The programmer should always remember that BASIC is always available as an aid when using the BASIC assembler 3 10 User Zero Page 32 bytes of zero page locations are reserved by BASIC for the users machine code programs These locations are from amp 70 to
231. he operating system Read character attempted This call to the net vector is only made when enabled by OSBYTE amp CF The net system must provide a character for processing on exit in the accumulator OSBYTE attempted This call to the net vector is only made if enabled with OSBYTE amp CE The entry parameters of the OSBYTE call are held in amp EF amp FO and amp F1 for A X and Y registers respectively If on exit the overflow flag is set the user is prevented from making the call OSWORD attempted Exactly as call 7 OSBYTE only an OSWORD call was attempted A line has been entered with OSWORD 1 and is now complete This is a warning to the net system that it can now take over the read character input without too much mess 10 8 The VDU extension vector VDUV amp 226 7 This vector is used whenever an unrecognised VDU command occurs This happens in three situations 1 VDU 23 n has been issued with n in the range 2 31 The vector is entered with the carry flag set The accumulator contains n Locations amp 31C amp 323 contain the eight parameters always sent with the VDU 23 command 2 A plot command has been issued in a non graphics mode 3 An unrecognised PLOT number has been used 261 In cases 2 and 3 the vector is entered vvith the carry flag clear The accumulator contains the PLOT number Locations amp 320 to amp 323 contain the X and Y co ordinates sent via the PLOT command If the comman
232. her on one vector each taking over one or two functions of that vector Wherever possible this strategy should be used as it maximises the possible flexibility of the system The main operating system vectors reside in page two of memory There is also an extended vector space in page amp D for use by paged ROMs see the section on vector entry to paged ROMs number 15 1 3 for details of the use of extended vectors The operating system vectors are amp 200 1 USERV The user vector This is used to take out certain unrecognised operating system calls including instructions CODE and LINE amp 202 3 BRKV The break vector This is used to trap errors within the system This vector is normally set by the current language 254 amp 204 5 amp 206 7 amp 208 9 amp 20A B amp 20C D amp 20E F amp 210 1 amp 212 3 amp 214 5 amp 216 7 amp 218 9 amp 21A B amp 21C D IRQIV The primary interrupt vector All interrupts are directed through this vector IRQ2V The secondary interrupt vector All interrupts unrecognised by the standard interrupt processing routine or which have been masked out are passed through this vector CLIV The command line interpreter vector All operating system commands commands are passed through this vector BYTEV The OSBYTE indirection vector All OSBYTE calls are passed through this vector VVORDV The OSWORD indirection vector All OSWORD calls are
233. histicated serial system which can be used for a variety of purposes These include controlling external serial printers other devices with an RS423 or RS232 interface and running the BBC microcomputer from or as a serial terminal The serial interface has two channels one for output and one for input Using OSBYTE calls 2 and 3 the input channel can be used to replace the keyboard input and the output channel can be connected to the computer s output stream The R5423 system can also be used to control the cassette port 14 1 OSBYTE calls relating to the serial system The following OSBYTE calls all relate to the serial system amp 02 2 Select input channel Keyboard RS423 amp 03 3 Select output channels including RS423 amp 05 5 Select printer type amp 07 7 Select receive baud rate amp 08 8 Select transmit baud rate amp 9C 156 Direct 6850 control amp B5 181 RS423 mode amp BF 191 RS423 use flag amp CO 192 RS423 control do not use amp CB 203 RS423 handshake control amp CC 204 RS423 input ignore amp CD 205 RS423 cassette select amp E8 232 6850 interrupt mask The buffer management OSBYTEs and event control OSBYTEs 13 and 14 are also relevant because an event is generated if an RS423 error occurs number 7 309 14 2 Uses of the RS423 system 14 2 1 RS423 printers RS423 printers can easily be interfaced to the BBC microcomputer using the RS423 system Simply select the RS423 pri
234. hnique is good for text but may produce jumpy movements in graphics due to the limited resolution in the screen position On a mode 0 screen each sideways scroll moves the screen by 8 pixels On a mode 2 screen each 6845 character only represents 2 graphics pixels so a fairly effective hardware scroll can be used 18 11 3 Mode 7 scrolling Hardware scrolling in mode 7 is slightly more complex than in modes 0 6 To calculate the value to put into registers 12 and 13 first of all calculate the required start address in RAM e g amp 7C28 Take the high byte and subtract amp 74 then EOR the result with amp 20 This new value should be put into R12 R13 contains the low order address byte A similar correction factor should be applied when working out the cursor register contents 372 18 12 FAST ANIMATION 18 12 1 Fast animation using mode 2 Mode 2 has several advantages over all of the other modes for fast animation It is for this reason plus the fact that all 16 colours are available that this mode is used in most fast graphics games Provided that the programmer is prepared to put up with a 2 pixel at a time movement instead of a 1 pixel at a time movement moving objects simplifies to moving complete bytes in memory Consider for a moment the layout of each byte on a mode 2 screen P2d Pld P2c Ple P2b Plb Pda Pia BIT 7 6 5 4 3 2 i 0 Pla P1d are 4 bits defining the colour of pixel 1 P2a P2d are 4 bits defining the colour of pi
235. ich are read and write R14 R15 and 2 of which are read only R16 R17 In order to gain access to any of these registers the register address must be written into the 6845 address register This is situated at Sheila address amp 00 Having written a 5 bit number into the address register the selected internal register may be written to or read from at Sheila address amp 01 The best way of programming the 6845 is by using the VDU23 command For example VDU23 0 R V 0 0 0 0 0 0 will put the value V into register R In BASIC programs it can be shortened by using semicolons instead of commas A semicolon causes a 2 byte word to be included in the VDU command For example VDU23 R V 0 0 0 has the same effect as the first example 360 18 3 THE HORIZONTAL TIMING REGISTERS The horizontal timing registers define all of the horizontal timing for the screen layout The point of reference for these registers is the left most displayed character position The registers are programmed in character time units relative to the reference point 18 3 1 Horizontal total register RO This 8 bit write only register determines the horizontal sync frequency It should be programmed with the total number of displayed plus non displayed character time units across the screen minus one Nht on figure 18 1 Note that the number of displayed characters is not necessarily the same as the number of characters per line This is because of the variable n
236. ilename terminated by RETURN amp 0D Load address of the file Low byte first Execution address of the file Low byte first Start address of data for save length of file otherwise Low byte first End address of data for save file attributes otherwise Low byte first 335 The value in the accumulator has the following interpretations A 0 Save a block of memory as a file using the information provided in the parameter block A 1 Write the information in the parameter block to the catalogue entry for an existing file i e file name and addresses A 2 Write the load address only for an existing file A 3 Write the execution address only for an existing file A 4 Write the attributes only for an existing file A 5 Read a file s catalogue information with the file type returned in the accumulator The information is written to the parameter block A 6 Delete the named file A amp FF Load the named file the address to which the file is loaded being determined by the lowest byte of the execution address in the control block XY 6 If this byte is zero the address given in the control block is used otherwise the file s own load address is used File attributes are stored in four bytes the most significant three bytes are filing system specific The least significant byte is defined as Bit 1 Means By 0 Not Readable You 1 Not Writable You 2 Not Executable You 3 Not Deletable You 4 Not Read
237. implementing a particular program idea or from utilising some machine facility At this stage the programmer must often seek recourse to the microprocessor s native language its machine code At the heart of any microcomputer is the microprocessor This microprocessor is the brain of the computer and provides the computer with all its computing power The BBC Microcomputer uses a 6502 microprocessor and the brief description of machine code given here applies specifically to the 6502 The microprocessor performs instructions which are contained in memory Each instruction which the microprocessor understands can be contained within a single byte of memory Depending on the nature of this instruction the microprocessor may fetch a number of bytes of data from the memory locations following the instruction byte Having executed this instruction the microprocessor moves on to the byte after the last data byte to get its next instruction In this way the microprocessor works its way sequentially through a program These single byte instructions are called operation codes or just opcodes although they are frequently referred to as machine instructions or just instructions The data used by the instructions are called the operands A program using the native machine instructions is called a machine code program An assembler is a software package a language or a program which enables a programmer to create a machine code program Machine code i
238. ine Name OSCLI OSBYTE OSWORD OSWRCH OSNEWL OSASCI OSRDCH OSFILE OSARGS OSBGET OSBPUT OSGBPB OSFIND NVRDCH NVWRCH GSREAD GSINIT OSEVEN OSRDRM Vector Address Name FFF FFF4 FFF1 FFEE FFE7 FFE3 FFEO FFDD FFDA FFD7 FFD4 FFD1 FFCE FFCB FFC8 FFC5 FFC2 FFBF FFB9 USERV BRRV IRQ1V IRQ2V CLIV BYTEV WORDV WRCHV RDCHV FILEV ARGSV BGETV BPUTV GBPBV FINDV FSCV EVNTV UPTV NETV VDUV KEYV INSV REMV CNPV IND1V IND2V IND3V Address 200 202 204 206 208 20A 20C 20E Summary of function The user vector The BRK vector Primary interrupt vector Unrecognised IRQ vector Command line interpreter FX OSBYTE call OSWORD call Write character Write LF CR to screen Write character amp 0D LF Read character Load save file Load save file data Get byte from file Put byte in file Multiple BPUT BGET Open or close file File system control entry Event vector User print routine Econet vector Unrecognised VDU commands Keyboard vector Insert into buffer vector Remove from buffer vector Count purge buffer vector Spare vector Spare vector Spare vector Non vectored read char Non vectored write char Read char from string String input initialise Generate an event Read byte in paged ROM 455 Appendix C Rey Values Summary Rey ASCII INREY Internal Rey No character dec hex dec hex dec hex SPACE 32 20 99 9D 98 62 33 21 34 22
239. ined 157 OSBYTE amp 86 134 Read text cursor position POS and VPOS No entry parameters On exit X contains horizontal position of the cursor POS Y contains vertical position of the cursor VPOS After call A is preserved C is undefined 158 OSBYTE amp 87 135 Read character at text cursor position No entry parameters On exit X contains character value 0 if char not recognised Y contains graphics MODE number After call A is preserved C is undefined 159 OSBYTE 2 88 136 FX 136 Execute code indirected via USERV CODE equivalent This call JSRs to the address contained in the user vector USERV amp 200 The X and Y registers are passed on to the user routine See CODE section 2 6 160 OSBYTE amp 89 137 FX 137 Switch cassette relay MOTOR equivalent Entry parameters x 0 relay off X 1 relay on The cassette motor LED will reflect the relay state The cassette filing system calls this routine with Y 0 for write operations and Y 1 for read operations After call A is preserved X Y and C are undefined 161 OSBYTE amp 8A 138 FX 138 Insert value into buffer Entry parameters X identifies the buffer See OSBYTE call with A amp 15 FX21 for buffer numbers Y contains the to be value inserted into buffer After call A is preserved C is undefined 162 OSBYTE amp 8B 139 FX 139 Select file options OPT equivalent Entry parameters X contai
240. is demonstration program uses the interval timer event to call the event handling routine at one second intervals The 5 byte clock value is incremented every centisecond and so the first task the routine must perform is to write a new value 100 to the timer to prepare for the next call Each time the routine is entered a count is incremented and the ASCII character printed up on the top right corner of the screen In this simple example repeating a count from amp 30 to amp 39 ASCII 0 to 9 makes the programming easy It does not require a lot of imagination to see how this concept could be expanded to provide a constant digital time read out 12 11 ESCAPE condition detected 6 When the ESCAPE key is pressed or an ESCAPE is received from the R5423 when RS423 ESCAPEs are enabled this event is generated When this event is enabled the ESCAPE state indicated by the flag at amp FF is not set when an ESCAPE condition occurs Therefore after a FX136 the ESCAPE key has no effect in BASIC 12 12 RS423 error 7 This event is generated when an RS423 error is detected see RS423 chapter 14 This event is entered with the 6850 status byte shifted right by one bit in the X register and the character received in Y 292 12 13 Network error 8 This event is generated when a network event is detected If the net expansion is not present then this could be used for user events 12 14 User event 9 This event number has been set a
241. ive flag not affected Addressing mode bytesused_ cycles implied 1 2 Example Explicitly clear the overflow flag CLV overflow now clear 60 CMP Compare memory and accumulator A M This is a very useful instruction for comparing the accumulator contents to the contents of a memory location The status register flags are set according to the result of a subtraction of the memory contents from the accumulator The accumulator contents are preserved but the status register flags may be used to cause branches depending on the values which were compared Processor Status after use C carry flag set if A greater than or equal to M Z zero flag set if AM I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of the result is set Addressing mode bytes used cycles immediate zero page zero page X absolute absolute X absolute Y indirect X indirect Y 1 if page crossed 2 3 4 4 4 4 1 if page crossed 6 5 NUNWWWNN N 1 if page crossed Examples Branching on the result of a comparison The test which if true is to cause the branch Code A gt M or M lt A BEQ over or BEQ P 4 no label BCS somewhere 61 A gt A lt A lt M 62 M M or OF or or BCC BEQ BCC som som som som som ewh e
242. ive immediately CR4 CR3 CR2 Function 0 0 0 7 bits even parity 2 stop bits 0 0 1 7 bits odd parity 2 stop bits 0 1 0 7 bits even parity 1 stop bit 0 1 1 7 bits odd parity I stop bit 1 0 0 8 bits 2 stop bits 1 0 1 8 bits l stop bit 1 1 0 8 bits even parity 1 stop bit 1 1 1 8 bits odd parity 1 stop bit 388 20 6 3 Transmitter Control Bits CR5 and CR6 Two transmitter control bits provide control of the interrupt from the Transmit Data Register Empty condition the RTS output and the transmission of a break level space CR6 CR5 Function 0 0 RTS low transmitting interrupt disabled 0 1 RTS low transmitting interrupt enabled 1 0 RTS high transmitting interrupt disabled 1 1 RTS low transmits a break level on the transmit data output Transmitting interrupt is disabled 20 6 4 Receive Interrupt Enable Bit CR7 The conditions of receive data register full overrun or a low to high transition on the Data Carrier Detect DCD signal line are enabled by a high level bit in this position 20 7 THE STATUS REGISTER Sheila amp 08 read only Information on the status of the 6850 is available from this register 20 7 1 Receive Data Register Full RDRF Bit 0 The RDRF register indicates that received data has been transferred to the Receive Data Register RDRF is cleared after the processor has read from the Receive Data Register or by a master reset DCD being high also causes RDRF
243. lable to the machine code in a parameter block at location amp 600 The USR function allows the machine code routine to return a value to the BASIC program made up from the register contents For more details of CALL and USR refer to the USER GUIDE 3 9 Conditional Assembly and Macros Working within the BASIC environment it is possible to use BASIC functions to implement these higher level assembly language structures Conditional assembly is a method of varying the code assembled according to a test All the facilities of BASIC are available for setting up the test criteria Typical applications for conditional assembly include the conditional incorporation of debugging routines and selecting different hardware specific sub routines from a number of alternatives A macro is a group of assembler statements which may be inserted into the assembler program when called A macro may be thought of as being a type of sub routine which is used to include a portion of assembler used more than once within a program A number of statements which are likely to be used more than once can be enclosed within assembler delimiters and placed within either a sub routine called using GOSUB and terminated by RETURN or a function definition or a procedure definition Using a procedure or a function is the best way to implement macros because the programmer is then able to pass parameters to the macro and the procedure function name serves to identify t
244. lag set if overflow in bit 7 Z zero flag set if A 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag set if sign bit is incorrect N negative flag set if bit 7 set Addressing mode bytes used cycles immediate zero page Zero page X absolute absolute X absolute Y indirect X indirect Y 1 if page crossed 1 if page crossed NUNWWWNN N 2 3 4 4 4 4 6 5 1 if page crossed Example Add 1 to a 2 byte value in locations amp 80 and amp 81 CLC LDA 1 clear carry flag load accumulator with 1 A A amp 80 carry set if overflow occurs place result of addition in amp 80 set accumulator to 0 carry unchanged A A amp 81 C add 1 if carry set store result back in 881 n D Rh co Dl 44 AND Logical AND A A AND M A logical AND is performed bit by bit on the accumulator contents using the contents of a byte of memory The truth table for the logical AND is Acc Mem Result bit bit bit 0 0 0 0 1 0 1 0 0 1 1 1 Processor Status after use C carry flag not affected Z zero flag set if A 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 set Addressing mode bytes used cycles immediate 2 2 Zero page 2 3 Zero page X 2 4 absolute 3 4 absolute X 3
245. lly takes as its instructions Each assembler mnemonic is described on a new page At the head of the page is the three letter mnemonic which the assembler recognises Beneath the heading there is a short phrase indicating the function of the instruction and the derivation of the mnemonic A short hand BASIC like description of the operation is given on the top right of the page The registers and flags are denoted by the abbreviations given on the previous two pages The initial M represents the data byte obtained using the selected addressing mode A brief description of the instruction and its operation is given beneath the headings Any changes to the status register are noted in a list of the status register flags All the available addressing modes are listed together with the number of bytes of memory which the instruction and its data will occupy when this mode is used The number of instruction cycles taken for the execution of the instruction in each addressing mode is also given 1 instruction cycle 0 5 microseconds A short example of the use of the instruction within an assembly language routine is given at the bottom of each page 43 ADC Add with Carry A C A M C This instruction adds the contents of a memory location to the accumulator together with the carry bit If overflow occurs the carry bit is set this enables multiple byte addition to be performed Processor Status after use C carry f
246. location pointed to by the contents of the user vector USERV at locations amp 200 amp 201 low byte high byte The command enters this code with the A 1 X least significant byte of string address and Y most significant byte of string address LINE provides 16 an easy method of incorporating a user function into the operating system command table 210 220 230 240 250 260 210 280 290 300 310 DIM MC 100 OSASCI 6 amp FFE3 USERV 6 200 FOR opt 0 TO 3 STEP3 PS MC OPT opt write CMP 1 is this LINE BEQ code execute machine code if LINE BRK otherwise print Out error message P 255 P P 1 REM error number P x PS PS L OPT BRK LINE only please ENSp3 opt reset OPT op code value 0 code STX 670 LINE code entry point and store STY LDY 811 string address lov byte high byt 0 set up Y register for indexing loop LDA amp 70 Y Post Indexed Indirect addressing JS N R X P E OSASCI print Out character increment index amp 0D test for end of string loop if not last character go round again RT I 8 ee N S EXT finished USERV write MOD 256 USERV 1 write DIV 256 This example program sets up the user vector to point to some machine code which prints out the string pointed to by X and Y After this program has been run typing in
