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ZX80 Operating Manual

1. 38 39 40 41 D 42 E 43 F 44 G 45 H 46 47 J 48 K 49 L 50 M 51 N 52 O 53 P 54 Q 55 R 56 S 57 T 58 U 59 V 60 WwW 61 x 62 Y 63 2 64 Unused 65 Unused 66 Unused 67 Unused 68 Unused 69 Unused 70 Unused 71 Unused 72 Unused 73 Unused 74 Unused 75 Unused 76 Unused 166 Inverse A 167 Inverse 168 Inverse 169 nverseD 170 Inverse E 171 Inverse F 172 Inverse 173 Inverse H 174 lnversel 175 nverse J 176 Inverse 177 L 178 Inverse 179 Inverse N 180 Inverse O 181 Inverse 182 Inverse Q 183 Inverse R 184 Inverse S 185 Inverse T 186 Inverse U 187 Inverse V 188 Inverse W 189 Inverse X 190 Inverse Y 191 Inverse Z 192 Unused 193 Unused 194 Unused 195 Unused 196 Unused 197 Unused 198 Unused 199 Unused 200 Unused 201 Unused 202 Unused 203 204 Unused 77 Unused 78 Unused 79 Unused 80 Unused 81 Unused 82 Unused 83 Unused 84 Unused 85 Unused 86 Unused 87 Unused 88 Unused 89 Un
2. line is output unless the lt list gt ends with a comma or semicolon sets the current line number to n and enters program input mode without waiting to display any printed output error code even if there is an error in evaluating n It is therefore only really suitable for use as a command is equivalent to LIST 0 causes error code 9 so that CONTINUE will carry on from the following statement Useful for displaying results when no input is required creates an array with name a and subscript range 0 to n inclusive If one already exists the new space will be reserved but the old array will continue to be used for all accesses If a variable a already exists it can still be used assigns to a ie an integer with an TO n single character name a FOR block as described in section 2 a The effect is that a subsequent NEXT a will increment a and if it is not now greater than the TO value jump to the statement following the FOR Note that we always enter the body of the loop at least once and that the association between FOR and NEXT is entirely dynamic ie not lexical jumps to line number n or to the next line with a line number greater than n or if neither exists stops showing code 0 n the first n is an address to which the value of the second modulo 256 is written as a single byte RANDOMISE n set the seed of the pseudorandom number generator to n RANDOMISE as above with n number of frames displa
3. Try a few other combinations by editing statement 80 like A B C A B C watch out for truncation A B A to the power B try using small numbers with C 0 A B A B Here s a golden rule when in doubt either use more than one LET statement and build up that way or use brackets Brackets make life much easier when it comes to complex arithmetic operations Take example Z A B C The normal sequence would be for the computer to evaluate B times C and then divide A by the result If we put brackets round A B thus 80LETZ A B C then the computer will carry out the operation within the brackets first even though it has a lower priority A more subtle example is Z A B C and Z A B C At first sight you would think that these would give the same answer Well they do sometimes Try them both using A 100 25 C 5 the answer is 500 Now try it both ways using A 100 B 3 5 the answer should be 60 What happened when you used the brackets Think about it The phantom truncator has struck again The computer evaluated 3 5 first and truncated it to ZERO then it multiplied 100 by ZERO and naturally got ZERO as a result All this emphasizes that you have to be very much on the alert when you use integer arithmetic for multiplication and division Multiplications can cause arithmetic overflow problems which will cause the program to stop whilst small numbers used in di
4. The program 10 LETX 4 20 PRINT THE ANSWER IS X UNITS expression X expression characters Would print THE ANSWER IS4UNITS The semi colon makes the computer print out the expressions without any spaces between them If you want spaces you have to include then in the literal strings to be printed The comma is used as a tab Each display line of 32 characters on the screen is divided into 4 fields each of which is 8 characters in length Each time the computer comes across a comma in a PRINT statement it starts printing the next expression at the beginning of the next available field The effect of this is to provide a display on the screen in 4 columns If a string variable or literal string is more than 7 characters long and is followed by a comma the computer uses two or more fields in which to print the string and starts the next expression at the beginning of the third field You can add extra commas to skip fields PRINT FIELD 1 FIELD 2 FIELD 3 would produce the output FIELD 1 FIELD2 FIELD3 but PRINT FIELD 1 FIELD 2 FIELD 3 extra comma would produce FIELD 1 FIELD2 FIELD3 this field skipped The statement PRINT THIS ISA FIELD FIELD1 FIELD2 would produce THIS ISA FIELD FIELD 1 FIELD2 The computer used 2 fields for the first string skipped a field and then continued It runs out of fields on the first line and has to continue in the first field of the seco
5. 1 221 30 2 94 print as 158 Inverse 2 222 P 31 3 95 print as 159 Inverse 3 223 32 4 96 print as 160 Inverse 4 224 AND 33 5 97 print as 461 Inverse 5 225 OR 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 u gt N N XX Ss sc TO ZETA Sf 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as print as 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 Inverse 6 Inverse 7 Inverse 8 Inverse 9 Inverse A Inverse B Inverse Inverse D Inverse E Inverse F Inverse G Inverse H Inverse Inverse J Inverse K Inverse L Inverse M Inverse N Inverse Inverse P Inverse Q Inverse R Inverse S Inverse T Inverse U Inverse V Inverse W Inverse X Inverse Y Inverse Z 226 227 228 229 230 231 232 233 23
6. 24 INPUT A 30 PRINT A 40 PRINT A because the computer always sorts out the lines in numerical order By leaving gaps between line numbers you can always do this It also makes it easy to see how you ve altered the program since the original lines will have line numbers in multiples of 10 unless the lines were added at the end of the program like statement 40 Now try running the program again FIRST it asks you for a string Enter a string THEN it asks you for a number Notice that when the ZX 80 is waiting for you to enter a number it displays L S and not L Enter a number FINALLY it prints out what you put in In chapter 3 the difference between string variables and integer variables is mentioned The program has two variables in it a string variable A and an integer variable A How does the computer react when you give it a different sort of variable to the one that it s expecting Easy enough to find out just run the program again When the computer asks you for a string give it anumber What happened It was accepted numbers are acceptable inside strings Now when it asks you for a number type in a letter Nothing much happened except that 2 24 appeared at the lower left hand corner of the screen This is another error code The 2 stands for the type of error VARIABLE NAME NOT FOUND because the computer knew that the letter was the name of an integer variable and you hadn t assigned a
7. 30 This lists 11 symbols together with their codes starting with the code entered at the start of the program Code Character 0 Space 1 Null String 2 Graphics 3 Graphics 4 Graphics 5 Graphics 6 Graphics 7 Graphics 8 Graphics 9 Graphics 10 Graphics 11 Graphics 12 13 14 15 16 17 18 19 20 ii 21 22 23 gt 24 lt 25 26 27 28 0 29 1 30 2 31 3 32 4 33 5 34 6 35 7 36 8 37 9 Code Character 128 Inverse Space 129 Inverse Quote 130 Inverse Graphics 131 Inverse Graphics 132 Inverse Graphics 133 Inverse Graphics 134 Inverse Graphics 135 Inverse Graphics 136 Inverse Graphics 137 Inverse Graphics 138 Inverse Graphics 139 Inverse Graphics 140 Inverse 141 Inverse 142 lnverse 143 nverse 144 Inverse 145 Inverse 146 Inverse 147 Inverse 148 149 Inverse 150 Inverse 151 Inverse gt 152 Inverse lt 153 Inverse 154 nverse 155 Inverse 156 Inverse 0 157 nverse 1 158 Inverse 2 159 Inverse 160 Inverse 4 161 Inverse 5 162 Inverse 6 163 Inverse 7 164 Inverse 8 165 nverse9
8. Even simple tasks can turn out to be more complicated than we think Most of the problems which we ve spotted are due to a lack of information and this can be coped with by adding extra instructions at the beginning of the program These could include 1 Ask where the shop is 2 Find clothes When you are writing programs for the ZX 80 it is particularly important to give the computer all the information it needs before it has to carry out a task For instance if you ask it to PRINT a variable the computer must know the value of the variable Now for some arithmetic CHAPTER FINDING THE ANSWERS FINDING THE ANSWERS Up to now you ve just been getting used to the feel of the computer Now we ll actually use the computer to do a few sums THE LET STATEMENT Nearly all arithmetic operations are done by using the LET statement It is of the form LET variable expression In this case the variable which can have any name you assign to it is what you want to find out The expression describes how you want to define the variable For instance LETA 2 4 variable expression means add 2 to 4 and set A equal to the result 6 You can use other variables LETA 2 B or LETA B C One condition the computer must already know what the values of B and C are before it comes to the LET statement The equal sign isn t used in quite the same way it is in ordinary arithmetic or algebra For instance L