247. lock high period from T to U Unfortunately two major problems arise from this mode of operation 442 2d49dNO 10 CO49dNO L04DdNO 10 L949dN2 049dN 10 939dN sna SS 3Hd0Y 3ZHW 931100 NMOHS Y20792 ZHWZ HILSVA ONIHOLIJH1LS 31949 HLIM 49019 Ado Figure 28 2 IMHz bus timing showing page select signals 443 28 5 1 Spurious address decoding glitches PROBLEM 1 Addresses on the system address bus will only usually change when the 2MHz processor clock is low However the 1MHz clock is alternately low then high when the CPU addresses change This gives rise to the address decoding glitches labelled P and Q in figure 28 2 The Q glitches are not normally important because the IMHZE clock is then low The P glitches can cause problems because the 1MHZE signal is then high Spurious pulses may therefore occur on the various chip select pins leading to possible malfunction of some devices 28 5 2 Double accessing of 1MHz bus devices PROBLEM 2 If a 1MHz bus device is accessed during a period when the IMHZzE clock is high point R in figure 28 2 that device will be accessed immediately The device will then be accessed again when 1MHZE is next high point V in figure 28 2 This is because the CPU clock is held high until the next coincident falling edge of the 2MHz and 1MHz clocks point U Double accessing a peripheral does not normally present a p
248. lock number of the last block read When the last block is reached further information is printed out If the default messages are selected then the length of the file will be added to the block number FILENAME 09 0904 If extended messages have been selected then the catalogue printout after the final block has been reached will look like this FILENAME 09 0904 FFFFOEOO FFFF801F This file is a BASIC level 1 program SAVEed from BASIC with PAGE amp E00 and the program is amp 904 bytes long The fourth field in the catalogue printout is the start address of the file and the file will LOAD to this address by default The fifth field is the execution address When using level 2 BASIC the execution address will be amp 8023 When an attempt is made to RUN a file the processor jumps using JSR to this address The two most significant bytes of the four byte fourth and fifth fields are set to the machine high order address see OSBYTE amp 82 130 For details about selecting extended messages see OPT 2 6 CODE x y CO OSBYTE with A amp 88 136 This command enables the user to incorporate his own command into the operating system command table CODE executes machine code indirected through the user vector USERV at locations amp 200 amp 201 low byte high byte The default contents of the user vector produce the Bad Command message The machine code at USERV is entered with A 0 X x and Y y 13 For example 10
249. locks it toggles this location between values of 5 and 10 These values represent offsets from amp 28D where the clock values are stored 237 OSBYTE amp F4 244 YFX 244 Read write soft key consistency flag lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If this location contains 0 then the soft key buffer is ina consistent state A value other than 0 indicates that the soft key buffer is in an inconsistent state the operating system does this during soft key string entries and deletions If the soft keys are in an inconsistent state during a soft break then the soft key buffer is cleared otherwise it is preserved 238 OSBYTE amp F5 245 FX 245 Read write printer destination flag lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This call is used by OSBYTE amp 5 FX 5 Using this call does not check for the printer previously selected being inactive or inform the user printer routine See Expansion vectors section 10 6 239 OSBYTE amp F6 246 FX 246 Read write character ignored by printer lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This call is used by OSBYTE amp 6 FX 6 240 OSBYTEs amp F7 247 amp F8 248 and amp F9 249 Rea
250. m JIM 1 MHz bus 95 Write to JIM 1 MHz bus 96 Read from SHEILA mapped I O 97 Write to SHEILA mapped I O 98 Examine buffer status 99 Insert character into input buffer 9A Write to video ULA control register and copy 9B Write to video ULA palette register and copy 9C Read write 6850 control register and copy 9D Fast Tube BPUT 9E Read from speech processor 9F Write to speech processor AO Read VDU variable value OSBYTE FX calls 161 amp A1 to 165 amp A5 are not used by OS1 20 112 166 167 168 169 A6 Read start address of OS variables low byte A7 Read start address of OS variables high byte A8 Read address of ROM pointer table low byte A9 Read address of ROM pointer table high byte 170 AA Read address of ROM information table low byte 171 AB Read address of ROM information table high byte 172 AC Read address of key translation table low byte 173 AD Read address of key translation table high byte 174 AE 175 AF 176 BO 177 B1 178 B2 179 B3 180 B4 181 B5 182 B6 183 B7 184 B8 185 B9 186 BA 187 BB 188 BC 189 BD 190 BE 191 BF 192 CO 193 Cl 194 C2 195 C3 196 C4 197 C5 198 C6 199 C7 200 C8 201 C9 Read start address of OS VDU variables low byte Read start address of OS VDU variables high byte Read write CFS timeout counter Read write input source Read write keyboard semaphore Read write primary OSHWM Read write current OSHWM Read vvrite RS423 mode Read characte
251. m event handler 270 280 NEXT I 290 2 EVNTV MC AND amp FF 300 EVNTV 1 MC DIV amp 100 310 sound_pars amp FFF50001 320 sound_pars 4 amp 00010000 REM set up SOUND 1 11 x 1 330 FX 14 2 340 REM enable keyboard event This example program illustrates how the keyboard event can be used When this program has been run each key press causes a sound to be made The pitch of this sound is dependent on the key pressed This event handling routine does not check the identity of the event calling it and so enabling events other than the keyboard event will have curious consequences 12 8 ADC conversion complete 3 When an ADC conversion is completed on a channel this event is generated The event handling routine is entered with the channel number on which the conversion was made in Y 290 12 9 Start of vertical sync 4 This event is generated 50 times per second coincident with vertical sync One use of this event is to time the change to a 6845 or video ULA register so that the change to the screen occurs during fly back and not while the screen is being refreshed This avoids flickering on the screen 12 10 Interval timer crossing zero 5 This event uses the interval timer see OSWORD calls amp 3 and amp 4 sections 9 5 and 9 6 This timer is a 5 byte value incremented 100 times per second The event is generated when the timer reaches zero For example 10 MODE7 20 OSWORD 6FFF1 30 OSBYTE amp FFF4 40 OS
252. m in accumulator 80 loop JSR OSWRCH perform VDU command 90 INX increment index count 100 LDA data X load next VDU parameter 110 CPX amp 20 has count reached 32 amp 20 120 BNE loop if not then go round again 130 RTS hack to BASIC 140 150 NEXT opt 160 data amp 04190516 170 data 4 amp 00C800C8 180 data 8 amp 00000119 190 data amp C amp 01190064 200 data amp 10 amp 000000C8 210 data amp 14 amp 00000119 220 data amp 18 amp 0119FF9C 230 data amp 1C s0000FF38 240 CALL entry 21 This program performs some simple graphics using the BASIC VDU method to select the screen MODE and perform PLOTting All the VDU codes are contained within the block of memory labelled data Using the operator does not make it immediately obvious what is going on Four bytes are inserted into memory with each operator The least significant byte being inserted at the address specified Each subsequent byte is inserted into the next byte of memory 1 e data amp 04190416 data 4 amp 00C800C8 will result in an equivalent to VDU amp 16 amp 04 amp 19 amp 04 amp C8 amp 00 amp C8 amp 00 or to separate it into its two components VDU amp 16 amp 04 VDU amp 19 amp 04 amp 00C8 amp 00C8 or VDU 22 4 select MODE 4 VDU 25 4 200 200 PLOT 4 200 200 move absolute X Y Any program which can be written in BASIC may also be implemented in machi
253. mal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing Mode bytesused cycles implied 1 3 Example See the example given for PHA above 81 PLA Pull accumulator off stack Pull A This instruction loads the accumulator with a value which is pulled from the stack This is usually a previous accumulator value which has been saved on the stack using a PHA instruction Processor Status after use C carry flag not affected Z zero flag set if A 0 I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of A set Addressing Mode bytesused cycles implied 1 4 Example Restore registers at the end of a routine PLA pull Y value from stack TAY put it back in Y PLA pull X value from stack TAX put it back in X PLA pull A value from stack PLP restore status register RTS back to calling routine 82 PLP Pull status register off stack Pull P This instruction loads the status register with a value which is pulled from the stack This is usually a previous status register value which has been saved on the stack using a PHP instruction Processor Status after use C carry flag bit 0 from stack Z zero flag bit 1 from stack I interrupt disable bit 2 from stack D decimal mode flag bit 3 from
254. mmand Note that most filing system commands should be intercepted via the filing system control entry see filing systems section 16 8 On entry the command to be interpreted is in the form of an ASCII string terminated by amp 0D and pointed to by the contents of amp F2 and amp F3 plus the Y register 321 05 06 07 08 322 Unrecognised interrupt When an interrupt occurs that is either not recognised by the operating system or has been software masked out the interrupt is first offered to the paged ROMs and then to the user via the IRQ2 vector A paged ROM accepting interrupts should interrogate the device s that it will respond to to see if any of them are responsible for the interrupt and if so process it If the interrupt is processed by a ROM it should set the accumulator to zero to prevent it being offered elsewhere Always return with an RTS instruction not RTI Break The user has executed a BRK instruction usually flagging an error Paged ROMs are informed before handing over the error to the current language via the break vector The Y register should be preserved during this call but only if the service routine intends to return and allow control to pass back to the current language On entry location amp FO contains the value of the stack pointer after the BRK instruction was executed Locations amp FD and amp FE point to the error number in memory Note that the error may have occurred in another
255. move the screen position upwards by one character line it is necessary to increment the current start address register R12 R13 by the number of characters per line Remember that these are characters as produced by the 6845 and not as seen on the screen There are 80 6845 characters per line in modes 0 3 and 40 characters per line in modes 4 5 and 6 The screen can be scrolled downwards by decrementing the screen start address register by the number of characters per line Note that you should not normally allow the screen start address register to contain a value less 371 than the official screen start address or greater than the official screen end address in the mode being used If this occurs then areas of the main system memory will be displayed directly on the screen This produces some interesting results especially if zero page is displayed Remember that the value put into R12 R13 is the actual memory address DIV 8 See the example program in section 18 14 18 11 2 Sideways scrolling The whole screen can be made to move left by one character as seen by 6845 by incrementing the screen start register It will move one character to the right by decrementing this register Note that each character which moves off the left of the screen will appear on the next line up at the right of the screen It is therefore necessary to move each of these characters down a line in software to maintain a true sideways scroll This scrolling tec
256. ms If a filing system wishes to issue an error it cannot do it in the normal way of executing a BRK instruction because when the language comes to process the message the message will be unavailable in another ROM The usual way of issuing errors is to copy to location amp 0100 the far end of the stack a BRK instruction the error number and error message and then jump to location amp 0100 The following workspace is reserved for filing systems Base page amp A0 amp A7 NMI workspace Can only be used whilst NMI claimed amp A8 amp AF Utilities area Can only be used within an operating system command can be a filing system command amp B0 amp BF Filing system scratch space Contents are not guaranteed to remain present from one call to another Do not use for interrupt routines amp C0 amp CF Filing system dedicated workspace This is guaranteed to remain intact as long as the filing system is active and as long as the absolute workspace is not claimed 329 Main memory amp 0D00 amp 0D5F NMI code area Filing systems using NMI code should copy the code to this space after claiming this space with service call amp 0C If the NMI code must access the ROM it should save the old ROM number to restore at the end of NMI service amp 0E00 amp ssss Absolute workspace The address ssss is set at reset by service call amp 01 Must be claimed before use by service call amp OA 15 4 ROM filing sys
257. n are returned in Y These locations are not used by the operating system Default values 0 234 OSBYTE amp F1 241 FX 241 Read write location amp 281 lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This call is not used by the operating system and is unlikely to be used by later issues either This location is reserved as a user flag for use with FX 1 Default value 0 235 OSBYTE amp F2 242 Read copy of the serial processor ULA register lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y See serial interface chapter 20 for the significance of this register This call should not be used for writing as it would place this copy of the register contents out of sync with the register contents themselves All the serial ULA functions can be controlled with OSBYTE with A amp 89 FX 137 motor control OSBYTE with A amp CD FX 205 cassette RS423 select OSBYTE with A amp 7 amp 8 FX 7 8 RS423 baud rate control 236 OSBYTE amp F3 243 Read timer switch state lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y The operating system maintains two internal clocks which are updated alternately As the operating system alternates between the two c
258. n which normally contains the error number See below Note that although a fully prepared exit from a BRK instruction is possible neither the operating system or BASIC expect a return from this vector Possibly fatal results may occur if such a return is made as paged ROM software typically stores the BRK error number and message in page one below the stack returning there is very hazardous The exception to this is when using the BRK instruction as a breakpoint in user supplied machine code and is not used as a standard error generating mechanism 257 Note also that the breaR vector only refers to the BRR assembler instruction it should not be confused with the BREAK key on the keyboard which causes a hardware reset This vector is changed to its default state during the course of processing such a reset The BBC microcomputer adopts a standard pattern of bytes following a BRK instruction this is A single byte error number An error message A zero byte to terminate the message 10 3 The interrupt vectors IRQ1V and IRQ2V amp 204 6 For the entry conditions to these vectors refer to the interrupts chapter number 13 10 4 Main vector zone vectors from amp 208 21F Entry conditions for these vectors are covered in the following sections OSBYTE calls chapter 8 for BYTEV OSWORD calls chapter 9 for WORDV Operating System calls chapter 7 for WRCHV and RDCHV and Filing systems chapter
259. n is used to place the 6502 in decimal mode This causes arithmetic operations to be performed in BCD mode See machine code arithmetic chapter 4 Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag set B break command not affected V overflow flag not affected N negative flag not affected Addressing mode Bytes used Cycles implied 1 2 Example Set decimal mode for arithmetic SED BCD from now on 90 SEI Set interrupt disable flag IF1 This instruction is used to set the interrupt disable flag When this flag is set maskable interrupts cannot occur See interrupts chapter 13 Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable set D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode Bytes used Cycles implied 1 2 Example Disable interrupts SEI No maskable interrupts 91 STA Store accumulator contents in memory M A This instruction is used to copy the contents of the accumulator into a memory location specified in the operand field Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow
260. n the instruction is executed The assembler automatically calculates the relative address from the address given and will cause an error if the address is out of range Used after a CMP instruction this branch occurs if A lt gt data Used after an LDA instruction this branch occurs if A lt gt 0 Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytesused_ cycles relative 2 2 1 if branch succeeds 2 if to new page Example Memory location to be written to if it contains zero i e IF amp 84 0 then amp 84 amp 7F LDA amp 84 load memory into A to set flags BNE round if not zero skip the next bit LDA amp 7F lead A with value to he written STA amp 84 write to location amp 84 round rest of program 52 BPL Branch on positive result Branch if N 0 Depending on the state of the negative flag a relative branch will be made The relative address is calculated by the assembler from an address provided by the programmer This address must be within the relative addressing range Branch occurs after a result which sets accumulator bit 7 to 0 Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D
261. name terminator amp 00 Load address of file four bytes low byte first Execution address of file four bytes low byte first Block number two bytes low byte first Block length two bytes low byte first Block flag one byte Bit 0 Protection bit The file can only be RUN if this bit is set Bit 6 Empty block bit This block contains no data if this bit is set An empty block is created when a file is opened for output and immediately closed again without BPUTting anything Bit 7 Last block bit This block is the last block of a file if this bit is set Four spare bytes amp 00 Header Cyclic Redundancy Check CRC two bytes high byte first A data block which is Data 0 to 65535 bytes the usual maximum is 256 as specified in the header Data CRC two bytes high byte first The header CRC excludes the synchronisation byte A cyclic redundancy check value CRC is a number which can be calculated from a block of data If the data is changed and another CRC performed the CRC value will probably be 347 different VVhen the cassette filing system saves data onto tape it calculates the CRC for each block of data and includes it in the cassette format When data is reloaded from cassette a CRC is calculated and compared with the CRC value stored with the data If the two CRC values are not the same then the data has been corrupted Cyclic redundancy checking is sensitive enough to detect single bit errors but ro
262. nd an error message into his code to identify the error The byte in memory following the BRK instruction should contain the error number The error message string should follow the error number and must be terminated by a zero byte The following lines set this up 240 error BRK cause an error 270 PS amp FF Error number 255 280 P P 1 290 SPS Wrong key pressed Error message 300 PS3 LEN SP 0 Terminating byte When a BRK is encountered in a machine code program called from BASIC the error message is printed out together with the line number from which the machine code was called Typing REPORT or printing ERR will reproduce the message and error number as with any BASIC error The user can provide his own BRK handling routine which may be useful when using machine code away from the BASIC environment see section 10 2 for more information about the BRK vector 3 8 Entering machine code from BASIC CALL and USR Machine code routines can be entered from a BASIC program using either the CALL statement or the USR function On entry to the machine code program using these instructions the accumulator the X register the Y register and the carry flag are set to the least significant bytes or bit of the resident integer variables A X Y and C A number of parameters may be passed to the machine code routine if the 28 CALL statement is used the addresses and data types of these parameters being avai
263. nd processor memory A 4 Start execution in the second processor Data is transferred to and from second processor memory over the Tube via a Tube register at location amp FEE5 Thus to read second processor memory a memory read is initialised with A 0 then successive reads are performed of location amp FEES 16 10 The Cassette filing system This filing system is provided as standard on all BBC microcomputers It is very basic and many entry points are not implemented or are only partially implemented The calls implemented are OSFILE OSARGS OSBGET OSBPUT OSGBPB OSFIND OSFSC 346 Partly implemented entry with A 0 is save all other entries are considered to be load file A amp FF Hardly implemented only filing system identification performed A 0 Y 0 Fully implemented Fully implemented Not implemented at all Implemented allowing one file to be open for input and one for output If an attempt is made to open a file for update it is opened for input Calls 0 through 6 are implemented though no extra commands exist ie code 3 just gives Bad command The file handles given are constant 1 is the input file 2 is the output file The cassette tape format is 5 seconds of 2400Hz tone to synchronise the ACIA see also bug fix in the hardware section in section 20 10 A header block which is 1 byte synchronisation amp 2A Filename 1 to 10 characters File