9. Here is a program which uses PEEK and POKE to gain access to the TV frame counter PEEK POKE REACTION TIMER 10 FOR I 1 to 20 RND 100 20 NEXTI 30 POKE 16414 0 40 POKE 16415 0 50 PRINT HIT RETURN 60 INPUTCS 70 LETA PEEK 16414 80 LET B PEEK 16415 90 PRINT YOUR REACTION TIME WAS B 256 A 4 20 MILLISECS 16414 and 16415 are the addresses of the two halves of a 16 bit number which counts the frames on the TV increasing by 1 every 1 50th of a second Lines 30 to 40 set the count to zero and the count is stopped when a null string is input to C There is a delay on all operations mainly between pressing the return key and this signal getting to the CPU of around 60 mS hence the 4 subtracted from the expression in line 90 This could have a repeat mechanism tacked on the end for example 100 PRINT DO YOU WANT ANOTHER GO 110 PRINT TYPE Y OR N 120 INPUT C 130 IF C Y THEN GO TO 10 140 STOP Another function which allows the user to communicate directly with the ZX 80 is USR A this calls a machine code subroutine at the address A The value is whatever the routine leaves in HL a storage register within the central processor of the ZX 80 or if the subroutine has not altered HL the result is A Typical form LET J USR A PEEK POKE and USR A are really facilities provided for very experienced users who understand the detailed working of the ZX 80 CLEAR is a statement which resets a
10. If you have read all the way through this book running the programs in it and writing your own you should be well on the way to becoming a fluent BASIC programmer Remember though that there are many things you can do with ZX 80 BASIC which you can t do using other BASIC S Equally some BASIC s have features which are not present in ZX 80 BASIC The appendices present a concise summary of the error codes and the ZX 80 BASIC Section 2 of Appendix 2 is written with experienced users in mind but most of it does not require a detailed knowlege of the working of the central processor From now on it s up to you APPENDIX ERROR CODES ERROR CODES When results are displayed a code n m is displayed also n is the error number m is a line number in the program Stmt type Code meaning 0 m next line that would have been executed BREAK pressed m 1 or 2 Command successfully completed m lt 0 or gt largest line number in program GO m was executed m largest line number in program end of program NEXT 1 NEXT lt id gt where lt td gt Is not the control variable of an active FOR loop Any 2 variable name not found any variable being used or array name of an array element being assigned to Any 3 subscript out of range or error of any kind while evaluating a subscript LET 4 no room to add new variable or to assign a longer INPUT string to a str
11. NEXTJ In chapter 9 it took 5 lines of program to get the same result using IF and GO TO statements Note the after the PRINT statement This tells the computer to print each immediately after its predecessor Try running this program It s a pity that one can t have a dollar bill for each sign printed In BASIC these loops are normally called FOR loops They are also widely known as DO loops because other languages such as FORTRAN use DO instead of FOR There is nothing to stop us from putting a loop inside another loop or putting several loops in Add the following lines to the program 30 FORI 1TO3 40 PRINT 50 NEXTI The Flowchart for this program is Major loop No Is 7 A 43 12 ae A A Y You can jump out of a loop at any time If you do this the loop control variable will remain at whatever value if had got to when you jumped out In our case the loop control variables were J for the major loop and I for the minor loop Try jumping out of the major loop at J 100 add 15 IF J 100 THEN GO TO 1000 1000 STOP Run this You ll see that you get fewer s and s Get into command mode Now type PRINT J NEWLINE The computer responds with 100 This shows two things 1 Jwas 100 when the program jumped out of the loop and stopped 2 You can use PR I NT to find out what state the control variables or other variable were in when the program stopped This is especia
12. always vii USR n call machine code sub routine at address n Value is whatever the subroutine leaves in HL or n if it doesn t alter ML ABS n if n 0 then n else n ix array element a n Example A I H May be used as dest for INPUT see3 b x n parenthesized integer expression Values of string expressions can be of any length and can contain any codes except 1 the closing quote Values of integer expressions must be in the range 32768 to 32767 any value outside this range causes a run time error number 6 Note that relations yield 1 for true and Ofor false and that isthe same as n OANDn is the same as 0 1ORn is the same as 1 0ORn is the same as n also NOTn is the same as n 1 so that for instance I AND I gt 0 OR I AND 1 lt 0 is the same as ABS I However constructions such as A gt B gt C do not have the obvious effect being parsed as A gt B gt C ie as A gt B AND lt 1 OR NOT A gt B AND lt 0 b Statements The statements available are NEW re initialise the computer to the state it has at initial switch on Loses all program variables etc LOAD read system variables program and variables from tape and re initialise input line and remake display file Does not affect any GOSUB blocks that may be on the stack although the effect of doing a RETURN is unlikely to be sensible BREAK after data has been found on the
13. effect When or 1f this happens delete the quotes using RUBOUT gt RUBOUT and enter say CHR 10 6 which causes arithmetic overflow giving a 6 N error message when N is the line number of the INPUT statement reading the string Generally speaking the BREAK key SHIFT SPACE is the first thing to resort to If the program is caught in an endless loop or if it is LOADing unsuccessfully the screen will grey or black for an indefinite period BREAK will get you back under these conditions If BREAK does not work in this situation there is nothing left to do but switch the ZX 80 off for a few seconds and then switch on You do lose whatever program was in the ZX 80 if you do this When it comes to faults in the program it is difficult to offer such specific advice Some problems arise if you type an O instead of a 0 LET J O would be accepted as a valid program line but would give a 2 N error code variable not found when the program was run This sort of thing can be very difficult to spot Similarly S and do get confused When a program stops unexpectedly or does something peculiar it may be difficult to work out exactly what went wrong It is possible to carry out a post mortem by using the immediate PRINT statement to find out what the value of variables especially loop control variables was at the time the program stopped If you type PR I NT J or whatever variable you want the ZX 80 will print it even after
14. in the latter case it will begin with a line number in the range 1 to 9999 and in the former case there should be no number although zero in practice counts as no number here Somewhere in the line a cursor is displayed This indicates two things the position in the line where symbols will be inserted and whether an unshifted alphabetic key will be treated as a keyword eg LIST or a letter eg A The cursor is in the form of an inverse video K for keywords or L for letters Note that this cursor although displayed in the line and occupying a character position on the screen does not form part of the line and is ignored by anything interpreting the line A second symbol similar in principle to the cursor may also be displayed this is in the form of an inverse video S and indicates that the line is not a syntactically correct BASIC state ment It is positioned such that the part to the left of it could be the beginning or the whole of a syntactically correct BASIC statement eg if 20 LET A B 5 is input from left to right then S displayed at the end of 20 LET and be absent from 2 20 LET A 20 20 LET A 20 LET A B 20 LET A B 20 LET A B 5 In most cases the symbol is displayed as far to the right as is consistent with the above description however there are a few circumstances where this is not quite so for instance in LETA ASC X although LET A ASC is a syntactically correct statement ASC here bei
15. of as many lines as have been written by PRINT and the lower part follows on rather than being placed at the bottom of the screen It is possible for the upper part to contain 24 lines in which case the lower part is not visible although the keys all function normally and provided the user is able to work blind the input can still be submitted The keys f and U have no effect the effect of EDIT is rather comical and not very helpful although it doesn t actually crash the system The INPUT statement cannot be used as a command because of the conflict in use of the working space however in this situation LET can be used instead Using INPUT as a command causes error code 8 2 If dest is an array element and there is an error in evaluating the subscript or the subscript is out of bounds the error is not reported until after the input value has been submitted PRINT lt list gt in which lt list gt may be lt empty gt or lt expr gt LISTn LIST STOP DIM a n FOR a nTOn GO n POKE n n or lt list gt lt list gt or lt list gt lt list gt and lt expr gt may be s or n writes the value of each lt expr gt to the upper part of the screen ie in the case of s writes the body of the string converting tokens into characters and in the case of n writes STR n Each comma causes the output to tab to the 9th 17th or 25th column on the line of to the 1st column on the next line A new
16. tape does the same as NEW BREAK during the lead in preserves the current program and variables SAVE Write system variables program and variables to tape for subsequent reading by LOAD Precedes data with 5 secs of silence starting the tape before executing SAVE which is recommended practice writes frame sync pulses to the tape The above can all be used in programs but are intended to be used as commands and use in programs is not particularly sensible RUNn RUN CONTINUE is the same as CLEAR followed by GOTO n is the same as RUN 1 is the same as GO TO n where n is the last number in an end of program message m n with m gt 0 see 1 and 3 c However after message 9 n CONTINUE is the same as GO TO n 1 REM any text no effect ie is comment IF n THEN statement INPUTdest executes the statement unless n statement is zero ie unless n is false where dest can take any of the forms marked above see 3 a Returns to the input mode with any output so far produced by the PRINT statement in the upper part of the screen and the input line initialised to contain the cursor alone if dest is an integer and the cursor inside a pair of quotes if dest is a string These quotes can however be deleted as the input can be any expression not necessarily a literal The expression is checked for correct syntax and NEWLINE is ignored if the S marker is present The upper part of the screen consists
17. that if you use the SHIFT key you get the symbol printed on the top right hand corner of the key You ll also notice that quite a lot of keys have labels above them such as NEW above the Q key These will save you a lot of time from now on One vital point it is very important to distinguish between the letter O and figure 0 or Zero In the book we will use O to mean letter O and 0 to mean zero On the screen the computer uses a square O O for letter O and a hexagonal 0 0 for zero The reason that we ve used 0 in the book is because it is easier for printing and because all other books on programming use it SINGLE KEY KEYWORD SYSTEM Hit the Q key Amazing The computer writes NEW on the screen Now hit NEWLINE You have cleared the computer ready to accept a new program This illustrates the single key keyword system Most of the words you ll have to use with ZX 80 BASIC can be typed with only one keystroke in this way In all this may save you up to 40 of the typing you d otherwise have to do Some BASIC S allow you to use single keystrokes to input words like NEW PRINT RUN etc but only the ZX 80 prints out the word in full This makes it much easier to keep track of what s going on in programs You ll see how it works as you go along The labels above the numeral keys NOT AND etc are not keywords They are called tokens and when needed are obtained by using the SHIFT key This also applies to EDIT on the NEWLINE key
18. the program has stopped running Sometimes there is not room to print the whole of the line number in an error message particularly with error numbers 4 and 5 In this case usually only the first digit of the line number is printed For example error 4 on line 250 may cause the message 4 2 Another useful technique is to put STOP statements into programs at key points When the program reaches the STOP statement it will stop of course and you will be able to see how it has performed up to that point You can then get back into command mode type CONTINUE key T NEWLINE and the program will continue from the STOP statement Luckily the ZX 80 is quite choosy about the program lines it will accept and this eliminates many of the problems which can happen with other BASIC s CHAPTER A RAGBAG OF FUNCTIONS A RAGBAG OF FUNCTIONS This chapter covers all those statements and functions that haven t already been dealt with elsewhere RND X provides a random number in the range 1 to X Typical statement using RND 10 LETJ RND X sets J equal to a random number Every time the ZX 80 executes the function RND it uses a random number generator to generate a pseudo random number It is called pseudo random because the numbers occur in a fixed sequence However the sequence is very long and hence appears random RANDOMISE sets the starting point of the sequence to a number equal to the number of frames supplied to the TV sinc
19. to MAXIMUM BASS to MINIMUM set the volume control to about 1 2 or 3 4 of MAXIMUM Have you got that program Unplug the MIC plug or DIN plug from the recorder and use the recorder s microphone to record the program s title if you haven t recorded your voice saying the program s title you will have trouble in finding programs when you ve got a lot of them on tape Stop the recorder Reconnect the plug to the recorder Get into command mode Start recording and then type SAVE E key and NEWLINE The screen will go grey for about 5 seconds then you ll see a series of horizontal streaks across the screen After a few seconds the screen will clear and will show the program listing You will want to check that the program has been SAVED Unplug the EAR plug Rewind the tape until you get back to your voice Now play back You will hear your title then you may or may not hear a short buzz followed by about 5 seconds of silence You will then hear a peculiar sound rather like a supercharged bumble bee This is the program being played back via the loudspeaker After a while the sound will change to a loud buzz Rewind until you are at the beginning of the 5 second silence Reconnect the EAR plug Start playing back and then type LOAD W key NEWLINE The screen will go grey or black and after a few seconds it may start looking grey but agitated After a few more seconds the screen should clear to show a listing of the program Note when you
20. value to it The 24 stands for the line number at which the error was found EDITING PROGRAMS Suppose that you want to alter something in a program that you ve already entered This is called editing a program It s easy on a ZX 80 Get into command mode Look at the listing at the top of the screen The gt cursor current line pointer is at line 40 Now see what happens when you hit SHIFT 7 f a few times the cursor moves up line by line SHIFT 6 U moves it down line by line These keys allow you to select any line in your program to edit A further key HOME SHIFT 9 sets the line pointer to line 0 Because there isn t a line 0 the cursor will vanish if you use HOME but when you hit SHIFT 6 the cursor will jump to the next line in this case line 10 and reappear Select a line line 22 for example Now hit SHIFT NEWLINE Line 22 or any other one you might have selected appears at the bottom of the screen Try SHIFT 8 you ll see the cursor jump past PRINT SHIFTS lt moves the cursor left to where it was before Try moving the cursor right a few times say to the position after ENTER Now type a few A s they are inserted into the string to the immediate left of the cursor A few RUBOUTS and you can erase them again This editing facility is very useful if you want to alter individual characters within lines However if you want to delete a whole line it would be rather tedious to go
21. waiting for a string variable to be entered the BREAK key SHIFT SPACE doesn t work As a result it is difficult to get out of the program except by entering a very long string variable a variable so long in fact that it makes the computer run out of storage space This is very tedious so it s better to use the IF statement Then if you enter any character the program will stop If you do ever get into a situation when nothing you do seems to have any useful effect on the computer try the BREAK key first If this has no effect give in and switch the ZX 80 off for a few seconds You lose the current program but this may be inevitable if you really have got yourself into a fix CHAPTER ITER WHAT ITER WHAT Very often we come up against problems which involve repeated operations The square rooting program in chapter 8 was one example of such a problem and the program was written so that the computer jumped back to line 50 if the test conditions in statements 70 or 80 were not met and tried again and again until the conditions were met Programs of this sort are called iterative By using iterative techniques it s possible to get the computer to do a lot of work with a very short program It s easy to make a program iterative all you have to do is to put a GO TO statement at the end of the program to jump the computer back to the first statement or any other suitable statement One snag is that such i