264. ndard interrupt routine can be made to ignore any of the above interrupts by use of OSBYTE amp E9 The status register is ANDed with this OSBYTE value Any bit masked by this is ignored and passed over to the user interrupt vector The user is then responsible for clearing the interrupt condition This is done by writing a byte to location amp FE4D with the bit corresponding to the interrupt to be cancelled set The standard interrupt routine uses the interrupts in the following ways A key pressed interrupt causes the operating system to mark the key pressed as the current key It will be processed on a subsequent timer interrupt A vertical sync interrupt is used to time the colour flashing and change the colours when required Modifying colours in this way ensures that the colours do not change halfway through displaying the screen It is also used to time out the cassette and RS423 systems when one of these has timed out after half a second of inactivity the 6850 can be claimed for use by the cassette or RS423 systems This interrupt also causes a frame sync event number 4 The shift register of the system VIA is not used and its interrupt is passed over to the user The analogue conversion completion interrupt is used to cause a the newly converted value to be read and stored in RAM The next channel to be converted is then initialised This interrupt also causes event number 3 The light pen interrupt is not used by
265. ne code although it is not always sensible to do so There now follows a detailed description of using the BASIC mnemonic assembler 3 1 The assembler delimiters and J All the assembler statements should be enclosed within a pair of square brackets When the BASIC program is RUN the assembler statements contained between the square brackets are assembled into machine code This code is inserted directly into memory at the address specified by P and P is incremented by the number of bytes in each instruction or directive 22 Within the assembler delimiters the text of the assembly language program may be written The assembly language program will consist of anumber of assembler statements separated by new lines or colons as in BASIC Each assembler statement should consist of an optional label followed by an instruction this will be a three letter assembler mnemonic or an assembler directive and an operand or address If a label is included it should be separated from the instruction by at least one space The operand need not be separated from the instruction Any character following the operand and separated by at least one space from it will be totally ignored by the assembler which will move onto the next colon or line for the next statement A comment may be placed after the operand field and should be preceded by an backslash Any text following an backslash in an assembly statement will be ignored by the assem
266. next location are returned in Y This call is equivalent to OSBYTE amp 9C FX 156 except that it does not update the 6850 chip This call should not be used to write the control flag as it would cause the operating system RAM copy to become inconsistent with the 6850 register contents 200 OSBYTE amp C1 193 FX 193 Read write flash counter lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains the number of 1 50th sec units until the next change of colour for flashing colours OSBYTE amp C2 194 FX 194 Read write mark period count lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y Equivalent to OSBYTE 8209 FX 9 OSBYTE amp C3 195 FX 195 Read write space period count lt NEW VALUE gt lt old VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y Equivalent to OSBYTE amp 0A FX 10 201 OSBYTE amp C4 196 FX 196 Read write keyboard auto repeat delay lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This call is used by OSBYTE amp 0B FX 11 OSBYTE amp C5 197 FX 197 Read write keyboard auto repeat period rate lt NEW VALUE gt lt OLD VALUE gt AND Y EO
267. ng an OR operation and then an FOR operation all the GCOL plotting operations can be taken into account by changing the data in these two bytes The graphics mask at location amp D1 is used to mask out the bits in these bytes when they are used 268 amp D6 and amp D7 contain the address of the top line of the current graphics character cell eight bytes long See location amp 31A amp D8 and amp D9 contain the address of the top scan line of the current text character amp DA F are used as temporary workspace amp E0 and amp E1 are used as a pointer to the row multiplication table high bytes first This table is used to calculate the offsets of a character row within memory thus as an eighty column row takes 80 8 640 bytes for eighty column modes this points to a 640 table This table is also used for the 320 operation needed by the 40 column modes the results being divided by two A 40 table is pointed to when in teletext mode The tables consist of 32 or 24 sequential entries for the 640 and 40 tables respectively Each entry consists of two bytes of the multiplied figure the high byte being stored first amp E2 is the cassette filing system status byte bit 0 Set if the input file is open bit 1 Set if the output file is open bit 2 Not used bit 3 Set if currently CATaloguing bit 4 Not used bit 5 Not used bit 6 Set if at end of file bit 7 Set if end of file warning given amp E3 is the cassette
268. ng diagram illustrates the format of an 8 bit word with odd parity and 1 stop bit selected O UUU START DATA DATA DATA DATA DATA DATA DATA DATA PARITY STOP BIT BITO BITI BIT2 BIT3 BIT4 BITS BIT6 BIT BIT Fig 20 1 Timing diagram for a serial character 20 2 Line termination If very long lines are used to connect devices over an RS423 link then it may be necessary to terminate the line This will only be necessary in exceptional circumstances where both high baud rates and long line lengths are in use Refer to Appendix I which describes the necessary links for more information 20 3 The 6850 ACIA Asynchronous Communications Interface Adapter This chip performs two basic functions It converts 8 bit parallel data to serial data which it then transmits It also converts a serial input stream into parallel data for the system bus automatically providing the formatting and error checking 386 20 4 Transmit data register TDR Sheila 4 09 write only A character may be written into the Transmit Data Register TDR if a status read operation has indicated that the TDR is empty OSBYTE amp 97 151 should be used to perform the write This character is transferred to a serial shift register where it is transmitted preceded by a start bit and followed by one or two stop bits Internal parity odd or even can optionally be added to the character and will appear after the last data bit and before the first stop bit After the fir
269. ng of the interrupt If this was not done the keyboard interrupt would last for ever It is the responsibility of the interrupt routine to clear an interrupt whether it is the operating system interrupt routine or a user provided one Note the direct poking of the I O devices This is necessary in interrupt routines which must operate fast and witha minimum of calls to external routines There is no need to worry about Tube compatibility since interrupt routines must always run on the I O processor An interesting example of a lightpen handler follows It detects invalid light pen strobe pulses such as might be generated when a lightpen is directed away from the screen It allows for a small amount of fluctuation in the output from the light pen such as could be generated by other light sources in the room Note that to keep the example as short as possible the code below assumes that there are no other claimers of IRQ2V The previous example should be consulted for the correct code to account for the other users of the vector 10 DIM M 150 20 olp amp 70 lpen amp 74 30 FOR opt 0 TO 3 STEP 3 40 P S M 50 60 OPT opts 70 Anit SEI Disable interrupts 80 LDA int MOD 256 Low byte of address 90 STA amp 206 IRQ2V low 100 LDA int DIV 256 High byte of address 110 STA amp 207 IRQ2V high 120 LDA amp 88 Interrupt change mask 130 STA amp FE4E Enable lightpen interrupt 140 CLI 150 RTS E
270. ng system command It is used by the disc system to implement a protection mechanism on dangerous commands by insisting that the previous operating system command was ENABLE On exit All registers are undefined where not defined as described above Interrupt state is preserved but may be enabled during operation 16 9 Filing systems and the Tube Filing systems are required to communicate with the Tube system and inform it of the following transactions Write to second processor memory Read from second processor memory Start execution in the second processor from a given address A filing system should only enter communication with the Tube system when the Tube is present checked with OSBYTE amp EA and when the address required is not in the I O processor The BBC microcomputer system uses a 32 bit addressing system and the I O processor s memory is normally selected when the top 16 bits are amp FFFF other addresses being second processor memoty When the Tube is present some machine code instructions will be present at location amp 406 If file addresses require data to be transferred over the Tube a call should be made to amp 406 with the following parameters X Y point to a control block in memory X low byte Y high byte The control block contains a four byte address 345 operation parameter this can take the following values A 0 Initialise read of second processor memory A 1 Initialise writing of seco
271. nning the house heating system for example would not wish to keep on checking that the temperature in every room is correct it would take up too much of its processing time However if the temperature gets too high or too low in any of the rooms it must do something about it very quickly This is where interrupts come in The thermostat could generate an interrupt The computer quickly jumps to the interrupt service routine switches a heater on or off and returns to the main program There are two basic types of interrupts available on the 6502 These are maskable interrupts IRQs and non maskable interrupts NMIS To distinguish between the two types there are two separate pins on a 6502 One of these is used to generate IRQs maskable and the other is used to generate NMIs non maskable 295 13 1 1 Non Maskable Interrupts When a non maskable interrupt is asserted by a hardware device connected to the NMI input on the 6502 a call is immediately made to the NMI service routine at amp 0D00 on the BBC microcomputer Nothing in software can prevent this from happening So that the 6502 is only interrupted in very urgent situations only very high priority devices such as the Floppy Disc Controller chip or Econet chip are allowed to generate NMIs They are then guaranteed to get immediate attention from the 6502 To return to the main program from an NMI an RTI instruction is executed It is always necessary to ensure that all of the
272. ns file option number and Y the option value required See OPT section 2 14 After call A is preserved C is undefined 163 OSBYTE amp 8C 140 FX 140 Select tape filing system TAPE equivalent Entry parameters X selects baud rate See TAPE section 2 19 After call A is preserved C is undefined 164 OSBYTE amp 8D 141 FX 141 Select ROM filing system ROM equivalent No entry parameters See ROM section 2 15 After call A is preserved X Y and C are undefined 165 OSBYTE amp 8E 142 FX 142 Enter language ROM Entry parameters X determines which language ROM is entered The selected language will be re entered after a soft BREAK The action of this call is to printout the language name and enter the selected language ROM at amp 8000 with A 1 Locations amp FD and amp FE in zero page point to the copyright message in the ROM When a Tube is present this call will copy the language across to the second processor 166 OSBYTE amp 8F 143 YFX 143 Issue paged ROM service request Entry parameters X service type Y argument for service On exit Y may contain return argument if appropriate See Paged ROM section 15 1 1 After call A is preserved C is undefined 167 OSBYTE 8 90 144 FX 144 Alter display parameters TV equivalent Entry parameters X vertical screen shift in lines Y 0 interlace on Y 1 interlace off On exit X and Y contain the previous settings of the
273. nter with OSBYTE call 5 and set the transmit baud rate with OSBYTE call 8 Note that R5232 printers are normally compatible with the RS423 interface 14 2 2 RS423 terminals It is possible to connect remote terminals to the BBC microcomputer By selecting RS423 input with OSBYTE 2 all input from the terminal will be treated by the operating system as if it had come from the integral keyboard Selecting RS423 output using OSBYTE 3 will ensure that all output is echoed back to the remote terminal Note that normally characters received from the R5423 input channel are not treated exactly as those received from the keyboard The differences are that neither softkey expansions nor the escape character are processed To enable this processing a OSBYTE amp B5 must be performed 14 2 3 Using the BBC computer as an intelligent terminal Using the BBC microcomputer as an intelligent terminal to another computer is not trivial This is because this application requires the RS423 channels to be connected in the opposite way to that normally assumed That is the RS423 input has to be directed to the VDU output stream and the keyboard input has to be directed to the RS423 output channel A simple example of use of the BBC microcomputer as a terminal to a remote computer is given here This program is extremely dumb and merely transfers input characters to the VDU output stream 310
274. ntrol vector cycle numbers directories files initialise paged ROM call paged ROM implementation Tube zero page work space FINDV OSFIND Fire buttons on joysticks Flag 6502 break 6502 carry 6502 interrupt disable 6502 negative 6502 overflow 6502 unused 6502 zero carry decimal escape indicating speech present indicating Tube presence interrupt disable overflow printer destination read RS423 control flag 289 273 292 293 291 258 288 374 383 136 13 18 281 326 494 336 203 204 163 333 342 335 18 333 343 334 333 333 325 328 345 268 342 418 42 41 41 42 42 42 41 32 89 33 90 147 231 230 59 91 32 60 239 200 RS423 input suppression RS423 use RS423 cassette selection soft key consistency user Flags determining ESCAPE effects Flashing control in ULA Flashing colours duration of 1st colour duration of 2nd colour read write flash counter read write mark period read write space period Floppy disc hardware upgrade Flushing buffers Font reading character definitions storage Font flags on page 2 Font explosion pages ROM service call read definition state FRED expansion bus reading and writing Function keys plus CTRL plus SHIFT plus SHIFT CTRL soft key status G Get Byte OSBGET Graphics OS ROM table Graphics byte mask zero page location Graphics colour bytes zero page locations Graphic colour cell Graphics cursor OSVVORD Graphic origin storage
275. o be passed to the event handling routine in X or Y this is specific for each event see below The event handling routine should preserve all registers The event handling routine is entered with interrupts disabled and should not enable interrupts Because interrupts are disabled an overly long routine will have dire consequences and the routine should be terminated after no more than about 2 milliseconds 288 Great care must be taken when using operating system calls from within an event handling routine Many operating system calls may enable interrupts during execution see chapter 7 If an interrupt occurs while the user s event handling routine is being executed the interrupt may cause the user s routine to be re entered before it has finished processing the previous event It may be possible to write the event handling routine in such a way that this will not have any ill effects A routine which may be re entered in this way is described as re entrant or re enterable While the event facility has been designed to enable users easy access to a form of interrupt handling this aspect of their use may complicate the issue For more information see interrupts chapter 13 The event handling routine should never be exited using an RTI instruction Using the old contents of the event vector is a good way of leaving the routine Making a copy of the vector contents before changing this vector allows the user routine to JMP indirect wh
276. ocation Counter P When the assembler is creating the machine code program the code produced is placed in memory starting from the address in P one of the resident integer variables unless remote assembly has been selected using OPT see section 3 2 The programmer must set P to a meaningful value before the assembly begins The usual method for short programs is to DIMension a block of memory and to set P to this value at the beginning of each pass of the assembler as in the example above A classic problem is sometimes encountered when a programmer adds more code to a short program which has been allocated space by this method If the code created overflows the space DIMensioned for it and is over written by BASIC it will fail to operate as expected when tested alternatively the code may over write the BASIC dynamic storage and a No such variable error will be flagged during the second pass of the assembler The assembler updates P as it is assembling and when it reaches the end of a pass the value of P represents the address of the first free byte of memory after the machine code program 3 4 Labels Any BASIC numerically assignable item may be used as a label with the assembler such as a variable or an array element A label is defined by preceding the variable name with a full stop The full stop prefix causes the assembler to set up a BASIC variable containing the current value of P Once set up this variable is
277. ocess was using the desired routine at the time of the interrupt If the routine to be called is not re entrant the foreground process will be disturbed and may crash The user s interrupt routine should be re entrant This means that if any routines called by the interrupt routine re enable interrupts and a second interrupt occurs before the first is finished the interrupt routine should be able to handle it This is achieved by Pushing the original X and Y registers onto the stack Pushing the contents of amp FC onto the stack Not using any absolute temporary storage locations Note that enabling interrupts during a user interrupt routine is unofficial in that Acorn state that it should not be done but with care it is safe to do so 305 If a non re entrant zone is entered such as an operating system routine or an area that needs temporary storage keep a semaphore for that zone If another interrupt then occurs that area of code must not be used again until it has finished There now follows an example of a routine using interrupts which will stop all keyboard input entering the buffer until break is pressed 10 DIM M 100 20 FOR opt 0 TO 3 STEP 3 30 PS M 40 50 OPT opts 60 sinit SEI Disable interrupts 70 LDA amp 206 Save old vector 80 STA oldv 90 LDA 8207 100 STA oldvtl 110 LDA tint MOD 256 Low byte of
278. ode placed in memory at O Implemented in Level 2 BASIC only In the example program above OPT is set up using the FOR NEXT loop variable opt On the first pass of the assembler OPT 0 is used listing is suppressed and assembler errors are not enabled For the second pass an OPT 3 is used which switches on assembly listing and enables assembler errors BASIC errors will be flagged as normal The assembler errors which are suppressed are the Branch out of range error and the No such variable error These will normally be generated during the first pass when the assembler is resolving forward passes see section 3 5 Bit 2 allows a program to be assembled into one region of memory while being set up to run at a different address P the program counter see below should be set up as usual to provide the source of label values If bit 2 is set then O should be set up at the same time as P to point to the start of memory into which the machine code is to be assembled This facility is useful for assembling machine code where it is impossible to use the memory in which the program is eventually going to reside e g Assembling programs which are going to be blown into EPROM for paged ROMs This option is only available in Level 2 BASIC Each time the assembler is entered the OPT value is initialised to 3 This means that a second chunk of assembler in the same BASIC program must perform its own OPT selection 24 3 3 The L
279. of each block during a CAT can be enabled by issuing a OPT 1 2 command 349 ROM software may be resident in standard paged ROMs or if the speech chips are fitted in a PHROM PHrase Read Only Memory Storing data in paged ROMs will be covered in the section on paged ROMs Data stored in PHROMs is recognised as data as opposed to speech by an identification sequence This has the following format 00 ignored 01 amp 00 02 amp 28 03 amp 43 C 04 amp 29 05 ignored 3D address of data in internal format for the speech chip 3E indirection command 16 12 Disc filing system The disc filing system is not provided as standard with the BBC microcomputer Extra hardware and software are required to operate disc drives The disc filing system is complete except in some small areas It differs from the standard filing system protocol in the following ways A restricted form of file attributes is used the four bytes are compressed into one bit This bit is referred to as the lock bit A file is locked if either of the you cannot write or you cannot delete bits is set If either of these attributes is set see OSFILE section 16 2 then both attributes are set File types 0 or 2 are never returned instead a File not found error is issued The device identity is the drive number and is one character long Directory names are one ASCII character 350 File names are up to ASCII charact
280. oftware environment and the hardware facilities available to the assembly language programmer The authors have presumed that the readers of this Advanced Guide are reasonably familiar with the basic use of the BBC Microcomputer While every attempt has been made not to bury the facts under a mountain of computer jargon the use of technical terms is an inevitable consequence of attempting to condense a large number of facts into an easily accessible form All the information about the operating system is exclusively based on OS 1 20 The hardware information has been verified with an issue 4 circuit board but where possible differences on earlier issue boards have been noted While this book gives the programmer full access to all the BBC microcomputer s extensive software and hardware facilities using techniques which the designers of the machine intended programmers to use it also opens the door to a multitude of illegal programming techniques For the enthusiast direct access to operating system variables or chip registers may enable him to perform the bizarre or even the merely curious For the serious programmer on the other hand attention to compatibility and machine standards will enable him to write software which will run on BBC Microcomputers of all configurations The responsibility rests with YOU the user The value of your machine depends on continued software support of the highest quality unlike many machines the BB
281. olution The graphics resolutions available on the screen in the various modes are Mode ND OP WNF United States Horiz x Vert 640 x 200 320 x 200 160 x 200 text only 320 x 200 160 x 200 text only Teletext G 4 Video frame period Britain Horiz x Vert 640 x 256 320 x 256 160 x 256 text only 320 x 256 160 x 256 text only Teletext In the U K the frame sync occurs at 50Hz In the U S the frame video frame sync occurs at 60Hz G 5 Memory usage The amount of memory used by the screen memory is different between U K and U S machines in some modes The value of HIMEM the top of user memory is Mode ND OF PWN EF United States amp 4000 amp 4000 amp 4000 amp 4000 amp 6000 amp 6000 amp 6000 amp 7CO00 Britain amp 3000 amp 3000 amp 3000 amp 4000 amp 5800 amp 5800 amp 6000 amp 7CO00 479 Appendix H Disc Upgrade Ensure that the following ICs are present IC 78 8271 ICs 79 80 7438 IC 82 74LS10 ICs 81 86 7415393 ICs 83 84 CD4013B IC 85 CD4020B IC 87 74LS123 Disc Filing System in paged ROM socket OS 1 00 or greater IC 51 On issue 1 or 2 circuit boards it will be necessary to connect the two pads of link S8 with a wire link A switch mode power supply must be present i e not a linear supply in a black case On issue 1 2 or 3 issue boards carefully cut the leg of IC 27 pin 9 do not totally remove the leg Cut the track connected to
282. ond 3 6 The EQUate Facility in Level 2 BASIC One of the improvements made to Level 2 BASIC was the incorporation of some EQU pseudo operation commands These allow the incorporation of data by reserving memory within the body of the assembly language program The EQUate operations available are EQUB equate byte reserves 1 byte of memory EQUW equate word reserves 2 bytes of memory EQUD equate double word reserves 4 bytes of memory EQUS equate string reserves memory as required These operations initialise the reserved memory to the values specified by the address field The address field may contain a string in double quotes or string variable for the EQUS operation or a number or numeric variable for the other EQU operations The assembler will use the least significant part of the value if too large a value is specified 26 The example program written in Level 2 BASIC could have been written with lines 30 and 170 to 240 replaced with 4 4 4 14 14 4 4 4 DCAIG UB UV N FP Q i w EQU EQU EQU EQ EQU EQU EQU EQU EQU ONO Oe OOo amp 04 90516 amp 00C800C8 amp 00000119 amp 011 90064 amp 000000C8 amp 00000119 amp O1 OFFIC 8 0000FF 38 In Level 1 BASIC one vvay to reserve space for data vvithin the body of a machine code program is to leave the assembler using a right hand square bracket and insert the data using the
283. ontains 0 then the ESCAPE key has its normal action Otherwise treat currently selected ESCAPE key as an ASCII code 227 OSBYTE amp F6 230 FX 230 Read write flags determining ESCAPE effects lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If this location contains 0 then when an ESCAPE is acknowledged using OSBYTE amp 7E FX 126 then ESCAPE is cleared EXEC file is closed if open Purge all buffers including input buffer Reset VDU paging counter If this location contains any value other than 0 then ESCAPE causes none of these 228 OSBYTE amp E7 231 FX 231 Read write IRQ bit mask for the user 6522 lt NEW VALUE gt lt OLD VALUE gt AND Y EOR N The old value is returned in X The contents of the next location are returned in Y See User VIA chapter 24 Default value amp FF OSBYTE 2 E8 232 FX 232 Read write IRQ bit mask for 6850 RS423 lt NEW VALUE gt lt OLD VALUE gt AND Y EOR N The old value is returned in X The contents of the next location are returned in Y See serial interface chapter 20 Default value amp FF OSBYTE amp E9 233 FX 233 Read write interrupt bit mask for the system 6522 lt NEW VALUE gt lt OLD VALUE gt AND Y EOR N The old value is returned in X The contents of the next location are returned in Y See system 6522 chapter 23 Default value amp F
284. onversion is initiated by writing to the Data Latch AD start register at Sheila amp CO Bits DI and DO together define which one of the four input channels is selected Bit D3 defines whether an 8 bit resolution or a 10 bit resolution conversion should occur If set to 0 an 8 bit conversion occurs if set to 1 a 10 bit conversion occurs 8 bit conversions typically take 4 ms to complete whereas 10 bit conversions typically take 10 ms to complete Unless high resolution is required it is often better to use the fast 8 bit conversion If enabled an interrupt will be generated when the conversion is complete This indicates that valid data can be read from the ADC 429 26 1 1 Channel Summary Bit 1 Bit 0 Channel Designation control control number in MOS and BASIC Master Joystick 0 0 1 Left Right low right 0 1 2 Up Down low down Secondary Joystick 0 3 Left Right low right 1 4 Up Down low down Relevant OSBYTES which write to the ADC are amp 10 amp 11 and amp BD WRITING TO THE ADC There is one register in the analogue to digital converter which can be written to 26 1 2 Data latch and conversion start Sheila amp C0 Write only Writing to this register will select the current input channel and select an 8 or 10 bit conversion The operation of writing to this register automatically initiates a conversion BitO and Define the input channel as shown in Bit 1 section 26 1 1 Bit 2 Flag input normally se
285. otor off 450 MOTOR 0 460 REM Restore RS423 470 FX 205 0 480 REM Tidy up serial input 490 FX 2 0 500 FX 21 1 14 2 5 Reading from the cassette port Reading from the cassette port also cannot be achieved directly by reading from the RS423 system This is because the RS423 system does not use the DCD line in the same way as the cassette system The RS423 system does not expect a DCD condition to occur as the carrier is assumed always to exist and no pin connection is made for it The RS423 system thus considers a DCD interrupt as an error condition an causes an RS423 receive error event The cassette system however uses a carrier detection circuit in the ULA to detect the gaps between blocks on the cassette A block start will only be recognised as the first thing after the detection of a carrier Note that after receiving an invalid block start the motor is blipped This has the effect of breaking the carrier So a new carrier detect interrupt will be caused as soon as a 2400Hz tone is detected 313 Thus a simple event must be set up that detects a DCD interrupt In the example program it only marks the condition in a byte in the base page Another problem that occurs when reading from tape is that the data separator on the ULA chip needs the baud rate to be set to 300 baud for it to work Thus to read at 1200 baud the 6850 must convert the 300 baud clock into a 1200 baud clock This is achieved at line 420 by changing
286. p 41 To read from the slow data bus DDRA must set all lines as inputs by writing amp 00 to Sheila address amp 43 A direct read from input register A at Sheila amp 41 can then be made NOTE that any reading or writing over this slow databus will have to be done from machine code with ALL 6502 interrupts disabled This is because the interrupt routines themselves will make extensive use of the system VIA and keep changing the register values CA1 input This is the vertical sync input from the 6845 CA1 is set up to interrupt the 6502 every 20 ms 50 Hz as a vertical sync from the video circuitry is detected The operating system changes the flash colours on the display in this interrupt time so that they maintain synchronisation with the rest of the picture 417 CA2 input This input comes from the Reyboard circuit and is used to generate an interrupt whenever a key is pressed See the keyboard circuit diagram in Appendix J for more details PBO PB2 outputs These 3 outputs form the address to an 8 bit addressable latch IC32 on the main circuit diagram See the following Addressable Latch section PB3 output This output holds the data to be written to the selected addressable latch bit PB4 and PB5 inputs These are the inputs from the joystick FIRE buttons They are normally at logic 1 with no button pressed and change to 0 when a button is pressed OSBYTE amp 80 can be used to read the status of the joystick fir
287. peech PHROM or ROM filing system ROM number For the ROM filing system if it is negative it refers to a PHrase ROM and if positive to a paged ROM amp F6 and amp F7 are used as an address pointer into a paged ROM or a speech PHrase ROM These locations must be used by any paged ROM service processors for service types amp 0D and amp 0E see paged ROM section 15 1 1 and 15 4 amp F8 and amp F9 are not used by OS 1 2 amp FA and amp FB are used as general operating system workspace 271 amp FC is used as an interrupt accumulator save register This location is only used temporarily at the very beginning of an interrupt routine while it is setting up the stack amp FD and amp FE point to the byte after the last BRK instruction or to the language version string after a language has been selected See the section on the BRK vector section 10 2 for details of the standard layout of post BRK data See the paged ROMs chapter 15 for details of language ROM version strings amp FF is the escape flag Bit 7 is set if an unserviced escape is pending Programs that could hang up or take a very long time should poll this bit and exit if it is set The tidiest way to perform such an exit is to execute a BRK with error number amp 11 and the message Escape 11 2 Page one amp 100 amp 1FF Page one is used by the 6502 stack Locations amp 100 upwards are also used by some service paged ROMs to save error messages in
288. pen file The sequential pointer always points to the next byte to be read or written In the disc filing system it is possible to change the value of this pointer to give random access within a file Only serial access is available with the tape filing system 16 1 2 Directories On the disc and network filing systems each file is resident in a directory A file is identified by its directory followed by its name Directories are separated from filenames by a full stop For example in A NAME A is the directory name and NAME is the file name For convenience the user may specify a current directory which is assumed if the directory name is omitted from the file identification 333 The user may also specify a library directory which is similar to a current directory The library directory is searched for unrecognised commands unless a different directory is included in the command name 16 1 3 Cycle numbers The network and disc filing systems also use a cycle number associated with the disc The cycle number represents the number of times that a particular disc catalogue has been written to It is of use in helping to identify a disc with greater reliability 16 1 4 Filing systems A filing system is a program that manages files on a particular storage medium Filing systems are entered through vectors held in page 2 of memory Filing systems upon selection must provide a series of seven
289. played Character rows Nvd on figure 18 1 on the CRT screen and is programmed in character row times Mode 0 1 2 3 4 5 6 7 character lines 32 32 32 25 32 32 25 25 18 5 4 Vertical sync position R7 This 7 bit write only register determines the vertical sync position with respect to the reference It is programmed in character row times Mode 0 1 2 3 4 5 6 7 Sync position 34 34 34 27 34 34 27 27 363 18 6 Interlace and delay register R8 This 6 bit write only register controls the raster scan mode and cursor display delay The interlace options are 18 6 1 Interlace modes bits 0 1 Interlace mode Description register Bit1 Bit 0 0 0 Normal non interlaced sync mode figure 18 2a 1 0 Normal non interlaced sync mode figure 18 2a 0 1 Interlace sync mode figure 18 2b 1 1 Interlace sync and video figure 18 2c All BBC microcomputer screen modes are interlaced sync only except for mode 7 which is interlaced sync and video The default values can easily be changed using TV FX 144 followed by a 0 to turn interlacing on or a 1 to turn interlacing off SCAN LINE ADDRESS 000 oo 2 __ ______ Figure 18 2a Normal Sync 364 SCAN LINE ADDRESS _ eee Figure 18 2b Interlace Sync SCAN LINE ADDRESS eee 7 _ _ Figure 18 2c Interlace Sync amp video 18 6 2 Display blanking delay bits 4 5 Bits 4 and 5 control t
290. pointer Get media title and boot up option The data returned is Single byte giving the length of the title The title in ASCII character values Single byte start option The start option meaning is filing system dependant Read the currently selected directory and device Single byte giving the length of device identity Device identity in ASCII characters Single byte giving the length of directory name Directory name in ASCII characters The device identity is the name of the device containing the directory On the disc filing system it is the drive number but is not applicable on the network filing system so its length is zero Read the currently selected library and device The data format is the same as that used for A 6 Read file names from the current directory The control block is modified so that the file handle byte contains the cycle number see section above 16 1 3 and the sequential pointer is adjusted to ensure that the next call with A 8 gets the next file name On entry the number of bytes to transfer is interpreted as the number of file names to transfer for the first call the sequential pointer should be zero The data returned is Length of filename 1 Filename 1 Length of filename 2 Filename 2 341 The requested transfer cannot be completed if the end of the file has been reached or there are no more file names to be transferred In this case the C flag is set on exit If a trans
291. presenting 0 all bits clear e g Using 8 bits to store a value 5 00000101 5 11111010 111111011 and 5 5 11111011 5 00000101 5 00000000 0 ignore the carry from the last bit Numbers in the range 128 10000000 to 127 01111111 can be represented using 8 bit 2 s complement values Using 2 s complement arithmetic the same addition and subtraction operations work identically on negative and positive numbers Negative numbers can be always be recognised by the state of the most significant bit this is always set for negative numbers 31 The 6502 microprocessor can only perform its arithmetic operations using 8 bit values This limitation can lead to errors when a carry is generated on the most significant bit so that the result cannot be stored in 8 bits The sign bit may also be wrongly changed when a carry occurs into it Two flags in the status register are set when certain conditions occur These flags are the carry flag and the overflow flag The carry flag is set when a carry is generated during an addition operation if a carry is generated from bit 7 i e the carry flag is a ninth bit of the result The carry flag is cleared if a borrow occurred into bit 7 during a subtraction The addition and subtraction instructions on the 6502 include the carry bit in the operation Using the carry bit makes it possible to perform multi byte arithmetic The examples for ADC and SBC in the mnemonics section illustrat
292. processing the operating system may generate a number of events which hand the processor over to an event handling routine provided by the user Thus the user is able to append some code of his own to the operating system s interrupt handling routine The events available are event number cause of event 0 Output buffer becomes empty Input buffer becomes full Character entering input buffer ADC conversion complete Start of vertical sync Interval timer crossing zero ESCAPE condition detected RS423 error detected Econet generated event User event WO COND OI HB WN Mm 267 12 1 OSBYTE calls amp D FX 13 and amp E FX 14 Disable enable event These calls are used to switch particular events on and off On entry both these calls require the event to be specified by its event number in X OSBYTE amp D FX 13 can be used to disable a particular event and OSBYTE amp E FX 14 can be used to enable each event Even though events continue to be generated when disabled they will not be passed on to the event handling routine 12 2 The Event Vector EVNTV Address amp 220 When any event is enabled the operating system jumps using JSR to the address contained in EVNTV To interface the user s event handling routine the entry address of the routine must be placed in the EVNTV 12 3 Event handling routines The user s event handling routine is entered with the accumulator containing the event number Other information may als
293. put OSWORD Link options Load file OSFILE Location counter in assembler programs Logical colours LPSTB signal LSR Logical Shift Right M Machine code Arithmetic Macros in assembler programs Maskable interrupts Masks for interrupts Memory mapped for I O reading and writing from Memory refresh Memory usage Messages default extended filing system Mnemonic Mnemonics MODE 8 implementation 25 267 166 243 279 327 325 328 495 74 75 76 495 161 140 368 369 418 248 482 335 25 380 418 77 495 31 29 296 229 170 359 267 13 13 18 18 495 43 383 N Negative flag Negative numbers in machine code Net printer station identity register NETV NMI handling routine address paged ROM service calls zero page work space Noise generator Non maskable interrupts NOP No operation 0 On error abort prompt for re entry One megahertz bus cleaning up FRED for peripherals hardware requirements introduction JIM memory expansion signal definitions timing Opcode Open collector Open files OSBGET OSBPUT OSFILE OSGBPB Opening files OSFIND Operand Operating system OSBYTES version number Operating system GSINIT OSREAD input non vectored OSRDCH non vectored OSWRCH OSASCI OSCLI OSEVEN OSNEWL OSRDCH OSRDRM OSWRCH output VDU character output 42 31 121 260 428 260 296 281 323 267 421 296 78 18 18 444 437 44
294. r USERV amp 200 1 The user vector exists to allow the user some expansion facilities without needing to take full control of one of the main vectors Two operating system commands CODE and LINE are passed through the user vector as are 32 OSWORD calls When a routine which is pointed to by the user vector is entered the contents of the accumulator describe the type of entry required 256 A 0 CODE has been executed or OSBYTE amp 88 X and Y contain the two parameters See OS commands section 2 6 A 1 LINE has been executed X and Y point to the rest of the command line See OS commands section 2 11 A amp E0 amp FF OSWORD has been entered with the accumulator in the range amp EO amp FF Registers as on entry to OSWORD 10 2 The break vector BRKV amp 202 3 The break vector is primarily used to trap errors On entry to the break vector the following conditions prevail a The registers A X and Y are unchanged from when the BRK instruction was executed b The stack is prepared ready for an RTI instruction to return to the instruction following the BRK instruction For this purpose the BRK instruction is taken as a two byte instruction ie the instruction pointed to is two bytes after the BRK instruction This is because many of the 6502 instructions that might be replaced by a BRK instruction are two bytes long c Locations amp ED and amp FE contain the address of the byte after the BRK instructio
295. r definition explosion state Read vvrite cassette ROM filing system switch Read RAM copy of video ULA control register Read RAM copy of video ULA palette register Read vvrite ROM number active at last BRK error Read write number of ROM socket containing BASIC Read current ADC channel Read write maximum ADC channel number Read ADC conversion type Read write RS423 use flag Read R5423 control flag Read write flash counter Read write mark period count Read write space period count Read write keyboard auto repeat delay Read write keyboard auto repeat period Read write EXEC file handle Read write SPOOL file handle Read write ESCAPE BREAK effect Read write Econet keyboard disable 202 CA Read write keyboard status byte 203 CB 204 CC Read write RS423 handshake extent Read write RS423 input suppression flag 113 205 CD 206 CE 207 CF 208 DO 209 DI 210 D2 211 D3 212 D4 213 D5 214 D6 215 D7 216 D8 217 D9 218 DA 219 DB 220 DC 221 DD 222 DE 223 224 225 Fl 226 227 E3 228 229 230 231 232 233 234 235 236 237 ED 238 FE 239 EF 240 FO 241 F1 242 F2 243 F3 244 F4 245 F5 114 Read write cassette RS423 selection flag Read write Econet OS call interception status Read write Econet OSRDCH interception status Read write Econet OSWRCH interception status Read write speech suppression status Read write sound suppression status Read write BELL channel Read write BELL envelope num
296. r each pixel in a display byte to be changed The leftmost pixel is the highest bit Index this table with the number generated to find the mask If this mask were stored directly in screen memory all the changed pixels will be in colour 3 and all the unchanged pixels in colour 0 By simple masking operations with the AND ORA and EOR instructions the mask can be used to only change those bits required or to set a screen byte to an appropriate mix of chosen foreground and background colours 282 amp C32F amp C332 is a similar lookup table to that at amp C31F amp C32E but is used for sixteen colour modes The table is only four entries long as only two pixels can be fitted into a byte The mask byte given in the table contains colour 15 for set bits in the index and colour 0 for cleared bits amp C333 amp C374 contains an address table for decoding VDU codes 0 through 31 This should not be used by the user as it is even more prone to changes than the other tables amp C375 amp C3B4 this contains 32 entries of a 640 multiplication table The high byte of each entry is held first This table can be used to find the address of the first byte of the first character on each screen row in the 20K and 16K modes and by dividing by 2 for 10K and 8K modes amp C3B5 amp C3E6 this contains 25 entries of a 40 multiplication table The high byte of each entry is held first This table can be used to find th