22. 10 address of next item in syntax table very unlikely to be useful U2 16412 seed for the random number generator This is set by the RANDOMISE statement q v and updated each time RND is called see note below U2 16414 number of frames displayed since the ZX 80 was switched on more exactly the remainder when this 1s divided by 65536 While a picture is on the screen this number is incremented 50 times per second in the UK version 60 times per second in the US version N2 16416 address of 1 character of 1 variable name in last LET INPUT FOR NEXT or DIM statement Thus in LET ABC PEEK 16416 ABC is assigned the remainder when the address of the A is divided by 256 Not likely to be very useful N2 16418 value of the last expression or variable in practice this is the parameter of PEEK so that PEEK 16418 yields 34 and PEEK 16419 yields 64 always 16420 position on line of next line character to be written to screen 33 lefthand column 32 second from left etc up to 2 righthand column Also 1 1st column in next line because current line is full 0 Ist column on next line because end of line has been signalled Thus after aPRINT not ending a comma or a semi colon PEEK 16420 always yields 0 It only yields 33 if the screen is empty eg after CLS 16421 position of current line on screen 23 top line 22 second line down etc X2 16422 address of the character after the closin
23. 194 print as GRAPHICS 67 print as 131 Inverse GRAPHICS 195 print as GRAPHICS 68 print as 132 Inverse GRAPHICS 196 print as GRAPHICS 69 print as 133 Inverse GRAPHICS 197 print as GRAPHICS 70 print as 134 Inverse GRAPHICS 198 print as GRAPHICS 771 print as 135 Inverse GRAPHICS 199 print as GRAPHICS 72 print as 136 Inverse GRAPHICS 200 print as 9 GRAPHICS 73 print as 137 Inverse GRAPHICS 201 print as 10 GRAPHICS 74 print as 138 Inverse GRAPHICS 202 print as 11 GRAPHICS 75 print as 139 Inverse GRAPHICS 203 print as OC A YO Na 12 76 print as 140 Inverse 204 print as 13 77 print as 141 Inverse 205 print as 14 78 print as 442 Inverse 206 print as 15 79 print as 143 Inverse 207 print as 16 80 print as 144 Inverse 208 print as 17 81 print as 145 Inverse 209 print as 18 82 print as 146 Inverse 210 print as 19 83 print as 147 Inverse 211 print as 20 84 print as 148 Inverse 212 21 85 print as 149 Inverse 213 THEN 22 86 print as 150 Inverse 214 TO 23 gt 87 print as 151 Inverse gt 215 24 lt 88 print as 152 Inverse lt 216 25 89 print as 153 Inverse 217 26 90 print as 154 Inverse 218 27 91 print as 155 Inverse 219 NOT 28 0 92 print as 156 Inverse O 220 29 1 93 print as 157 Inverse
24. 4 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 LIST RETURN CLS DIM SAVE FOR GO TO POKE INPUT RANDOMISE LET NEXT PRINT NEW RUN STOP CONTINUE IF GOSUB LOAD CLEAR REM not available from the keyboard Codes 38 63 only available when the cursor is L codes 230 255 only when cursor is K APPENDIX SYSTEM VARIABLES SYSTEM VARIABLES The contents of the first 40 bytes of RAM are as follows Some of the variables are 1 byte and can be POKEd and PEEKed directly The others are each 2 bytes and have the low order byte at the given address n say and the high order byte at the next address Thus to poke value v at address n do POKE n v POKE n t v 256 and to PEEK at the value in address n use the expression PEEK n PEEK n 1 256 if the value is known to be in the range 0 to 32767 and something like LET MSB PEEK n 1 IF MSB gt 127 THEN LET MSB MSB 256 LET VALUE PEEK n MSB 256 if it may be negative ie if n is 16412 or 16414 The notes at the left hand side of the table have the following meanings X The variable should not be altered by POKE as this could cause the BASIC to hang up N POKE will have no lasting effect as the variable will be rewritten either at the end of the POKE statement or the next time edit mode is entered either at the end of the run or to get some input lor2 number of bytes in va
25. 5 You can use PRINT to print out a variable Now run the program by typing RUN key R and NEWLINE The computer responds with ENTER YOUR STRING IL and waits The L means that it is waiting for you to do something in this case the quotation marks show that is is expecting you to enter a string variable Type in a short message or random string of letters don t use quote marks and press NEWLINE The computer prints out what you have just put in which was a string variable called A Get back into command mode by pressing any key Try running the program several times using more and more characters in your string Keep an eye on the cursor On the 15th line it will vanish RUBOUT shift 0 the cursor will reappear Now carry on adding characters the display will gradually get SMALLER as you use up the storage capacity of the processor Now hit NEWLINE a 4 20 error message appears The 4 tells you that the ZX 80 ran out of space to put the string variable into and the 20 tells you that this happened at line 20 of the program Get back into command mode You may have wondered why the line numbers have been 10 20 30 instead of say 1 2 3 This is because you may want to add bits to the middle of your program For instance enter 22 PRINT ENTER A NUMBER 24 INPUT A 40 PRINT A The program displayed at the top of the screen now reads 10 PRINT ENTER YOUR STRING 20 INPUT A 22 PRINT ENTER A NUMBER
26. 60 IF NOT Y THEN GO TO 1000 470 CLS 500 LET I RND 10 510 LETK RND 3 520 IF K 1 THEN LET 1 0 530 IF K 2 THEN LET B I 0 540 IN K 3 THEN LET C I 0 550 GO TO 200 1000 STOP When typing in a program like this where there are several similar lines the EDIT facility 1s very useful because you can edit line numbers For example after entering line 210 type EDIT lt lt RUBOUT 3 NEWLINE Statements 10 to 30 set up 3 arrays A B and C of 10 elements each Statements 100 to 140 set all the elements of all arrays to 1 200 to 240 examine each element of the array A in turn If an element is equal to 1 a is printed If an element is not 1 a space is printed Similarly for 300 to 340 and 400 to 430 On the first pass all the elements are set to 1 and is printed all the way through 445 to 470 print the instruction and if the user then hits NEWLINE the screen is cleared 500 to 540 select a random element of a random array to be set to 0 At 550 the program jumps back to print out all the arrays The element which has been set to 0 1s printed as a space or nibble As more nibbles are taken there is a greater chance that the element selected will already be 0 and thus the effective nibble rate slows down as the game progresses ABS n this function gives n if n gt 0 and n if n lt 0 n is any integer expression eg 5 5 5 ABS 5 5 CHAPTER OVER TO YOU OVER TO YOU
27. 79 FREDBLOGS N B no space AAAA Z123ABC QTOTAL Some illegal names are 4 4 4AD name must start with a letter FRED BLOGGS space not allowed A B other characters not allowed FRED BLOGGS not a variable but two variables one subtracted from the other String variables have names of the form letter The dollar sign tells you and the computer that the variable is a string variable Because only one letter is allowed this means that you can only have up to 26 string variables in a program Some allowable names for string variables AS P X Some illegal names 2 name must start with letter A2 too many characters AC too many characters All this may seem a little daunting at first The good news is that like so many things it s easier than it sounds CHAPTER TALKING TO THE ZX 80 THEN gt HOME RUBOUT NEW LOAD SAVE INPUT PRINT LIST POKE RAN NEW LINE The Keyboard LET S TALK Set up the computer as described in chapter 2 and switch on You will see a K sign appear at the bottom left hand comer of the screen This is the cursor and shows you where you are on the screen THE KEYBOARD If you look at the keyboard you will see that it looks quite like a typewriter keyboard but there are some differences There is no carriage return key instead there s a key called NEW LINE and the space bar is replaced by a key labelled SPACE All the letters are upper case capitals so
28. ATEMENT 30 40 PRINT END OF PROGRAM Now run the program The computer responded with THIS IS STATEMENT 10 END OF PROGRAM It skipped statement 30 That s not much use you must be saying what s the point of including a perfectly good statement which will never be executed OK Get back into command mode Now type GO TO 30 and then NEWLINE The ZX 80 came back with THIS IS STATEMENT 30 END OF PROGRAM This demonstrates two uses of GO TO first to jump unconditionally to an arbitrary statement in this case statement 40 second as a command which causes the computer to jump to the desired statement in this case 30 and start executing the program from there THE IF STATEMENT This is the most powerful control statement in BASIC It allows the user to incorporate decision making into his programs Here is a program for calculating square roots approximately It does this by multiplying a number by itself starting with 0 and comparing the result with the number whose square root is to be found If the product is less than the number to be rooted it increases the number by 1 and tries again 10 PRINT SQUARE ROOT ROUTINE 20 PRINT ENTER THE NUMBER TO BE ROOTED 30 INPUTX 40 LETJ 0 50 LET K J J 60 LET D X K 70 IF D 0 THENGO TO 110 Use SHIFT 3 to get THEN 80 IFD lt OTHENGO TO 130 lt is SHIFT N 90 LET J J 1 100 GO TO 50 110 PRINT THE ROOT IS J 120 GO TO 140 130 PRINT THE RO
29. ET N N 1 530 GO SUB 500 540 LET N N 1 550 GO SUB 100 560 PRINT N 2 ON 570 GOSUBS 2f0 575 LET N N 2 580 RETURN Your output for the case N 4 should look like 2 OFF 1 OFF 4 OFF 1 ON 2 ON 1 OFF 3 OFF 1 ON 2 OFF 1 OFF The GO SUB statement can be used in conjunction with a variable e g 10 GO SUB G whereupon the computer will jump to the subroutine at line whose number is the value of G if there is such a line As before an expression can also be used In general it is not a very good idea to use variables or expressions in this way because the program may alter the variable a way you hadn t foreseen and thus cause problems CHAPTER HOW TO PRINT HOW TO PRINT Up to now we have used the PRINT statement as and when it was needed to print out headings and variables In fact PRINT is a very versatile statement indeed The general form ofthe PRINT statement is PRINT expression ch expression ch expression ch and so on ch stands for control character and this can be or or nothing at all at the end of the statement only Expressions can be literal strings in quotes e g THIS IS A LITERAL STRING or string variables such as A Alternatively they can be integer variables or arithmetic expressions e g A2 or B 2 C The object of using control characters is to be able to control the spacing of the line to be printed We ve already come across the use of in PRINT statements
30. ETJ J 1 Obviously J isn t equal to J 1 What the LET statement means is Take the existing value of J add one to it and then set J to this value The sign is called an operator and defines the operation you wish to be performed The other arithmetic operators are minus multiply there is no x sign to avoid confusion with letter divide raise to the power For the moment we ll leave and out of it Here is a program for multiplying two numbers together 10 PRINT MULTIPLICATION 20 PRINT ENTER FIRST NUMBER 30 INPUT A 40 PRINT ENTER SECOND NUMBER 50 INPUT B 60 LETC A B 70 PRINT THE ANSWER IS C Notice line 70 It s a PRINT statement with a string between inverted commas but it s got a C added to it This form allows you to economize on PRINT statements Enter the program and try running it using two quite small numbers such as 17 and 25 The computer will come up with the answer THE ANSWER IS 425 Run the program a few more times increasing the size of the input variables At about 255 x 129 or just a bit larger you will get another error message 6 60 The 6 is the code for arithmetic overflow because the answer is larger than 32767 which is the largest number the computer can hold as an integer variable Now try editing the program so that it does addition instead just change the to a in line 60 You can also try subtraction and check that you get nega
31. G would use 12 2 x 6 24 bytes Apart from these two cases the stack is only used for subroutine calls and for saving registers b Actions The actions taken by the computer in response to the user s keystrokes are as follows Each time a symbol or token is inserted into or deleted from the input line also each time the cursor is moved this change is put into effect in the input line held in working space after deleting the lower part of the display file viz that part from DF_EA to DF_END note that during this period the display file may be incomplete in that less than 25 newline characters are present although the display file is never allowed to become large enough that there will not be room to add the remaining newline characters Then the input line is checked to see if it is syntactically correct The input line contains an inverse video K at the point where the cursor is the syntax checker notes the address of the cursor in the variable and sets variable to point to the first wrong symbol or to zero if there is none It also notes whether the cursor should be displayed as K or an L Finally the lower part of the display file is rebuilt inserting the S symbol and changing the cursor from K to L if required as well as converting tokens into characters If there is now insufficient room for the display file the display file area is cleared and the upper part is remade with fewer lines b
32. GO TO 1000 400 PRINT D 405 PRINT E 410 PRINT B 415 PRINT E 420 PRINT C 430 GO TO 1000 500 PRINT A 505 PRINT E 510 PRINT E 515 PRINT 520 PRINT A 530 GO TO 1000 600 PRINT A 605 PRINT E 610 PRINT B 615 PRINT ES 620 PRINTAS 630 GO TO 1000 700 PRINTAS 705 PRINT E 710 PRINT A 715 PRINT E 720 PRINTAS 1000 STOP It s a bit of a strain to enter this program but it s quite fun to run Note that lines 140 to 190 could have been replaced by 140 GOTO X 1 100 If you want to make the program restart itself try editing the program 1000 PRINT HIT NEWLINE TO THROW AGAIN 1100 INPUT X 1200 CLS CLS is on key C 1300 IF X THEN GOTO 120 Set up output strings Let X RND 6 DIE THROWING pe eal zt Print No c Print Fe M E Print Two points CLS clears the screen If you didn t put this in the screen wouid fill up after 3 goes and the program would stop giving an error message such as 5 305 run out of screen at line 305 Secondly the use of a INPUT statement to halt program execution is a useful trick The string variable X is used in statement 1300 when it is tested to see that only NEWLINE has been pressed any other entry will stop the program There is nothing between the quotation marks in 1300 The reason that there is a IF statement at 1300 rather than a TO statement is that while the ZX 80 is
33. Getting data in and out of the ZX 80 computer The PRINT statement allows you to get data out of the computer This can be done while a program is running or afterwards Type exactly 10 THIS IS A STRING using SHIFT Y for That should have come out as 10 PRINT THIS IS A STRING The computer provided the word PRINT as well as the space between 10 and PRINT Did you notice what happened to the K cursor First of all it followed what you were typing in appearing just after the last character typed After PRINT it changed to a L sign this showed that the computer was not expecting any more single key keywords just then After you typed you may have noticed that another character appeared as well a S sign What was actually on the screen was 10 PRINT S This S 1s the syntax error marker and occurs if there is a syntax error in the line being typed in In this case it came up because the line needs the closing quotation marks if it is to be correct The S followed the cursor as you typed in THIS IS A STRING and vanished when you typed the second Now erase the by using the RUBOUT key The S reappears Try hitting the NEWLINE key nothing happens because the computer won t accept a line containing a syntax error Type and hit the NEWLINE key The computer accepts the line and displays it at the top of the screen together with a gt cursor which shows that it is the last and in th
34. IF A Z Z B B THEN GO TO 10 Texpression is quite valid and so is IF A B gt C D THEN GO TO 20 The condition to be met can be even more complex because logical operators can be used For example 10 IF NOT A 2 THEN GOTO 100 logical operator use SHIFT 1 for NOT This NOT operator negates the succeeding condition so that the ZX 80 will jump to 100 if A does not equal 2 This program illustrates the use of the AND operator 10 INPUT A 20 INPUT B 30 INPUT C 40 IF A 1 AND B 1 AND C 1 THEN PRINT OK T TAnother logical operator T Use SHIFT 2 for AND The condition to be met is that ALL three variables must be 1 if this is so it will print OK You can chain together as many conditions as you like in this way Try running this program and see if you can make it print OK in any way other than by entering three 1 s The other logical operator is the OR operator Edit the program by changing the AND s to OR s OR is SHIFT B When you run the program now you will find that it prints OK as long as one of the numbers entered is a 1 You can use brackets to group things together Consider the following statements a 40 IF A 1 1 AND C 1 THEN PRINT OK b 40 IF A 1 OR B 1 AND C 1 THEN PRINT OK If you remember in chapter 6 we said that some operators had priority over others multiplications were carried out before divisions or additions for instance The same applies to logica