297. r has a number n such that the vector normally exists at location amp 0200 2 n Any vector can be made to point into a sideways ROM by a Making the main vector at location amp 0200 2 n point to amp FF00 3 n This is the operating system s entry point for processing extended vectors b Ascertaining the extended vector space by issuing OSBYTE with A amp A8 X amp 00 Y amp FF this returns in X and Y the address of the extended vector space Call this address V In OS1 20 V amp 0D9F c Setting the extended vector at location V 3 n to address in ROM low byte address in ROM high byte ROM number held in amp F4 326 15 2 Languages A language on the BBC microcomputer is not necessarily a language at all but could be any self contained machine code program that is independent of language Examples are Text editors and vvord processors Terminal emulators Teletext systems Languages can only be started up by issuing an OSBYTE call with A amp 8E X ROM number excepting BASIC which is a special case This is normally inconvenient and dependant on ROM position A service entry should therefore be provided which uses the unrecognised command entry service type 4 to recognise the language name as a command Upon recognising the language name the ROM should issue the OSBYTE call to properly start up the language When starting up a language the operating system does the following a Records the ROM number to resele
298. r interrupt causes a byte to be taken from the 6850 and stored in memory A Data Carrier Detect interrupt is used to mark the end of a data block when skipping to find files or during a cataloguing process 13 8 2 The RS423 serial system The RS423 system uses the interrupts in the following ways A transmitter interrupt causes a character to be sent to the 6850 from the RS423 transmit buffer or the printer buffer if the RS423 printer is selected If both buffers are empty the RS423 system is flagged as available see OSBYTE amp BF and transmitter interrupts are disabled A receiver interrupt is used to cause a character to be read from the 6850 and inserted into the RS423 receive buffer if enabled by use of OSBYTE amp 9C If there is a receive error which can be ignored by use of OSBYTE amp E8 event number 7 is generated and the character is ignored The character is also ignored if OSBYTE amp CC has been made non zero The RTS line is pulled high if the receive buffer is getting full The number of characters which need to be in the buffer to cause RTS to go high is set by OSBYTE amp CB A DCD interrupt cannot occur unless the RS423 has been switched to the cassette connector by use of OSBYTE amp CD The DCD interrupt is normally cleared by reading from the 6850 receive register An event number 7 RS423 receive error event is then generated 301 The RS423 system can be made to ignore any of the above interrupts by us
299. rcuits to add on to the one megahertz bus the Not page amp FC NPGFC signal together with the lower 8 address lines should be decoded to select the add on circuit Note that a clean up circuit will be required on the NPGFC signal in most applications This is described in section 28 5 For very keen constructors who require more than the 63 page amp FC locations reserved for User Applications either page amp FD can be used for memory mapped peripherals or other FRED locations can be used Using reserved FRED locations in this way will mean that the hardware add ons specified for those locations cannot be added in future if user hardware is already using the slot 28 3 JIM and 64K Paged Memory 28 3 1 General description of JIM Page amp FD in the BBC microcomputer address space is used in conjunction with the paging register in FRED to provide an extra 64K of memory This memory is accessed one page at a time The particular page being accessed is selected by the value in FRED s paging register and is referred to as the Extended page number Note that a Not page amp FD NPGFD signal is available on the one megahertz bus connector Accessing memory through the 1MHz bus will generally be about twenty times slower than accessing memory directly 28 3 2 Extended page allocation Extended pages amp 00 amp 7F in JIM are reserved for use by Acorn The other pages amp 80 amp FF are reserved for user
300. red databus and the lower 8 bits of the address bus are connected to this socket together with a series of useful control signals Whilst the designer could use the one megahertz bus in innumerable different configurations Acorn has defined how the bus should be used to maintain compatibility with other devices The standard uses of the one megahertz bus allow up to 64K bytes of paged memory to be used as well as 255 direct memory mapped devices plus the paging register FRED is normally assigned as the memory mapped I O page and JIM is normally assigned as the 64K memory expansion page Communication between FRED JIM and programs should be implemented using OSBYTEs amp 92 amp 93 amp 94 and amp 95 28 2 FRED and Memory Mapped Hardware Page amp FC in the BBC microcomputer is reserved for peripherals with small memory requirements The initial allocations of space in FRED are amp FC00 amp FCOF Test Hardware amp FC10 amp FC13 Teletext amp FC14 amp FCIF Prestel amp FC20 amp FC27 IEEE 488 Interface amp FC28 amp FC2F Acorn Expansion currently unused 437 amp FC30 amp FC3F Cambridge Ring Interface amp FC4O amp FC47 Winchester Disc Interface amp FC48 amp FC7F Acorn Expansion currently unused amp FC80 amp FC8F Test Hardware amp FC90 amp FCBF Acorn Expansion currently unused amp FCCO amp FCFE User Applications amp FCFF Paging Register for JIM When designing ci
301. ress Cursor position Light pen position R Q Es gon D amp 28 A wa A Hi MN R GC oOo Oo amp 26 amp 00 WO Ow XX xx amp 00 amp amp oo io NS NA NA N Ce a amp 67 amp 07 3 N N 41 l SL E a E E a LL LS LL LAN LO LL LT AN Q Aa amp amp O m Gy El amp 08 8 Variable Variable Variable MODE 5 RS EE CIE RE gn ee en ee ne eee Bes gop Sha ihe E Siete aN EE DE CEE ee aah et ey eae EE Eril ae a ta AM LOE SORT ess a 85940 e m ss ee u a amp 7D86 TAR TABER EE EEE Oe TA datos dant amp 7ECO a A EE SEC EE a a EE ache ca ae De DE ea Ca ey a a IR ot seis Ne eee Sle ca seen a ie ee a recs 8 7ECD ae Pi ee ate Sa ce oe Sh 4 PIXELS 2 BITS PIXEL N B The screen layout is only as shown after a CLS it will change when the screen is hard scrolled amp 4000 should be subtracted for each screen location on a model A 473 MODE 6 Screen layout Graphics Colours Text Registers RO R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 474 Not available 2 40 x 25 Horizontal total Characters per line Horizontal sync position Horizontal sync vvidth Vertical sync time Vertical total Vertical total adjust Vertical displayed characters Vertical sync position Interlace mode bits 0 1 Display delay bits
302. ressing is indicated to the assembler by enclosing the address within brackets e g 5 LDA te 40 load accumulator with 8440 STA amp 1900 store low byte of indirection LDA amp 28 load accumulator with amp 28 STA amp 1901 store high byte of indirection JMP amp 1900 goto address in amp 1900 and amp 1901 N B A JMP amp 2840 instruction would have been more sensible in this case There is a bug in the 6502 When the indirect address crosses a page boundary the 6502 does not add the carry to calculate the address of the high byte i e JMP amp 19FF will use the contents of amp 19FF and amp 1900 for the JMP address Indexed Addressing The following 5 addressing modes use the X or Y registers as an offset which is used to modify another address specified in the operand field These addressing modes give the program access to a table of memory locations specified in terms of a base address to which is added the 8 bit offset value 5 7 Absolute X or Y addressing using an absolute address X These are the simplest indexed addressing modes An absolute 16 bit address is specified in the operand field This should be followed by acomma and either X or Y The address used by the instruction will be the 16 bit address the contents of the register specified 37 The X and Y register contents are always taken as positive values in the range 0 to 255 and so only forward offsets are available c f Relative
303. restore all files still open when the filing system shut down Tube system post initialisation This call is always issued after OSHWM has been set up It is intended to allow the Tube system to explode the character set or make other use of the main memory On systems without the Tube this call can be used for nefarious break trapping Tube system main initialisation This call is issued only if the Tube hardware has been detected and is called prior to message generation and filing system initialisation 15 1 2 The language entry point The language entry point is used to start up a language and should only be entered with amp 01 in the accumulator on the BBC microcomputer No return is expected from a language The language entry point is always entered with a JMP instruction The stack state on entry is undefined and the stack pointer should be reinitialised 325 The actual entry codes in the accumulator are 0 No language present on break No language ROM is entered this vvay but the Tube language entry point might be 1 Normal language start up 2 Electron only Request next byte of soft Rey expansion Key number set by call with A 3 Byte out in Y 3 Electron only Request length of soft key expansion Key number in Y Length out in Y 15 1 3 Vectored entry into paged ROMs As many filing systems are paged ROM resident a mechanism has been provided for changing vectors to point into paged ROMs Each vecto
304. rior to closing the SPOOL and EXEC files when changing filing systems This call is made primarily to allow any users of these files to tidy them up prior to their closure with the disappearance of the filing system If a ROM responds to this call and returns with the accumulator zero the SPOOL and EXEC files will not be closed Care is necessary to prevent accidental access to files belonging to another inactive filing system 11 12 FE FF Font implosion explosion warning Each time the fonts are exploded or imploded the high water mark will change This call exists to inform languages that the high water mark has changed and that the Y register contains the new value This call should be used to get your precious data out of the way before being destroyed by the character set BASIC it should be noted ignores this warning as it has no service entry Initialise filing system If a program is transferring files from one filing system to another it may need to have files open in more than one filing system To do this filing systems need to be re activated before an access all filing systems allow files to be open whilst inactive This call exists to initialise a filing system without the fuss of issuing operating system commands A filing system should respond to this call if the value in the Y register is its identification number for filing system numbers see OSARGS section 16 3 The filing system should initialise and
305. roblem However if reading from or writing to a device has some other function such as clearing an interrupt flag a problem may occur 28 5 3 Clean up circuit 1 This standard clean up circuit for the page select signals is shown in figure 28 3 Three NOR gates are used to create a standard R S flip flop with a gated input The clean page select output CNPGFC1 can only be set low if IMHZE is low The net effect of the circuit is illustrated in figure 28 2 Both of the problems outlined above are overcome since the P glitches are removed and the page select only goes low at 5 after the IMHZzE clock has gone low The Q glitches due to spurious addresses whilst 1MHZE is low are still there In most applications this will not affect circuit operation but occasionally a totally glitch free page select will be required Circuit 2 will provide this type of page select 444 1MH4E CLEAN NPGFC NPGFC or CLEAN NPGFD or NPGFD 74LS02 CIRCUIT TO REMOVE GLITCHES FROM PIN CONNECTIONS NPGFC OR NPGFD ON THE 1MH3BUS Figure 28 3 Clean up circuit 1 445 28 5 4 Clean up circuit 2 In situations in which a 100 clean page select signal is required circuit 2 which is illustrated in figure 28 4 should be used Before CNPGFC can go low a valid page address with 1MHZE low must occur The page low is then latched into a D type flip flop on the rising edge of the 1MHZE clock
306. routine Any routine terminated in this way should be called using a JSR instruction which places a return address on the stack The top two stack values are placed in the program counter and execution is resumed at the point in the program after the JSR instruction During a subroutine the same number of items pushed on the stack must be removed before the RTS instruction is reached if the subroutine is to return to the correct address Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing Mode Bytes used Cycles implied 1 6 Example Last instruction in a subroutine ste body of subroutine RTS return to calling routine 87 SBC Subtract memory from accumulator with carry A C A M 1 C This instruction subtracts the contents of the specified memory from the accumulator contents leaving the result in the accumulator If the carry flag is used as a borrow source and if clear then an extra unit is subtracted from the accumulator This enables the borrow to be carried over in multi byte subtractions see example below Processor Status after use C carry flag cleared if a borrow occurs Z zero flag set if result 0 I interrupt disable not affected D decimal mode flag not affected B break command
307. rs parameters are placed in a parameter block the address of which is sent to the OSWORD routine in the X low byte and Y high byte registers 9 1 OSWORD OS call specified by contents of A taking parameters in a parameter block Call address amp FFF1 Indirected through amp 20C On entry A selects an OSWORD routine X contains low byte of the parameter block address Y contains high byte of the parameter block address OSWORD calls which are called with accumulator values in the range amp EO 224 to amp FF 255 are passed to the USERV amp 200 The routine indirected through the USERV is entered with the register contents unchanged from the original OSWORD call See Vectors section 10 1 and Operating system commands sections 2 6 and 2 11 for more information about the user vector Other unrecognised OSWORD calls are offered to the paged ROMs see Paged ROM section section 15 1 1 reason code 8 OSWORD summary Read line from currently selected input into memory Read system clock Write system clock Read interval timer Write interval timer Read byte of I O processor memory Write byte of I O processor memory Perform a SOUND command Define an ENVELOPE ll OU NBEO ll CON ON OI OE En N 247 9 Read pixel value RA Read character definition amp B_ Read palette value for a given logical colour amp C Write palette value for a given logical colour ASED Read previous and current graphic
308. rs to specific tasks allows both systems to operate at maximum efficiency For example complex VDU plot and colour fill commands can be sent via the largest FIFO from the parasite to the host processor The parasite processor will not then have to wait for the host to finish processing the laborious VDU commands before continuing with its language processing 27 2 Some second processors and their uses Currently three second processors can be used on the Tube These are a 6502 Z80 and 16032 27 2 1 The 6502 Second processor The 6502B on the Tube is a faster 3MHz version of the 2Mhz 6502A used in the main BBC microcomputer It is provided with a full 64K of RAM When the machine is powered up the default language is copied across from the main BBC to the Tube processor From then on the main BBC microcomputer 6502A is turned into a servant Its purpose is that of handling all input from the R5423 keyboard joysticks etc and output to the screen sound generator etc It sends its input across the Tube to the fast 6502B and executes laborious tasks which the fast 6502B sends back All of the processing for languages or applications packages are performed by the Tube processor The advantage is that the work load is shared out Because the fast 6502B doesn t have to worry about interrupts from any of the devices connected to a BBC microcomputer it can process at a very fast speed The old 6502A does all output like plotting graphics on th
309. s Keyboard circuit diagram Main circuit diagram Bibliography Glossary Index 353 359 377 385 395 397 417 425 427 429 433 437 449 455 456 459 460 462 478 480 482 489 Inside Back Cover 491 493 499 Introduction The Advanced User Guide for the BBC Microcomputer has been designed to be an invaluable supplement to the User Guide Information already contained in the User Guide is only repeated in this book in sections which contain much new information and where omitting the duplicated details would have left the section incomplete Some parts of the User Guide are factually inaccurate or incomplete and where details in this book are at variance with corresponding information in the User Guide the reader will find that a more accurate description is usually found in these pages This reference manual contains a considerable amount of information about 6502 assembly language programming the operating system and the BBC Microcomputer hardware The intention has not been to provide the inexperienced user with a tutorial to guide him or her through the complexities of these advanced concepts However it is hoped that the information has been presented in a way that enables users new to assembly language programming and unfamiliar with hardware topics to develop their understanding of the machine and to expand the scope of their programming Contained within this book is an extensive description of the s
310. s debugging aids file utilities etc Hardware drivers for personalised hardware on the 1MHz bus or the lightpen A paged ROM must be recognised by the operating system To be recognised the first few bytes must conform to 00 02 JMP language entry 03 05 JMP service entry 06 ROM type 07 Copyright offset pointer nn 08 Binary version number 09 Title string printed on selection as a language VV Optional version string preceded by amp 00 nn nn 3 amp 00 amp 28 amp 43 C 8229 317 nn 4 Copyright message XX Copyright message terminator 800 xx 1 xx 5 If applicable second processor relocation address a The language entry is called upon initialising the ROM as a language and after copying over to the second processor if applicable b The service entry is called regularly whenever a service is needed by the MOS c The ROM type is a flag byte informing the MOS as to what the ROM is expected to do bit 7 bit 6 bit 5 bit 4 bit 1 If set this indicates that the ROM has a service entry Note that a ROM with no service entry is assumed to be the BASIC ROM ALL user ROMs should have a service entry If set this indicates that the ROM has a language entry and wishes to be considered for selection on a hard reset If not set the ROM may still have a language entry but it must be started up by an operating system command such a language if selected will still be s
311. s 1 5 half of a displayed character Mode 2 quarter of a displayed character Note that the screen should be cleared before using a light pen and not scrolled whilst the pen is in use If it is scrolled the position of the start of the screen will have to be taken into account as well 18 10 Displayed screen start address register R12 R13 This 14 bit write only register determines the location in memory which corresponds to the upper left hand character displayed on the screen R13 8 bits is the low order address and R12 6 bits is the high order address It can often be useful to know what the current contents of this register are Unfortunately being a write only register it is not possible to read the value directly However OSBYTE amp AO can be used to get these parameters from the operating system workspace in page amp 03 The start of screen address is stored in locations amp 350 and amp 351 Call OSBYTE amp A0 with X amp 30 The contents of amp 350 will be returned in the X register and the contents of amp 351 will be returned in the Y register Note that the actual screen start address must be divided by 8 before being sent to R12 R13 because there are 8 lines per character modes 0 6 In mode 7 a rather more complex correction has to be applied See section 18 11 3 on mode 7 scrolling at the end of this chapter The ability to define the start of the screen to be anywhere in memory is very useful because it allows fas
312. s already present on the computer and those who wish to add their own hardware to the computer All of the standard hardware features available on the BBC microcomputer are therefore outlined in detail from a programmer s point of view Wherever possible it is better to use operating system routes for controlling the hardware These are very powerful and will be referred to whenever relevant In certain specialised cases it is necessary to directly access hardware but even in such cases OSBYTES amp 92 amp 97 should be used This will ensure that the software will still operate on machines fitted with a Tube processor For those who wish to add their own hardware full details on using the USER port and 1MHz BUS are supplied The hardware on the BBC microcomputer consists of a large quantity of integrated circuits resistors capacitors transistors and various other electronic components All of these are shown on the full circuit diagram inside the back cover of this book In order to help those who are not familiar with the general layout of a computer circuit and the devices attached to it the rest of this introduction is devoted to analysing the hardware as a series of discrete blocks interconnected by a series of system buses 353 THE SYSTEM BLOCR DIAGRAM Fig 17 1 EE YH HOF U3QOON3 Tid SOLV INQOM BE Es 1U0d H3SN gO SS3J0Nd OHAIN Yz059 1Xx313731 osos win BOSS3508Hd 1vIH3S 311385V2 u3 TOH