35. OT LIES BETWEEN J 1 AND 140 STOP See the diagram on the next page The form of the I F statement is IF CONDITION TO BE MET THEN DO THIS 70 IF D OTHENGOTO 110 In line 70 the condition to be met was D 0 and if this is so then J is exactly the square root of X so we want to print THE ROOT IS J In this case DOTHIS is GO TO 110 which is the number ofthe PRINT statement we want In most BASIC S the only thing you can do in a IF statement is to GO TO a line number The ZX 80 lets you do other things as well In fact it lets you do almost anything Some examples 70 IF D OTHEN PRINT NUTS 70 IF D OTHEN INPUT G 70 IF D 0THEN LET A 10 Print headings Input number to be rooted x FLOWCHART FOR SQUARE ROOT PROGRAM Print J 1 and J Any keyword can follow THEN in an IF statement even things like LIST or RUN The results can be rather peculiar if you use commands such as LIST The examples listed above can be useful though The condition to be met is in its simplest version of the form Expression relational operator expression This sounds rather a mouthful A relational operator can be equal to A less than gt greater than Examples A A equal to 2 lt 10 less than 10 A B gt C D A B greater than C D In these cases 2 10 A B and C D are the expressions The expressions need not be a simple integer or integer variable The statement
36. SAVE a program you also SAVE all the data and variables You can avoid deleting these RUN clears them by using the command GO TO 1 If it doesn t use BREAK SHIFT SPACE to stop LOADING If this doesn t work unplug the power from the ZX 80 for a few seconds Try repeating the procedure using different volume control settings If this doesn t work are you using the cassette recorder on mains power Try again using battery power If this fails check the wiring to MIC and EAR Good luck CHAPTER NOT SO BASIC NOT SO BASIC Nearly all digital computers such as the ZX 80 talk to themselves in binary codes We talk to each other in English This means that either we have to learn binary codes or we have to teach the computer English Binary codes are about as difficult to learn as Chinese but it can be done Forty years ago you had to learn them if you wanted to use a digital computer at all But why should we do the work when we ve got a computer to do it for us So we teach the computer English There is a snag though Computers so far aren t bright enough to learn English something which a child of two does relatively easily This is mainly because we have so many words and ways of saying things in English A computer would have to learn every single one We have to compromise half way between binary and English Sometimes we compromise at a low level by using assembly codes which are more like binary than English Sometime
37. X 80 BASIC These are READ DATA RESTORE END ON Nearly everything that can be done by using these statements can be done in ZX 80 BASIC by using other statements The END statement is not needed at all in ZX 80 BASIC In a BASIC program all statements or lines are preceded by a statement number or line number which labels that particular line Line numbers can vary between 1 and 9999 The computer carries out or executes statements in the order in which they are numbered It makes life easier by always displaying programs in order of increasing statement numbers so that the order in which the program is listed is also the order in which it would be executed VARIABLES All the pieces of information stored in the computer for use in a program are labelled so that the computer can keep track of them Each piece of information or variable has a name There are two sorts of variable l Numbers integer variables which can take any whole number value from 32768 to 32767 inclusive Example 142 2 Strings which can be any sequence of any characters except of any length Example AROUND THE WORLD IN 80 DAYS Integer variables have names which must always start with a letter and only contain letters and digits Integer variable names can be any length so if you wish the name could be a mnemonic for the variable Some allowable variable names for integer variables are A A2 AB3 ANSWER Y Y8 YZ
38. ZX80 Operating Manual First issue developed during June 2003 The Most up to date Version of this manual is always available at http www zxsoftware co uk http www worldofspectrum org HTML and PDF versions of this document are derived from the original word Document Any errors and omissions will be corrected in future versions The Author of this document can be contacted via http www zxsoftware co uk The original document is copyright Science of Cambridge 1980 Document developed using Word 2000 Enjoy Tony Barnett Malta June 2003 Digitally signed by Tony Barnett Y DN cn Tony Barnett ony Barnett Date 2003 06 26 Signature No 16 59 28 Z rifi A Course in BASIC Programming CONTENTS Chapter 1 INTRODUCTION Chapter 2 GETTING STARTED Chapter 3 NOT SO BASIC Chapter 4 TALKING TO THE ZX 80 Chapter 5 SO YOU VE GOT PROBLEMS Chapter 6 FINDING THE ANSWERS Chapter 7 DECISIONS DECISIONS Chapter 8 BRANCHING OUT Chapter 9 ITER WHAT Chapter 10 LOOPING THE LOOP Chapter 11 HOW TO PRINT Chapter 12 COPING WITH CHARACTERS Chapter 13 HELP OR WHAT TO DO WHEN DESPERATE Chapter 14 A RAGBAG OF FUNCTIONS Chapter 15 OVER TO YOU Appendix I ERROR CODES Appendix II 4K BASIC FOR ZX 80 Appendix SYSTEM VARIABLES Index 1980 CHAPTER INTRODUCTION INTRODUCTION READ CHAPTER TWO FIRST This book is the user s manual for the ZX 80 personal computer In it is everything you need to know in order t
39. ad or notebook handy when you are writing programs and an even better one to draw a flow diagram of your program before you start writing it Turn to the next chapter to find out how the ZX 80 can take decisions GETTING UP There are three sorts of symbols or boxes Begin or end Processing block this encloses anything which can be done without making any decision Look at clock Decision diamond encloses a decision with branches to either side depending on whether the decision is yes Read book until jt is time Do need clean clothes Get clean clothes CHAPTER BRANCHING OUT BRANCHING OUT Before we go on to real decisions lets have a look at the GO TO statement This is ofthe form GO TO n where n is normally a line number It can be a variable though only the ZX 80 BASIC has the power to do this When the ZX 80 comes to a GO TO statement it jumps straight to statement number n and executes that statement If n is a variable X say the computer will look at the value of X and jump to the statement with that number assuming that there 1s a statement with that number in your program A further refinement of the GO TO statement is that n can be any integer expression such as A 10 Here again this is possible with the ZX 80 Here is a program which illustrates the use of GOTO 10 PRINT THIS IS STATEMENT 10 20 GO TO 40 30 PRINT THIS IS ST
40. ains a current line number RAM stands for random access memory or store for editing the program and the display is always organised so that the line with that number or the preceding line If no line with that number exists is on the screen if at all possible If there is a line with the current number it is displayed with a symbol consisting of a reverse video gt between its line number and the text of the line if there is none then the reverse video gt will not appear Three keys are provided for changing the current line number fl changes it to the line number of the preceding line U changes it to the line number of the following line and HOME resets it to zero If there is no preceding line fl sets it to the line number of the first line in the program similarly U will set it to the last line There are two other ways which the current line number can change inserting a line into the program sets it to the line number of that line and the command LIST n will set it to n When the current line is off the top of the screen the window moves up so that it becomes the first line When it is at or just off the bottom the window moves down a line If it is well beyond the bottom of the window the window moves down so that it becomes the second line on the screen b Input area The lower part of the screen contains the line the user is currently typing in This line may be a command or a line of program
41. and tokens codes CCfh to FFh although reverse video characters codes 80h to BFh have also been allowed for The variables take the forms shown on the next page They are not stored in any particular order in practice each new variable is added onto the end When a string variable is assigned to the old copy is deleted and a new one created at the end Created first LET A 5 does work Note that apart from the ms bit of the first byte a single character integer is the same as the controlled variable of a FOR loop The characters in a name being all alphanumeric have 6 bit codes as in the character code table The first character in a name being perforce alphabetic ie in the range 26h to 3Fh effectively has a 5 bit code The variables area is terminated by a single byte holding 80h which can t be the name of a string The working space holds the line being input or edited hence E LINE except when statements are being obeyed when it is used for 1st Byte Subsequent bytes ETY 011 Ls byte Ms byte Integer 1 char name Letter Value ES 010 00 10 5 byte Ms byte 1 Integer gt 1 char name lali LLL EC UP E Letter 20h 2nd char Last char 80h Value TTT ES 100 00 00 0001 String body may be null Letter 20h Body of String Quote code 1 ro n Ls byte Ms byte I Ls byte Ms byte Array Lett
42. e NEXT J An example of the use of subroutines is given below The Chinese Ring Puzzle As you will see one subroutine can call another or even call itself this is called recursion THE CHINESE RINGS PUZZLE This is a program which will say what moves are required to remove N rings from the T shaped loop The mechanics of this wire puzzle are not important roughly speaking one manipulates the rings until they all come off the loop For an arbitrary number of rings the rules are as follows 1 Each ring can be either the loop or off it 2 Only one ring may be moved from on to off or vice versa at a time 3 The first ring may be moved at any time 4 The ith ring 121 may be moved if and only if a All the rings numbered 1 2 or lower are off b Ringi l is on the rings higher up the loop number gt i may be in any state so To remove the first i rings 1 Remove the first 1 2 rings 2 Remove the ith ring 3 Replace the first 1 2 rings 4 Remove the first i 1 rings To replace the first i rings 1 Replace the first 1 1 rings 2 Remove the first 1 2 rings 3 Replace the ith ring 4 Replace the first 1 2 rings CHINESE RINGS Recursive procedure 10 INPUTN 20 GO SUB 100 30 STOP 100 IF N lt 1 THEN RETURN 120 LET N N 2 130 GO SUB 100 140 PRINT N 2 OFF 150 GO SUB 500 160 LETN N 1 170 GO SUB 100 180 LETN N 1 190 RETURN 500 IF 1 THEN RETURN 520 L
43. e the machine was turned on unless POKE is used to alter the count see below RANDOMISE n sets the starting point of the sequence to n unless n is 0 when it behaves as RANDOMISE The RANDOMISE statement allows the random sequence to be initialised at any time in the program Typical forms 10 RANDOMISE 6 10 RANDOMISE RANDOMISE n n 4 0 generates a sequence depending only on the value of n which will be the same from run to run if n is unchanged RANDOMISE generates a different sequence each time One highly useful statement is POKE This is of the form where is the ADDRESS of a location in store and is an expression the value of which should be less than 256 for sensible results Typically A might be the address of one of the two bytes which form the variable which acts as the frame counter This is demonstrated in the next example program Thus LET X PEEK A sets X equal to the contents of address A PEEK A is always less than 256 in range 0 to 255 The reason that the variables associated with PEEK and POKE should always be less than 256 255 is maximum is that everything in the ZX 80 is stored in 8 bit bytes Bit is short for Binary digit The maximum which can be stored in 8 bits is 255 and so all the integer variables use up 2 bytes which allows numbers up to 32 767 to be stored Each half of a variable is stored in one byte and every byte has an address of its own
44. ee Variables Iterative programs Jumps 26 22 104 22 104 22 104 26 103 89 105 9 106 108 36 26 25 99 35 110 109 71 32 43 61 115 64 115 47 115 78 48 113 24 113 57 47 Keyboard Keyword LET Line numbers LIST LOAD Literal number Literal string Loops Multiplication NEW NEWLINE NEXT NOT Operators arithmetic relational logical OR PEEK POKE Power connections PRINT Priority of operators Program listing RAM RANDOMISE Recording programs on tape REM RETURN RND RUBOUT RUN SAVE SHIFT SPACE Stack Statement numbers see line numbers STOP String variables see Variables STR Subroutines Subscripts Subtraction Syntax error marker System variables TL Token Truncation USR Variables integer variables string variables 21 22 22 105 35 17 26 114 11 112 111 71 61 35 22 112 23 61 115 50 35 35 50 50 50 88 111 87 115 9 71 114 37 111 26 103 87 115 11 63 64 115 87 112 24 105 23 113 11 113 22 105 110 107 85 114 79 64 89 36 23 104 107 121 T1 22 105 36 89 112 17 17 111 17 110 Zero confusion between letter and number 0 22 Printed by Heffers Printers Ltd Cambridge England
45. er 120 LET D3 10 R3 Y Last decimal place 130 PRINT THE ANSWER IS 3Z D1 D2 D3 This program does a long division in exactly the same way as we would do it on paper Try running it Notice the PRINT list in line 130 it contains several literal strings and variables separated by semicolons The semicolons tell the ZX 80 that each thing to be printed must be printed immediately after the preceding item without spaces between them Also notice the LET statements from line 70 on There s no fiddle you can use more than one operator per statement There is a snag though Operations are not necessarily carried out in order from left to right The order in which they are carried out depends on what sort of operations are present in the statement Here is a list of priorities for arithmetic operations First A to the power Second A Negation or multiplication by 1 Third A B Fourth A B Fifth A B or Now for some experiments 10 REM TEST PROGRAM REM is a keyword key Y 20 PRINT ENTER A 30INPUT A 40 PRINT ENTER B 50 INPUT B 60 PRINT ENTER C 70 INPUT C 80LETZ A B C 90 PRINT Z Try running this with A 1 B 2 C 3 but before you start use the priority rules to predict what answer the ZX 80 will give 7 or 9 O K run it now The answer was 7 First the computer multiplied 2 by 3 to get 6 then it added 1 to get the final answer
46. er Max subscription Element Element n Controlled 111 Ls byte 3 Ms byte Ls byte Ms byte Ls byte Ls byte variable 1 en um ofafor 7 See loop Letter Value Limit ie value after TO 1 line of for stmt temporary strings e g the results of CHR and STR and any other similar requirements The subroutine X TEMP is called after each statement to clear it out so there is no need to explicitly release space used for these purposes The display file always contains 25 newline characters hex 76 the first and last bytes are always 76h and in between 24 lines each of from 0 to 32 inclusive characters DF EA points to the start of the lower part of the screen The stack pointed to by register SP has at the bottom high address end a stack of 2 byte records GOSUB adds a record to this stack consisting of 1 its own line number RETURN removes a record and jumps to the line number stored therein The last 2 bytes of RAM contain a value which RETURN recognises as not being a line number The expression evaluator which is also used to check the syntax of expressions pushes 4 bytes onto the top of the stack for each intermediate result and pops them again when the appropriate operator is found eg A B C D E F G X B CP Y A X E stack C X Y F A B A Y Thus the above expression uses a maximum of 12 bytes of stack Parentheses use an additional 6 bytes each eg A B C D E F
47. g bracket of the call of PEEK or of the newline at the end of the POKE statement The first line of program begins at address 16424 The function RND generates a pseudo random number from the current seed as follows let n be the seed if n is zero take n 65536 instead let m be the remainder when n 77 is divided by 65537 if m is 65536 take m o instead the result of RND x is now x m 65536 the new seed is m The TAB function in printing can be implemented as follows 1000 REM GO SUB 1050 TABS TO 1001 REM COLUMN I 1010 REM GO SUB 1040 TABS TO 1011 REM COLUMN I OUTPUTTING 1012 REM AT LEAST ONE SPACE 1030 1040 PRINT 1050 IF IHPEEK 16420 33 THEN RETURN 1060 IF I 0 THEN IF PEEK 16420 lt 2 THEN RETURN 1070 GO TO 1040 INDEX ABS Addition AND Arithmetic overflow Arrays BASIC Bit Brackets use of Branches BREAK Byte Character set CHR n CLEAR CLS CODE Codes for different characters Commands Command mode Connections to tape recorder Connection to television CONTINUE Control character Control variable Current line pointer Cursor keyword cursor letter cursor Cursor control keys DIM Display Display file Division EDIT Element see Array Error codes or messages Expressions Expression evaluator Field Flowchart FOR TO GO SUB GOTO Graphics characters HOME see Cursor control keys IF THEN INPUT Integer variables s