313. s cursor positions 9 2 OSWORD call with A amp 0 Read line from input This routine takes a specified number of characters from the currently selected input stream Input is terminated following a RETURN or an ESCAPE DELETE amp 7F 127 deletes the previous character and CTRL U amp 15 21 deletes the entire line If characters are presented after the maximum line length has been reached the characters are ignored and a BEL ASCII 7 character is output The parameter block XY 0 Buffer address for input LSB 1 MSB 2 Maximum line length 3 Minimum acceptable ASCII value 4 Maximum acceptable ASCII value Only characters greater or equal to XY 3 and lesser or equal to XY 4 will be accepted On exit C 0 if a carriage return terminated input C 1 if an ESCAPE condition terminated input Y contains line length including carriage return if used 9 3 OSWORD call with A amp 1 Read system clock This routine may be used to read the system clock used for the TIME function in BASIC The five byte clock value is written to the address contained in the X and Y registers This clock is incremented every hundredth of a second and is set to 0 by a hard BREAK 248 9 4 OSWORD call with A amp 2 Write system clock This routine may be used to set the system clock to a five byte value contained in memory at the address contained in the X and Y registers 9 5 OSWORD call with A amp 3 Read interval timer This routine may be used to
314. s substantially different from a higher level language such as BASIC The machine code programmer is limited to three registers for temporary storage of data while in BASIC he has unlimited use of variables For more permanent storage in machine code programs the register values can be copied to bytes of memory Only very limited arithmetic is available there are no multiply or divide instructions There are no automatic loop structures such as FOR NEXT or REPEAT UNTIL and any loops must be explicitly set up by the programmer using conditional branches these approximate to IF THEN GOTO in BASIC The range of instructions available are sufficient to enable extremely complex programs to be written but a lot more effort is required to implement the program One of the most grave disadvantages of machine code is that very little error checking is made available to the programmer A well designed assembler will help the programmer but once the machine code program is running the only error checking is that which is provided within the program itself At first glance it may appear that there is little to be gained from writing a machine code program The principal advantage is that of speed While assigning a value to a variable in BASIC will take about 1 millisecond in machine code assigning a similar value will only take 10 microseconds This is why fast moving arcade games have to be written in machine code Some of the facilities a
315. s the VDU queue The queue is organised so that whatever the number of characters queued the last byte queued is always at amp 323 amp 324 amp 327 contain the current graphics cursor in internal co ordinates amp 328 amp 349 is used as general graphics co ordinate workspace amp 34A and amp 34B contain the text cursor position as an address as sent to 6845 amp 34C and amp 34D contain the text window width in bytes ie the number of characters wide the number of horizontal bytes per character 8 for graphics modes or 1 for teletext This is used to control the number of bytes which are soft scrolled for each line of scrolling amp 34E contains the high byte of the address of the bottom of screen memory amp 34F contains the number of bytes of memory taken up by a single character This is 8 for 2 colour modes 16 for 4 colour modes 32 for 16 colour modes and 1 for teletext mode amp 350 and amp 351 contain the address of the top left hand corner of the displayed screen as is sent to the 6845 276 amp 352 and amp 353 contain the number of bytes taken per character row of the screen This is 40 for teletext mode 320 for 8K and 10K modes and 640 for 16K and 20K modes amp 354 contains the high byte of the size of the screen memory in bytes amp 355 contains the current screen mode amp 356 contains the memory map type The contents indicate the size of the screen memory It has the value 0 for 20K modes
316. se ROM logical number storage Physical colours Pixel value OSVVORD PLA Pull A from stack PLOT numbers expansions summary PLP Pull Status from stack POINT read pixel value Poll POS Post indexed indirect addressing Power up Pre indexed indirect addressing Printer buffer status destination flag empty printer buffer ignore character ignored character networked output enabled by VDU 2 port select output destination sections for output used defined Printer driver going dormant warning Printers RS423 Program counter Put Byte OSBPUT 322 322 322 326 324 319 319 251 252 379 496 121 28 434 25 496 80 81 271 382 250 82 261 460 83 250 496 158 39 356 38 151 239 138 122 240 260 139 425 121 119 258 146 309 42 339 R RAM RAM available Raster scan display Read byte from an open file OSBGET OSGBPB Read byte in paged ROM Read character OSRDCH Read character from string Read file attributes OSAGS OSFILE Read I O processor memory Read line OSWORD Read user flag Refresh Register Relative addressing Relay for cassette motor REMV REPORT Reset soft keys ROL Rotate Left Rollover Rollover on keyboard input ROM current language ROM number number containing BASIC ROM active at last BRK ROM filing system address pointer data format logical ROM number storage paged ROM service calls software select switch ROM information table ROM pointer table ROR Rotate Ri
317. sent indicates to the tube system that the language part of the contents of this ROM have been assembled to an address other than amp 8000 The address given is the address to which the program must be copied in the second processor The service call code should still be assembled into the amp 8000 space because for service entries the ROM is executed within the I O processor 15 1 Entering Paged ROMs Paged ROMs are entered via one of three methods by a service call via an extended vector or through the language entry point Generally a ROM should never be executed at all until it has responded to at least one service call the exception to this is BASIC which has no service entry point Always when within a paged ROM location amp F4 contains the ROM number of the ROM being executed 319 15 1 1 Service Call entries When the paged ROMs are asked to provide a service the highest priority ROMs the ones in the sockets to the right of the board are offered the chance first They are entered at the service entry point with the registers set up thus A Service type requested X ROM number of the current ROM Y Any parameter required for the service If a ROM wishes to issue further service calls to other ROMs for example to claim memory it should issue such calls using OSBYTE call amp 8F with the service type in the X register and the parameter in the Y register If a ROM does not wish to provide the service it should exit
318. should do next A variety of different instructions exist on the 6502 The basic functions available are reading or writing data to memory or an input output device and performing arithmetic and logical operations on the data Once the MOS machine operating system program is entered this piece of software gains full control of the system SHEILA and the system hardware All of the main blocks connected to the 6502 in the block diagram figure 17 1 together form the system hardware In 6502 systems the hardware is memory mapped which means that any hardware device registers appear in the main memory address space Page amp FE the 256 bytes of memory starting at amp FE00 is reserved especially for the system hardware in the BBC microcomputer The special name of SHEILA has been assigned to this page of memory Two other special pages are amp FC called FRED and amp FD called JIM FRED and JIM are concerned with external user hardware attached to the one megahertz bus They are dealt with in chapter 28 on the one megahertz bus In the following chapters all of the devices attached to Sheila are described in detail The table below shows the memory map of Sheila the function of the devices attached to it and the sections in which they are described 356 SHEILA Integrated Description address circuit offset from amp FE00 amp 00 amp 07 6845 CRTC Video controller amp 08 amp 0F 6850 ACIA Serial controller
319. side for the user event This is most usefully generated from a user interrupt handling routine to enable other user software to trap an interrupt easily e g an event generated from an interrupt driven utility in paged ROM An event may be generated using OSEVEN see section 7 11 293 294 13 Interrupts 13 1 A brief introduction to interrupts An interrupt is a hardware signal to the microprocessor It informs the 6502 that a hardware device somewhere in the system requires immediate attention When the microprocessor receives an interrupt it suspends whatever it was doing and executes an interrupt servicing routine Upon completion of the servicing routine the 6502 returns to whatever it was doing before the interrupt occurred A simple analogy of an interrupt is a man working hard at his desk writing a letter a foreground task Suddenly the telephone rings an interruption The man has to stop writing and answer the telephone the interrupt service routine After completion of the call he has to put the telephone down and pick up his writing exactly where he left off return from interrupt In a computer system the main objective is to perform foreground tasks such as running BASIC programs This is equivalent to writing the letter in the above example The computer may however be concerned with performing lots of other functions in the background equivalent to the man answering the telephone A computer which is ru
320. size amp 2E1 amp 2E9 are the buffer end indices They contain the offset of the last byte to be entered into each buffer If this value is the same as the start offset the buffer is empty If this value is less than the start offset it means the buffer has wrapped around to the start amp 2EA and amp 2EB contain the block size of currently resident block of the open cassette input file amp 2EC contains the block flag of the currently resident block of the open cassette input file see section 16 10 for the cassette format and details of the flag byte amp 2ED contains the last character in currently resident block of the open cassette input file amp 2EE amp 2FF are used as an area to build OSFILE control blocks for LOAD and SAVE 11 4 Page three amp 300 amp 3FF Page three is used for the VDU workspace the cassette system workspace and the keyboard buffer Locations amp 300 amp 37F provide the VDU workspace In examining these locations it should be noted that there are two forms of graphic co ordinate internal and external The external graphics co ordinate is exactly that used by the PLOT 274 command in BASIC The internal graphics co ordinate is derived from the external by taking into account the graphics origin and scaling so that it is measured in pixels horizontally and vertically Graphics co ordinates are stored in four bytes with the low byte of the X co ordinate first VDU workspace is laid o
321. specialised pieces of hardware which require very fast response from the 6502 NMIs are not used on a standard system They are used in disc and Econet systems An NMI causes a jump to location amp 0D00 to be made 13 4 Using Maskable Interrupts Most of the interrupts on the BBC microcomputer are maskable This means that a machine code program can choose to ignore interrupts if it wishes by disabling them Since all of the operating system features such as scanning the keyboard updating the clock and running the serial system are run on an interrupt basis it is unwise to disable interrupts for more than about 2ms There are two levels of priority for maskable interrupts defined by two indirection vectors in page amp 02 The priority of an interrupt indicates its relative importance with respect to other interrupts If two devices signal an interrupt simultaneously the higher priority interrupt is serviced first 13 5 Interrupt Request Vector 1 IRQ1V This is the highest priority vector through which all maskable interrupts are indirected This is nominally reserved for the system interrupt processing routine This operating system 298 routine handles all anticipated internal IRQs Anticipated IRQs include interrupts from the Reyboard system VIA serial system and the analogue to digital converter Any interrupt vvhich cannot be dealt vvith by the operating system routine such as an interrupt from the user VIA or a piece of sp
322. st character has been written to the TDR the Status register can be checked for a Transmit Data Register Empty condition If the register is empty another character can be loaded for transmission even though the first character is in the process of being transmitted The second character will automatically be transferred into the Shift Register when the first character transmission is complete 20 5 Receive data register RDR Sheila amp 09 read only Data is received from a peripheral via the cassette or RS423 serial interfaces A divide by 64 clock ratio is provided to select the baud rate from the serial ULA and divide by 16 or 1 ratios are provided as well The 6850 waits until a full half of the start bit has been received before it synchronises its clock to the bit time As a character is being received parity odd or even will be checked and any errors will be available in the Status register When parity has been selected for an S bit word 7 bits plus parity the 6850 sets bit 8 to 0 so that only the 7 bit data is transferred to the 6502 To read from the RDR OSBYTE amp 96 150 should be used 20 6 THE CONTROL REGISTER Sheila amp 08 write only The 8 bit 6850 control register determines the function of the transmitter receiver interrupts and the RS423 Request to send RTS Since this is a write only register it is not possible to read its current contents directly There is a way of determining its current contents via th
323. state read write ESCAPE BREAK effects read write location amp 280 read write user OSBYTE location read write location amp 28A read write location amp 28B read write available RAM version number read address of OS variables select O P stream read write O P status enter language ROM issue ROM service request read address of ROM pointers read address of ROM information read ROM active at last BRK BASIC ROM number read write current language no printer destination set printer char ignored printer driver going dormant read write printer destination read write character ignored dec hex 20 14 130 82 131 83 132 84 133 85 179 B3 180 B4 182 B6 200 C5 240 FO 241 FI 250 FA 251 FB 254 FE 0 0 166 A6 167 A7 3 3 236 EC 142 8E 143 8F 168 A8 169 A9 170 AA 171 AB 186 BA 187 BB 252 FC 5 5 6 6 123 7B 245 F5 246 F6 RS423 serial soft keys sound speech timer Tube brief description RX baud rate TX baud rate read write RS423 mode read write RS423 use flag read RS423 control flag read write RS423 handshake read write I P suppression flag read write RS423 cassette select read write 6850 control reg read write IRQ mask 6850 read RAM copy of serial ULA reg disable cursor editing reset soft keys read write key string length read write amp CO to amp CF status read write amp D0 to amp DF status read write amp E0 to amp EF status read write amp FO to amp FF status read wri
324. stem commands during execution amp BO amp BF are allocated as filing system scratch space but are not exclusively used by the currently active filing system amp C0 amp CF are allocated to the currently active filing system This area is nominally private and will not be altered unless the filing system is changed or the absolute workspace is claimed see paged ROMs chapter 15 amp D0 amp E1 are allocated to the VDU driver amp D0 is the VDU status as returned by OSBYTE amp 75 amp D1 contains a byte mask for the current graphics point This byte indicates which bits in the screen memory byte correspond to the point For example for the rightmost pixel in a two colour mode this byte would contain amp 01 and for a sixteen colour mode amp 55 amp D2 and amp D3 are the text colour bytes to be ORed and EORed into memory respectively When writing text to the screen in modes 0 to 6 the pattern byte to be written to the screen is first ORed with the contents of amp D2 and then EORed with the contents of amp D3 The pattern byte contains a bit set where the pixel is to be the foreground colour and a bit clear where the pixel is to be the background colour In four and sixteen colour modes the pattern byte is expanded before using these locations to take account of the extra bits per pixel amp D4 and amp D5 are similar in function to locations amp D2 and amp D3 only they are the graphics colour bytes By performi
325. t blink type Cursor start bits 0 4 Cursor blink bit 6 Cursor type bit 5 R11 Cursor end R12 R13 Screen start address R14 R15 Cursor position R16 R1 Light pen position 462 amp 50 amp 62 amp 00 amp 20 amp 00 amp 07 Variable Variable Variable MODE 0 83000 83008 83278 Ermites eee Die ES Py Ra Tata tag nia Der DE ES ME ESE SR rey Rae aa area Ale i one aa ara eu da il bel ena a EE ae ct ek TERRI OE EE eee ee ee amp 7B06 TOR 1 ne te ee 87D80 amp 7D88 amp 7FFB 87081 Bed ame eta ERE a amp 7D82 amp 7D8A P 87D83 amp 7D8B FB CEE ee amp 7FFC amp 7D85 amp 7D8D A amp 7FFD EG ties ee eee ee amp 7FFE amp 7FFF 8 PIXELS 1 BIT PIXEL N B The screen layout is only as shown after a clear screen it will change when the screen is hard scrolled 463 MODE 1 Screen layout Graphics Colours Text Registers RO R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 464 320 x 256 4 40 x 32 Horizontal total Characters per line Horizontal sync position Horizontal sync vvidth Vertical sync time Vertical total Vertical total adjust Vertical displayed characters Vertical sync position Interlace mode bits 0 1 Display delay bits 4 5 Cursor delay bits 6 7 Scan lines per character Cursor start blink type Cursor start bits 0 4 Cursor blink bit
326. t Decimal mode SEI Set interrupt disable flag Select input stream Select output stream Serial Serial ROM reading of Serial ULA read register Service call entry paged ROMs Service call types 508 360 276 246 359 462 168 139 374 371 139 139 89 435 434 435 90 91 118 119 497 177 236 320 320 SHEILA address amp 20 address amp 30 addresses amp 00 amp 07 addresses amp 08 amp 1F addresses amp 20 amp 21 addresses amp 40 amp 7F addresses amp 80 amp BF addresses amp C0 amp C2 addresses amp E0 amp FF introduction reading and writing SHIFT BREAK action Slow data bus Sockets for paged ROMs Soft BREAK Soft character RAM allocation Soft character explosion state Soft character RAM explosion Soft key KEY buffer storage string length Soft keys 11 15 consistency flag function key status page two expansion pointer plus CTRL plus SHIFT plus SHIFT CTRL resetting using TAB key Sound buffer storage channel status chip empty a sound channel buffer input on 1MHz bus suppression status Sound command OSWORD Sound semaphore page two location Spare vectors Speech buffer status buffer storage empty speech buffer presence flag processor read from speech processor suppression status write to speech processor Spooling selection of 428 395 356 385 377 397 427 429 433 356 170 246 417 395 166 244 136 191 188 15 281 219 120 238 225 273 225 225 22
327. t NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This call returns the address of a table which contains the ASCII values of each key where the offset into the table is the internal key number see OSBYTE with A amp 78 FX 120 Values returned for non ASCII keys are f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 128 amp 80 129 amp 81 130 amp 82 131 amp 83 amp 84 133 amp 85 134 amp 86 135 amp 87 amp 88 amp 89 137 amp 89 COPY LEFT CURSOR RIGHT CURSOR DOWN CURSOR UP CURSOR TAB CAPS LOCK SHIFT LOCK ESCAPE 139 amp 8B 140 amp 8C 141 amp 8D 142 amp 8E 143 amp 8F 0 1 2 27 amp 1B Non valid key numbers and SHIFT or CTRL return values with no significance On exit X amp 2B Y amp FO i e address is amp F02B for OS 1 20 183 OSBYTEs 8 AE 174 and amp AF 175 Read VDU variables origin NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This call returns with the address of the table of internal VDU variables See memory section 11 4 for list of these On exit x amp 00 Y amp 03 i e address is amp 300 for OS 1 20 184 OSBYTE B0 176 Read write CFS timeout counter lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of
328. t line to provide information on a particular area If the rest of the line is blank the ROM should type its name and a list of keywords to which it will respond Claim static workspace When a paged ROM requires use of the absolute workspace starting at location amp E00 it should ask the current owner to relinquish it by issuing this call On receiving this call a ROM should copy its valuable information to its private area and update its flag in its private area to indicate that it no longer owns the workspace When it again needs the absolute workspace it should itself issue this call to get it back NMI release This call when issued means that the current user of the NMI space no longer requires it and it may be claimed The Y register on entry should contain the ROM number of the previous owner usually the net system and each ROM should compare it with their own ROM number in the X register and amp F4 before reasserting control over the NMI space NMI claim This call should be made with Y amp FF and if a ROM is currently the owner of the NMI space it should return in the Y register its ROM number and clear from the NMI space any important data Y should not be altered if the NMI space was not previously in use The claimer of NMI should store the returned ROM number for use when releasing the NMI claim 323 OD OE OF 10 324 ROM filing system initialise This call is issued when the ROM filing sys
329. t of all since it applies to both units Separate sections on the MOS VIA and the printer user VIA then follow on 22 1 6522 Versatile interface adapters in general Each VIA chip is housed inside a large 40 pin package It contains two fully programmable bi directional 8 bit I O ports These are designated port A and port B each one of which has its own handshaking capability There are two 16 bit programmable timer counters a serial parallel or parallel serial shift register and latched input output registers 22 1 1 PIN DESCRIPTIONS PAO PA7 peripheral port A These 8 lines can be individually programmed as inputs or outputs under control of a Data Direction Register The logic level on the output pins is controlled by an output register and input data can be latched into an internal register under control of the CA1 line These various modes of operation are all controlled via internal control registers which are programmed by the 6502 397 CA1 CA2 port A control lines These two lines can act either as interrupt inputs or as handshake outputs Each line controls an internal interrupt flag with a corresponding interrupt enable bit In addition CA1 controls the latching of data on port A input lines PBO PB7 peripheral port B The 8 bi directional port B lines are controlled by an output register and a data direction register in a similar way to port A The logic level of the PB7 output signal can also be controlled
330. t scrolling of the 370 screen up down left and right Provided that the start address is inside the screen memory of the mode being used a hardware wrap around feature will also operate Characters which would have scrolled off the top of the screen will therefore reappear at the bottom The wrap around circuit simply detects whenever the 6845 tries to get video data from a ROM an address above amp 7FFF and adds an offset to that address This has the effect of bringing the address back inside the video RAM Since the screen sizes are different in the various modes 2 bits on the SYSTEM VIA are used to define the length of the hardware scrolled screen see section 23 2 18 11 HARDWARE SCROLLING Scrolling the screen fast in any direction can be of immense use in a large number of applications Text can be scrolled in word processing applications landscapes can be made to rush by ina horizontal direction see games such as Acornsoft Planetoid If it were not for the hardware scroll feature it would be necessary to move every byte on the screen to perform a scroll This is very time consuming for 20000 bytes and therefore slow In order to make effective use of the hardware scrolling facilities available it is necessary to understand both the advantages and the limitations which are imposed Modes 0 6 will now be analysed in detail followed by mode 7 which is slightly different 18 11 1 Modes 0 6 vertical scrolling In order to
331. t stream selection Output stream status Overflow flag P Page Page mode Paged ROM select register zero page RAM copy Paged ROMs HELP service call ROM software absolute workspace claim address pointer auto boot BRK service call claim static work space copyright string EXEC SPOOL closure warning expanded vectors location filing systems font explosion warning information table initialise Filing system language entry point language ROMs NMI service calls OS command OS commands private work space addresses private work space claim read byte read ROM info table address read ROM pointer table address recognition bytes ROM filing system ROM type byte select register service call entry service call types service request sockets title string Tube service calls 248 251 250 248 250 247 256 256 249 249 252 249 271 101 104 119 232 32 42 60 232 496 220 271 323 330 320 271 321 322 323 319 324 281 328 325 273 325 325 327 323 321 11 281 321 106 182 181 317 324 317 393 320 320 167 395 319 325 unrecognised interrupt unrecognised OSBYTE unrecognised OSVVORD vectored entry vectors claimed service call version number version string Palette read OSWORD write OSVVORD Palette selection in video ULA Parallel Parallel printer Parameter block Parasite processor Pass in assembling programs Peripheral PHA Push A onto stack PHP Push status onto stack Phra
332. t to 0 Bit 3 8 bit mode 0 10 bit mode 1 Bits 4 7 not used 430 READING FROM THE ADC There are three registers in the ADC which can be read directly the status register and two data registers 26 1 3 Status Register Sheila amp CO Read only BitO and These define the currently selected input channel Bit 1 as in the AD start register Bit 2 not used Bit 3 8 bit mode 0 10 bit mode 1 Bit 4 2nd most significant bit MSB of conversion Bit 5 MSB of conversion Bit6 0 busy 1 not busy Bit 7 0 conversion completed 1 conversion not completed 26 1 4 High data byte Sheila amp C1 Read only This byte contains the 8 most significant bits of the analogue to digital conversion 26 1 5 Low data byte Sheila amp C2 Read only Bits 7 to bit 4 define the four low order bits of a 12 bit conversion In 8 bit only mode all four bits are inaccurate In 10 bit mode bits 7 and 6 are accurate Bits 5 and 4 are likely to be inaccurate but this will depend upon the particular qualities of individual 7002 chips Bits 3 0 are always set to low OSBYTES amp 80 amp BC amp BD and amp BE are relevant when reading from the ADC 431 26 2 Hardware connections for a joystick Oz Qi ana ANALOGUE ov 5V BUTTON GROUND FIRE E c PB1 LPSTB BUTTON Q VIEW INTO ANALOGUE PORT CONNECTOR SHOWING CONNECTIONS FOR BOTH JOYSTICKS Figure 26 1 Connections for Joysticks A 15 way D type connector is provide
333. ta 330 ninc7 data LDA TAY INC BNI INC PLA LDA RTS Fl amp F6 Y amp F6 ninc7 857 0 a a an a an an Get the data In the Y register ready for exit Increment the address for next time No need to increment amp F Discard service type Set service type to no operation Exit Data onward from here 331 332 16 Filing systems 16 1 Filing systems in general 16 1 1 Files A file to the BBC microcomputer is a sequence of bytes A file has a number of attributes a load address an execution address a length and if open a sequential pointer Every file is given a name when it is created When a file is used it is identified by this file name When a file is opened for single byte access the filing system allocates the file a channel The channel can be considered to be a window into a file A handle is assigned to each channel to identify it internally A file may be opened more than once for input and so more than one channel can be associated with one file at any one time When the file is closed and no further access is required then the channel is released and is no longer valid The channel that was in use is now available to be assigned to another file if required Any filing system can only have a limited number of open channels at one time and so there are only a limited number of handles available A sequential pointer is maintained for each o
334. te function key status read write SHIFT fn key status read write CTRL fn key status read write CTRL SHIFT fn status read write consistency flag read write sound suppression read from speech processor write to speech processor read write speech suppression read speech presence read write timer switch state fast Tube BPUT read Tube presence dec hex 7 7 181 B5 191 BF 192 CO 203 CB 204 CC 156 9C 232 E5 242 F2 4 4 18 12 216 D8 221 DD 222 DE 223 DF 224 EQ 225 E1 226 F2 227 E3 228 F4 244 F4 210 D2 158 9E 159 9F 209 DI 235 EB 243 F3 157 B9 234 EA 453 user vertical sync VDU VIA 6522 video 454 brief description user OSBYTE call read write user OSBYTE location wait for vertical sync read VDU status read VDU variable value read address of VDU variables read write VDU queue length read write IRQ mask user VIA read write IRQ mask system VIA flashing col mark duration flashing col space duration wait for vertical sync write to ULA palette register write to ULA control register read RAM copy of ULA ctrl reg read RAM copy of ULA pal reg read write flash counter read write mark period count read write space period count read write lines from page halt dec hex 1 1 241 F1 19 13 117 75 160 AO 174 AE 175 AF 218 D8 231 E7 233 E9 9 9 10 A 19 13 154 9A 155 9B 184 B8 185 B9 193 C1 194 C2 195 C3 217 D9 Appendix B Operating System calls summary Rout
335. tem is active and the paged ROMs are being scanned for a particular file On entry the Y register contains 15 minus the ROM number of the next ROM to be scanned If that adjusted ROM number is less than the number of the ROM receiving the call the call should be ignored Otherwise the number should be replaced by the ROM number of the active ROM and stored at amp F5 after adjusting it The current ROM should mark itself as active by returning with the accumulator zero and store in locations amp F6 and amp F7 a pointer to the data within the ROM The alteration of location amp F5 is necessary because the ROM filing system will abort a search if it gets no response from any of the ROMs for any particular ROM number Altering amp F5 causes non existent ROMs to be skipped over See the example on ROM filing system use in section 15 4 ROM filing system byte get This call is issued after initialising the ROM with service type to retrieve one byte from the ROM A ROM should only respond to this if the ROM number in amp F5 is 15 ROM s physical number in amp F4 The fetched byte should be returned in the Y register See the example on ROM filing system use in section 15 4 Vectors claimed When a new filing system is initialised it should after writing its new vectors issue this service call This is to inform all the paged ROMs that there is a filing system change imminent SPOOL EXEC file closure warning This call is issued p
336. tem software ROM filing system RFS software that is resident in paged ROMs should contain header code to provide data which has been requested by service calls amp 0D and amp 0E Each paged ROM should contain an end of ROM code amp 2B after the last block Data should be formatted as specified in the ROM Filing System section number 16 11 A simple ROM filing system service code block could be serve CMP amp 00 Is it a ROM initialising call BNE ninit No branch PHA Save the service call type TYA Get logical ROM number EOR amp 0F Adjust it do 15 x CMP amp F4 Compare with this ROM s number BCC notus N umber lose this ROM already been don LDA data MOD 256 Low byte of data address STA amp F6 Location amp F6 and amp F7 reserved for this LDA data DIV 256 High byte of data address STA amp F7 Save high byte LDA amp F4 This ROM s number EOR amp 0F N Adjust it back 15 X STA amp F5 Pass over any higher non filing system N ROMs PLA Discard service type LDA 0 Set service type to no operation RTS Exit notus PLA Restore service typ exit RTS Exit ninit CMP amp 0E Is it a byte requests call BNE exit o exit PHA Save service typ LDA amp F5 Find the current ROM number EOR 6 amp 0F Adjust it CMP amp F4 Compare it with this ROM BNE notus Not the same must be for another ROM LDY 0 Prepare to get the da
337. ter Scan line Scan line Scan line Data bit 7 x 6 5 43 2 1 0 x x A4 A3 A2 Al AO D7 D6 D5 D4 D3 D2 D1 DO D7 D6 D5 D4 D3 D2 D1 DO D7 D6 D5 D4 D3 D2 D1 DO v3 cl 17 17 17 h3 h2 h1 h0 v2 vl vO D6 D5 D4 D3 D2 D1 DO x x D4 D3 D2 D1 DO D6 D5 D4 D3 D2 D1 DO D6 D5 D4 D3 D2 D1 DO VS d1 d0 c0 D4 D3 D2 D1 DO s4 s3 s2 sl sO D4 D3 D2 D1 DO x h5 h4 h3 h2 hi h0 16 15 14 13 12 11 10 x h5 h4 h3 h2 hi ho 16 15 14 13 12 11 10 x hd h4 h3 h2 hi h0 16 15 14 13 12 11 10 375 376 19 The video ULA Sheila address amp 20 amp 21 The Video ULA is a special chip designed by Acorn especially for use in the BBC microcomputer It provides all of the video timing for the rest of the system including the 6845 determines the relationship between logical and physical colours controls the cursor width and provides Red Green and Blue R G B video outputs This section explains how the ULA is programmed for the various modes 0 7 and then gives an example of creating a totally new mode with 16 colours but only 10 characters across the screen This only uses 10K of memory and therefore allows a 16 colour mode on model A micros or large games programs on a model B which require lots of memory and full 16 colour graphics 19 1 THE VIDEO CONTROL REGISTER SHEILA amp 20 WRITE ONLY This 8 bit register controls which flashing colour is present at any one time whether teletext is selected the num
338. text describes how the different addressing modes work and the assembler syntax which is necessary N B Not all addressing modes are available for all instructions Details of which addressing modes can be used with which instructions are contained in the Assembler Mnemonics section 6 2 5 1 Implicit addressing Many instructions do not require any addressing mode to be specified in the operand field In such cases the addressing is implicit in the instruction itself For example an RTS instruction will always cause the processor to jump to the location addressed by the top two bytes of the stack 5 2 Accumulator addressing Some instructions may operate on either a memory location or the accumulator The accumulator is specified by putting a capital A in the operand field e g ASL A shift accumulator contents one bit left ROR A rotate accumulator contents one bit right Note that the variable A cannot therefore be used as an operand 35 5 3 Immediate addressing using a data constant If at the time of programming the data required for a machine code instruction is known then immediate addressing may be used Immediate addressing is indicated to the assembler by preceding the operand with a character The assembler uses the least significant byte of the value given to define the operand The machine code instruction actually uses the byte of memory immediately following the instruction in program memory e g
339. the serial chapter 20 In the RS423 system this line is used to inform the remote device that the computer s RS423 input buffer is getting full and that transmission should cease until the buffer has been emptied somewhat 311 The cassette uses the RTS line to control the carrier Data on the cassette system is frequency modulated one tone is used to represent a 1 state and another is used for 0 A third state is also required silence this is used to indicate the gap between blocks When transmitting silence the carrier is said to be absent When the RTS line is at logical zero the 2400Hz carrier tone is enabled The RTS line cannot be controlled directly within the RS423 system Control has to be achieved by rendering the RS423 input buffer non empty and enabling receiver interrupts This has the effect of allowing the RS423 system to make changes to the RTS line The state of the RTS line depends on the fullness of the RS423 input buffer Program control of the RTS line can thus be achieved by changing the tolerance of the buffer to being full when one byte is present from being full when 247 characters are present in the buffer the default state When the buffer still has space in it the RTS line is low and the 2400Hz tone is enabled There follows an example program to output to the cassette port via the RS423 system Note the use of ADVAL to sense the buffer going empty This is needed because if the
340. the clock divide bits in the 6850 control register to divide by 16 instead of 64 10 REM Insert assembler code to handle RS423 event 20 DIM M 40 30 FOR PASS S 0 TO 2 STEP 2 40 P S M 50 OPT PASS 60 event CMP 7 Check for RS423 event 70 BEQ rsev If it is then branch 80 RTS Otherwise exit 90 rsev PHA Save the accumulator 100 TXA And test the status byte 110 AND 2 X for the DCD bit 120 BEQ ntdcd Branch if not a DCD error 130 SEA amp 70 If DCD mark it in amp 70 140 ntdcd PLA Restore accumulator 130 RTS And exit 560 170 NEXT PASS Set event vector RE 190 amp 220 event MOD 256 200 amp 22l event DIV 256 210 RE Enable the RS423 event 220 FX 14 7 230 REM indicate that the RS423 system is connected to cassette 240 FX 205 64 250 REM Select band rates note they are 300 baud 260 FX 7 3 270 SEX 8 3 280 REM reset 6850 290 FX 156 3 252 300 FX 156 2 252 310 REM Turn motor off then on to cause a break in the data 320 REM carrier signal 330 MOTOR 0 340 MOTOR 1 350 REM Ensure that the computer is left tidy if user escapes 360 ON ERROR GOTO 560 370 REM Wait for DCD interrupt 380 2870 0 390 REPEAT UNTIL amp 70 lt gt 0 400 REM Set for RS423 input and do a bit adjust for 1200 band 410 FX 2 1 420 FX 156 1 252 430 REM Search for synch character see previous example 440 amp 70 0 450 REPEAT X INKEY 0 460
341. the operating system as it has no software to support the light pen This interrupt is always passed over to the user An example lightpen interrupt processing routine concludes this chapter 303 Timer 2 is used by the operating system to count transitions Of the speech ready output When an interrupt occurs there is an attempt to speak another word Timer 1 is used to provide regular 100Hz interrupts On receipt of this interrupt the following happens a The interrupt is cleared b The TIME clock is swapped with its alternate and incremented There are two 5 byte clocks provided by the operating system They are updated alternately This ensures that any program which is in the middle of reading the clock when an interrupt occurs can still read a valid value c The interval timer is incremented and if zero an event is caused number 5 See section 12 9 d The INKEY timer is decremented e One element of sound processing is performed f If a new key has been pressed its code is entered into the buffer and auto repeat processing is begun g Then three emergency back door operations are performed 1 The speech chip is checked in case a speech interrupt has been missed 2 The 6850 is checked because the transmitter interrupts must be disabled to set the RTS line high 3 The analogue converter is checked to ensure that an interrupt is not missed If system VIA interrupts are disabled sound spee
342. the operating system s workspace For details of the effects of writing to this register see Video Hardware chapter 19 This call also sets the flash counter stored in location amp 251 to the mark value stored in amp 252 After call A X Y and C are preserved 173 OSBYTE amp 9B 155 FX 155 Write to video ULA palette register and OS copy Entry parameters X contains value to be written This call writes to register 1 of the video ULA and also stores a copy of this value at location amp 249 The actual value written to the register and the internal copy is X EOR 7 See chapter 19 The Video ULA for further details After call A X Y and C are preserved 174 OSBYTE amp 9C 156 FX 156 Read update 6850 ACIA control register and OS copy Entry parameters X and Y determine action sregister contents gt AND Y EOR X are written to the register i e if Y amp FF and X 0 then no change is made N B Using this call has no effect on the cassette interface operation and so this is the best way of implementing non standard RS423 formats See serial interface hardware chapter 20 On exit X old register contents After call A and Y are preserved C is undefined 175 OSBYTE amp 9D 157 FX 157 Fast Tube BPUT entry parameters X byte to be output Y file handle In OS 1 2 this is channelled through the standard BPUT routine A fast BPUT routine may be implemented in other software After call A
343. themselves inactive with OSBYTE amp 7B after they have finished printing the last character in the buffer This has the following effects a b c The user is allowed to select a new printer type with OSBYTE 5 The operating system no longer offers interrupts to the user print driver The printer driver will be warned with an entry code 1 when a new character is printed 10 7 The Econet vector NETV amp 224 5 The net vector is used for various network effects In non networked machines this vector can be used for a variety of purposes The user can be entirely disconnected from the operating system with this call or have all his actions vetted This vector has rather more program protection applications than main line uses Usually other vectors which allow more specific control of functions would be used The effects are specified by the value in the accumulator The net routine should preserve registers The net effect codes are 0 1 2 3 5 These codes are used to control the networked 260 printer The printer is in every respect the same as a user printer see the previous section on the user print driver Printer type number 4 is nominally allocated to the networked printer amp 0D Write character attempted This call to the net vector is only made when enabled by OSBYTE amp D0 On entry Y is the character to be output On exit if the carry flag is set the output of the character is not passed on to t
344. throughout the rest of this chapter It is most relevant to the RS423 interface A serial interface will usually consist of a data IN line data OUT line and a ground connection This is the barest minimum for a bi directional serial link Handshaking lines are often supplied as well The RTS Request to send goes low when the BBC micro is ready to accept data on the RS423 it acts as a data carrier enable for the cassette The other device can control when data is sent to it via the CTS Clear to send line The device pulls this low to indicate that it can receive data and pulls it high to indicate that it is unable to receive data eg because its input buffer is full Data should therefore only be sent out from the BBC microcomputer if the CTS input is low OSBYTE amp CB controls the serial handshaking and OSBYTE amp CC suppresses all RS423 input 385 Each character is transmitted serially according to a predefined format A start bit indicates that a character will follow 7 or 8 bits of character are then sent followed by a parity bit if parity is selected This parity bit may be set to either 1 or 0 to indicate whether the number of high bits in the character were odd or even The parity bit if selected is then followed by one or two stop bits The idea behind using a parity bit is that the receiving device can check that parity is correct If it is incorrect then an error has occurred between the transmitter and receiver The followi
345. to indicate empty 20 7 2 Transmit Data Register Empty TDRE Bit 1 This bit goes high to indicate that the Transmit Data Register contents have been transferred and that new data may now be entered The low state indicates that the TDR is full 20 7 3 Data Carrier Detect DCD Bit 2 When DCD goes high it indicates that the carrier is not present from the cassette input It will always be low when the RS423 interface is selected 389 20 7 4 Clear To Send CTS Bit 3 This is always low when using the cassette On the RS423 this bit indicates that the RS423 is Clear To Send data out while this bit is low Master reset doesn t affect the CTS bit since it is an external input 20 7 5 Framing Error FE Bit 4 The Framing Error indicates that the received character was incorrectly framed by a start and stop bit The error persists throughout the time that the associated character is available in the RDR 20 7 6 Receiver Overrun OVRN Bit 5 This indicates that a character or number of characters were received but not read from the receive data register Character synchronisation is maintained during overrun The overrun indication is reset after the reading of data from the Receive Data Register or after a Master reset 20 7 7 Parity Error PE Bit 6 This bit indicates that the number of ones in the character doesn t agree with the preselected odd or even parity The parity error is present whilst the character is in the
346. umber of bits attributed to each pixel depending upon the number of colours available The table for R1 contents illustrates this Mode 0 1 2 3 4 5 6 7 RO 127 127 127 127 63 63 63 63 18 3 2 Horizontal displayed register R1 This 8 bit write only register determines the number of displayed characters per horizontal line Nhd on figure 18 1 Mode 0 1 2 3 4 5 6 7 No of CHRS as 80 80 80 80 40 40 40 40 seen by 6845 Nhd No of CHRS as 80 40 20 80 40 20 40 40 seen on the screen No of bits used 1 2 4 1 1 2 1 1 to store colour information 361 18 3 2 Horizontal sync position register R2 This 8 bit write only register determines the horizontal sync pulse position on the horizontal line The specification is in terms of character widths from the left hand side of the screen Mode 0 1 2 3 4 5 6 7 R2 98 98 98 98 49 49 49 51 Increasing the value of this register pushes the entire screen left whilst decrementing it pushes the whole screen right 18 4 The sync width register R3 This 8 bit write only register defines both the horizontal and the vertical sync pulse times 18 4 1 Horizontal sync pulse width The lower 4 bits contain the horizontal sync pulse width in number of characters Any number between 1 and 15 can be programmed but 0 is not valid It is however not advisable to change this register since most monitors and televisions require the standard sync width to operate properly Mode 0 1 2 3 4 5 6
347. using the OSCLI call amp FFF7 from machine code see section 7 12 for details of OSCLI Any command offered to the command line interpreter but not recognised as a command resident in the command table is offered to paged ROMs for possible action If it is not claimed by a paged service ROM it is presented to the currently selected filing system ROM The filing system may recognise the command as one of its internal file commands or attempt to load and execute a file specified by the may command name the cassette and ROM filing systems excepted i e it is equivalent to RUN lt command name gt Each command name may be abbreviated by using enough letters to identify the command terminated by a full stop The minimum abbreviations for each command are noted below with the description of each command The command line interpreter does not distinguish between upper and lower case characters in the command name cat has the same effect as CAT Where a string or filename is specified as a parameter the text need not be enclosed within paired quotation marks but must be separated from the command name by at least one space Several of the operating system commands invoke OSBYTE calls and so their action mimics these calls Where there is an equivalent OSBYTE call this has been indicated 11 A number of commands are filing system dependent Any command which creates or uses files is described for the ROM and cassette
348. ut thus amp 300 amp 307 contain the current graphics window in internal co ordinates amp 300 1 Left hand column in pixels amp 302 3 Bottom row in pixels amp 304 5 Right hand column in pixels amp 306 7 Top row in pixels amp 308 amp 30B contain the current text window in absolute characters offset from the top left of the screen amp 308 Left hand column amp 309 Bottom row amp 30A Right hand column amp 30B Top row amp 30C amp 30F contain the current graphics origin in external co ordinates amp 310 amp 313 contain the current graphics cursor in external co ordinates This is used for calculating relative PLOTs amp 314 amp 317 contain the old graphics cursor in internal co ordinates This is used for the generation of triangles amp 318 contains the current text cursor X co ordinate amp 319 contains the current text cursor Y co ordinate 2 9 amp 31A contains the line within current graphics character of the current graphics point Because the BBC microcomputer has a non linear address space for the graphics screen it is simpler to calculate the address of the byte at the top of the character cell that contains a point and then calculate the row within the character Thus the location of the byte containing the current graphics point is amp D6 256 amp D7 amp 31A amp 31B amp 31E is used either as graphics workspace or as the first part of the VDU queue amp 31F amp 323 i
349. vailable on the BBC Microcomputer can only be used when programming in machine code For example a user printer driver can only be implemented in machine code Of no less importance than the design of the hardware or the choice of microprocessor in the machine is the operating system This is a large and highly complex machine code program which governs the machine The operating system consists of a large number of routines which perform operations such as scanning the keyboard updating the screen performing analogue to digital conversions for the joysticks and controlling the sound generator All these functions are performed by the operating system and are made available to other machine code programs A machine code program with which all users will be familiar is BASIC This program which provides the user with an easier way of using the microprocessor s computing power constantly uses the operating system routines to get input from the keyboard and to reflect that input on the screen BASIC recognises words of text and when it wants to use a hardware facility it calls a machine code routine within the operating system 8 The great advantage of this independence of the language program from the direct use of hardware is that the same facilities can be offered to different languages Writing a machine code program requires the programmer to place the appropriate values into successive memory locations corresponding to the opcodes and
350. value is returned in X The contents of the next location are returned in Y This location contains a value indicating the type of the last BREAK performed value 0 soft BREAK value 1 power up reset value 2 hard BREAK 244 OSBYTE amp FE 254 FX 254 Read write available RAM lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains a value indicating the available RAM value amp 40 16 K usually model A value amp 80 32 K usually model B 245 OSBYTE amp FF 255 FX 255 Read write start up options lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location is determined by the 8 links on the right hand front corner of the keyboard pcb following a hard BREAK bits 0 to 2 screen MODE selected following reset MODE number 3 bit value bit 3 if clear reverse action of SHLFT BREAK bits 4 and 5 used to set disc drive timings see below bits 6 and 7 not used by operating system reserved for future applications Disc drive timing links link link step settle head 3 4 time time load 1 1 4 16 0 1 0 6 16 0 0 1 6 50 32 0 0 24 20 64 246 9 OSVVORD CALLS The OSVVORD routines are a similar concept to the OSBYTE routines except that instead of the parameters being passed in the X and Y registe
351. value is returned in X The contents of the next location are returned in Y This location is updated by any character definition RAM explosion which may have been selected and returns with the high byte of the OSHWM address the low byte always being 0 See OSBYTE amp 14 20 189 OSBYTE amp B5 181 FX 181 Read write RS423 mode lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y If this location contains 0 then RS423 input is treated exactly the same as keyboard input i e ESCAPEs are recognised soft keys are expanded and each character entering the input buffer causes a keyboard event If this location contains 1 the usual situation then ESCAPEs are ignored soft keys are not expanded and no events are caused 190 OSBYTE amp B6 182 Read character definition explosion state lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains the state of font explosion as set by OSBYTE call with A amp 14 FX 20 191 OSBYTE B7 183 FX 183 Read write cassette ROM filing system switch lt NEW VALUE gt lt OLD VALUE gt AND Y EOR X The old value is returned in X The contents of the next location are returned in Y This location contains 0 for TAPE selection and 2 for ROM selection Other values are meaningless 192
352. vectors from location amp 212 to amp 21F These point to the relevant routines within the filing system The filing system vectors are amp 212 FILEV Operations on whole files amp 214 ARGSV Adjust file arguments amp 216 BGETV Get one byte from an open file amp 218 BPUTV Put one byte to an open file amp 21A GBPBV Get put a block of bytes to from an open file amp 21C FINDV Open close a file for byte access amp 21E FSCV Filing system control various actions Filing systems are selected either during a reset or by means of an operating system command thus filing systems must also trap the command line interpreter see section 7 12 to catch their selection command Filing systems resident in paged ROMs are automatically informed by the operating system of any unrecognised command attempted 334 16 2 OSFILE Read or vvrite a vvhole file or its attributes Call address amp FFCE Indirected through amp 212 Note the correct address seems to be GFFDD This routine is concerned with actions on whole files Actions performed by this routine are loading a file into memory saving a file from memory and writing file attributes On entry X and Y points to a parameter block in memory X low byte Y high byte The accumulator contains a number indicating the action to be performed The format of the parameter block is 00 01 02 03 04 05 06 07 08 09 0A 0B 0C OD OE OF 10 11 Address of f
353. wh ehw ewh ewh ere ere ere ere ere CPX Compare memory with X register X M This instruction performs a subtraction of the contents of the memory location from the contents of the X register the memory location and the register remain intact but the status register flags are set on the result Processor Status after use C carry flag set if X greater than or equal to M Z zero flag set if X M I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag set if bit 7 of the result is set Addressing mode bytesused cycles immediate 2 2 zero page 2 3 absolute 3 4 Example Clearing an area of memory max amp 100 bytes The number of bytes to be cleared is stored in count LDA 0 set accumulator to 0 TAX set loop index to 0 loop STA page X write 0 to byte pagetX INX increment loop index CPX count X count comparison BNE loop if not equal go round again 63 CPY Compare memory with Y register Y M This instruction performs a subtraction from the Y register of the specified memory location contents The memory location and the register remain intact but the status register flags are set on the result Processor Status after use C carry flag set if Y greater than or equal to M Z zero flag set if Y M I interrupt disable not affected D decimal mode flag
354. which is wrapped around is controlled by the system VIA as described in section 23 2 18 14 Hardware scroll example The program listed below uses the hardware scroll facilities in mode 0 A line of text can be moved around the screen using the cursor keys Note that as text moves off one side of the screen sideways scroll it reappears on the other side either one line up or one line down from its original position If a true sideways scroll is required it is necessary to move all of the bytes on the relevant side of the screen up or down one character position During motion of the line of text in a vertical direction there will be brief flashes of another line on the screen This is partially due to the delay in BASIC between setting register 12 and register 13 on the 6845 and also because the change occurs in the middle of a screen display The flashing will be reduced in machine code programs which wait until the frame sync period before changing any of the 6845 registers 10REM HARDWARE SCROLL EXAMPLE IN MODE 0 20 MODEO 30START amp 3000 40PRINT THIS TEXT CAN BE SCROLLED IN ANY DIRECTION USING THE CURSOR KEYS 50REM SET KEYS REPEAT RATE AND CURSOR KEYS TO GIVE 136 ETC 60 FX4 1 70 FX12 3 80REPEAT 90 A INKEY 0 100 IF A 136 THEN PROCMOVE 8 110 IF A 137 THEN PROCMOVE
355. wise the assembler will throw up an Out of range message Used after a CMP instruction this branch occurs when A lt DATA Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytes used cycles relative 2 2 1 if branch succeeds 2 if to new page Example Branch if contents of amp 80 lt 100 LDA 100 load accumulator with data CMP amp 80 A data comparison BCC finish goto finish if 728802100 47 BCS Branch on Carry Set Branch if C 1 A relative branch will occur if the carry flag is set The branch address given to the assembler must be within relative addressing range Used after a CMP instruction this branch occurs when A gt data Processor Status after use C carry flag not affected Z zero flag not affected I interrupt disable not affected D decimal mode flag not affected B break command not affected V overflow flag not affected N negative flag not affected Addressing mode bytes used cycles relative 2 2 1 if branch succeeds 2 if to new page Example Branch if contents of X register are greater than or equal to 5 CPX 5 X 5 compare BCS label branch to label if X gt 5 48 BEQ Branch on result zero Branch if Z 1
356. with 1 0 operating system or later The ROM filing system uses data stored in paged ROM or in serially accessed ROMs associated with the speech processor The socket on the left hand side of the keyboard is designed to accommodate ROM packs containing speech ROM devices these require the presence of a speech system upgrade This filing system is the same as the cassette filing system in every way except OSFILE Save is meaningless OSBPUT Writing to ROM is not sensible OSFIND Opening for output is not possible The handle given for input is 3 The internal format for ROMs is the same as the tape format with the following exceptions amp 2B is used as an end of ROM marker Files can span over an end of a ROM marker but care should be taken to ensure that the right ROM is read at the right time i e the next sequential block in the file must be in the next ROM to be read In the ROM filing system the whole header may be replaced by a single character amp 43 for all bar the first and last blocks If the header is abbreviated in this way it is assumed to mean that the header is unchanged from the last block except for the block number In the ROM filing system the four spare bytes in the cassette header block are used to contain the address of the byte after the end of the file enabling file searches to be a lot faster Fast searching is used by the ROM filing system by default Full CRC checking
357. would contain amp 80 and amp 01 and in a sixteen colour mode amp AA and amp 55 amp 364 and amp 365 contain the X and Y co ordinates of the text input cursor The input cursor is the position from which characters are COPYed amp 366 contains the character to be used as the output cursor in teletext mode this is normally the block character amp FF amp 367 contains the font flag This byte marks whether or not a particular font zone is being taken from ROM or RAM If a bit is set it indicates that that zone is in RAM See OSBYTE amp 14 20 for more information on fonts bit 7 characters 32 63 amp 20 amp 3F bit6 characters 64 95 amp 40 amp 5F bit5 characters 96 127 amp 60 amp 7F bit4 characters 128 159 amp 80 amp 9F bit3 characters 160 191 amp A0 amp BF bit 2 characters 192 223 amp C0O amp DF bit1 characters 224 255 amp E0 amp FF amp 368 amp 36E are the font location bytes These contain the upper bytes of the addresses of the fonts for each of the 7 zones mentioned above amp 36F amp 37E form the colour palette One byte is used for each logical colour That byte contains the physical colour corresponding to the logical colour The bytes are stored in numerical order of logical colour The area of page three from amp 380 to amp 3DF is used by the cassette filing system as working storage amp 380 amp 39C is used to store the header block for the BPUT file See the section on the cass
358. xel 2 To move graphics sideways by one pixel involves extracting Pla Pid from P2a P2d These removed bits must then be reinserted into the adjacent byte This process is tricky and consumes a lot of processing time leading to very slow movement in all but the simplest of cases It will be appreciated how much faster it is to simply move a byte 2 pixels at a time from one memory location to another which can be done very fast indeed 18 12 2 Fast animation using mode 0 Unlike mode 2 moving a byte at a time in mode 1 moves 8 pixels Animation moving 8 pixels at a time will generally produce very uneven motion However since the packing of pixels in mode 0 assigns one bit per pixel animation can be implemented by shifting all of the bits in a byte left or right by one position This uses a 6502 ROR or ROL instruction The bit which moves off the edge of one byte must be put into the adjacent byte 18 13 Wrap around To ensure that the hardware wrap around feature operates correctly the start of screen address must be kept within the screen boundaries If it goes below the start of screen address then add the length of the screen to it If it goes above the top of screen address then subtract the length of screen from it 373 eg in mode 0 the calculated start of screen address may be amp 8050 Since this is outside of the screen it should be changed to amp 3050 by subtracting amp 5000 the screen size The amount of memory
359. xel in the byte Each mask contains the value that would be stored in screen memory if the appropriate pixel were set to colour 1 and all the others to zero The leftmost pixel is stored first byte in the table and the rightmost in the last amp C414 amp C41B Note that this table overlaps the last This table contains one byte per mode containing the number of colours available minus one amp C41B amp C425 Note that this table overlaps the last This table contains four five byte tables used to process the five GCOL plotting options These tables are highly overlapped amp C424 amp C439 are the colour tables There is a colour table for each of 2 colours 4 colours and 16 colours Each table contains one byte per colour in ascending order of colour number The byte contains the value that would be stored in screen memory to give a fully coloured byte of that colour These colour bytes are used in conjunction with the various masks mentioned above amp C424 amp C425 The two colour table amp C426 amp C429 The four colour table amp C42A amp C439 The sixteen colour table amp C43A amp C441 contain one byte per display mode giving the number of pixels per byte on the screen minus one The value stored is zero for non graphics modes amp C440 amp C447 contain one byte per display mode giving the memory map type for the mode This is zero for 20K modes 1 for the 16K mode 2 for 10K modes
360. xit 160 int LDA amp FC 2 Do save 307 170 PHA 180 TXA 190 PHA 200 TYA 210 PHA 220 LDA amp FE4D 230 AND amp 88 240 CMP amp 88 250 BNE exit 260 DA amp FE40 270 LDX amp 16 280 STE amp FEOO 290 INX 300 DA amp FEO1 310 CMP olptl 320 STA olptl 330 BNE diffl 340 STE 6FE00 350 LDA amp FEOL 360 TAY 370 SBC olp 380 CLC 390 ADC 1 400 BMI diff2 410 CMP 3 420 BCS diff2 430 Have two values same 440 STY lpen 450 LDA olptl 460 STA lpentl 470 JMP exit 480 diffl STX amp FEOO 308 IN ELS SOON EE GES DE EE te BOO tes Ee ee Get system VIA interrupt status Mask out bits not interested in is it a lightpen interrupt No exit Clear interrupt Lightpen register 6845 address Ready for next read 6845 data old value Update with new value Next register Get low address Temporary store Is it nearly eq Nearly eq if 0 1 2 Compare with 2 4 gt 3 so not nearly eq so update lpen And depart Next register 14 The RS423 serial system This chapter describes how the RS423 serial interface system can be used The flexibility provided by the BBC microcomputer hardware and software enables the serial system to be reconfigured in a variety of ways Details on using the RS423 system to run the cassette port are also included in this chapter The BBC microcomputer is equipped with a fairly sop
361. y bits 4 5 Cursor delay bits 6 7 Scan lines per character Cursor start blink type Cursor start bits 0 4 Cursor blink bit 6 Cursor type bit 5 Cursor end Screen start address Cursor position Light pen position amp 13 19 Variable Variable Variable MODE 7 amp 7C00 amp 7C01 N B The screen layout is only as shown after a CLS it will change when the screen is hard scrolled amp 4000 should be subtracted for each location on a model A 477 Appendix G The American MOS differences This Appendix outlines the fundamental differences betvveen the English and American BBC Microcomputers The hardvvare is basically the same in both machines except for minor changes in the video circuitry The software in the MOS is however somewhat different to cope with the difference in television display frequency G 1 Text display Both versions of the machine have eight screen modes The text resolution in each mode is Mode United States Britain columns x rows columns x rows 0 80x25 80x32 1 40x25 40x32 2 20x25 20x32 3 80x22 80x25 4 40x25 40x32 5 20x25 20x32 6 40x22 40x25 7 40x20 40x25 G 2 Graphics modes In the U K version the MOS regards the screen as 1280 points horizontally and 1024 points vertically in all modes In the U S version the MOS regards the screen as 1280 points horizontally the same as in the U K and 800 points vertically compared with 1024 in the U K 478 G 3 Actual graphics res

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