48. hs of Y X and Z 24 X and plots them in bar chart form As you will see if you run the program it produces a very clear unambiguous display You can use the same sort of trick to achieve greater resolution along the line as well as from line to line Lines 85 to 125 decide what character will be printed depending on the relative size of J Z and Y These examples are only scratching the surface when it comes to character manipulation and graphics The possibilities are almost literally endless CHAPTER HELP OR WHAT TO DO WHEN DESPERATE HELP or What to do when desperate You may occasionally get into difficulties These can be divided into two sorts a Problems with the system b Problems with your programs Most problems with the system likely to occur when you re entering string variables into your programs One favourite is deleting the quotation marks round the string by accident When the computer is waiting for you to enter a string variable it prints L If you enter the wrong string and then use RUBOUT to delete it you may delete one or other of the quotation marks This will give rise to a S syntax error symbol If you see this check to see that both quotes are there If you have written a program which calls for a null string to be entered and the program is recursive you may find it very difficult to get out of the program back into command mode Whatever you enter seems to have no
49. ies to help you write programs From now on we can concentrate on how to write programs which actually do something like solving arithmetic problems for instance CHAPTER SO YOU VE GOT PROBLEMS SO YOU VE GOT PROBLEMS People are very good at disentangling confused instructions and solving complex problems A computer isn t all it can do is to follow a list of instructions and carry out the instructions as it comes to them As an example take the following instructions which a mother might give to her child Could you run down to the shop and buy some bread Take 50p which is in my purse on the kitchen table And for goodness sake get dressed It all seems quite clear the child knows where to go what to buy and where the money is A computer controlled robot would take the first instruction run down the shop and do that The second instruction buy some bread It can t do that no money and at this stage would probably just stop baffled The third instruction take 50p from the purse it can t do that The fourth instruction go to the kitchen table It returns home and goes to the kitchen table Then the fifth instruction get dressed It gets dressed A child would instinctively carry out the instructions in the following order 1 Get dressed 2 Go to the kitchen table 3 Take 50p from the purse 4 Go to the shop 5 Buy some bread He or she would come back home The mother in th
50. ing in the TV set At some position of the tuning control the screen will suddenly clear At the bottom left hand corner of the screen you will see a curious symbol a black square with a white letter K in it If you can t see the K turn the brightness control up until you can see it you may find that adjusting the contrast control improves legibility You may be planning to use your ZX 80 a lot If so it may be worth considering buying a second hand black and white TV to use with the ZX 80 They can be bought cheaply in the UK at least STORING PROGRAMS ON TAPE Before you can store programs on tape you ll have to write a program The rest of this book is all about writing programs so the first thing to do is to read on until you ve got a program to store and now that you have we can start The twin coax cable with 3 5 mm jack plugs is used to connect the tape recorder to the ZX 80 Most cassette recorders have 3 5 mm jack sockets for MICROPHONE and EARPIECE If you have one of those just connect the 3 5 mm jack plugs as shown in the diagram On other recorders DIN sockets are used DIN plug to 3 5 mm jack plug connecting leads are available from most Hi Fi shops Consult the handbook to find out how to connect up the plug or plugs Once you have done this connect up the cassette recorder to the ZX 80 Set the tone control if any to MAXIMUM Some recorders have separate TREBLE and BASS control In this case set TREBLE
51. ing variable or to screen DIM PRINT PRINT 5 no more room on Screen Any 6 arithmetic overflow result gt 32767 or lt 32768 also result 32768 in some circumstances RETURN T RETURN with no corresponding GOSUB INPUT 8 INPUT can only be used in a program not directly STOP 9 STOP statement executed m line number of offending statement CONTINUE is the same as GO TO m except after code 9 when it is GO TO m 1 APPENDIX 4K BASIC FOR ZX 80 4K BASIC FOR ZX 80 1 7 80 user s view a Program Listing The user inputs via the keyboard lines of BASIC for insertion into the program and commands for immediate execution While he is doing this he sees a display which is divided into two parts the upper part is a window on the program listing while the lower part displays the line or command he is currently outputting Normally the lower part is large enough to hold the whole line which will take more than one screen tine if it contains more than 32 characters there is a blank line between the two parts and the upper part occupies the remainder of the screen If however there is insufficient RAM to hold a disptay of this size each character on screen occupies 1 byte of RAM the upper part of the display will be shrunk line by line until the display file is small enough When the upper part has disappeared altogether the lower part shrinks character by character The computer maint
52. is case the first line entered The gt points to the current line Although there is only one line entered it is a program consisting of only one instruction Type R RUN and NEWLINE and the program will run after a brief flicker on the screen the words THIS IS A STRING appear at the top of the screen At the bottom you ll see 0 10 which tells you that the program ran successfully with no errors Type any key to get back into command mode What the computer did was to print all the characters between the quotation marks Any character except for quotation marks is legal when PRINT is used in this way In general this form of the PRINT statement is useful for things like titles which only have to be printed once in a program There are other forms which allow more complicated things to be done you ll come across these the whole way through this book Now let s try getting some data into the computer This is done by using the INPUT statement which is basically of the form INPUT variable name Hit NEW key Q and NEWLINE This tells the computer that a new program is being entered Type in the following program using NEWLINE at the end of each line If you make a mistake use SHIFT 0 RUBOUT to delete the mistake every time you hit RUBOUT the character keyword or token to the left of the cursor is deleted 10 PRINT ENTER YOUR STRING 20 INPUT 5 A is the name of a string variable 30 PRINT
53. is example missed out one vital instruction come home as well as putting the instructions in an illogical order Children use commonsense to interpret complex instructions but computers can t do this You have to do all the thinking in advance when you use a computer So remember THINK STRAIGHT One way of making sure that you are thinking straight is by drawing a flow diagram before you start to do any actual programming This helps you get things in the right order to begin with The flow diagram for the example above would look like this Each box contains an instruction and the arrows show the order in which the instructions are executed Well that looks pretty simple and neat Now that the flow diagram is drawn on paper we can check it by going through the boxes one by one and asking ourselves Can the child or the computer do this Starting with the first box get dressed we might think that it s pretty basic On the other hand the child may not be able to find his clothes because his naughty sister has thrown them out of the bedroom window Perhaps we should add an instruction find clothes at the beginning of the program Go to the kitchen table well that looks OK Take 50 from purse what happens if the purse is empty Go to the shop does the child know where the shop is Buy some bread what sort Return home nothing much wrong with that Maybe the flow diagram wasn t as good as we thought
54. its first character or the null string if s is null or contains only one character the decimal representation of n from 1 to 5 digits leading minus sign if negative no spaces hence from 1 to 6 characters in all parenthesized string expression Integer expressions are 1 integer varible 11 literal number first character alphabetic subsequent characters alphanumeric no embedded spaces no limit on length of name May be used as dest for INPUT see 3 b decimal unsigned value must be 32768 syntax error otherwise as many leading zeros as you like Examples 23 32767 0000032767 11 prefix and infix operations form priority notes n n 10 to the power n 9 n n 8 n n 7 n n n n n n behaves identically to n n n gt n n lt n 8 5 528 5 lt 8 Value is 1 if true 0 if false NOTn nANDn norn Bitwise Boolean operations III INN ad Ambiguites in parsing operations are resolved by considering the priority of the operators in question higher priorites bind tighter equal priorities associate from the left Example A B C D E F G H is the same as A B C D E F G H iv PEEK n the value at address n being a single byte in the range 0 to 255 v CODE s the code for the first character in strings vi RND n a pseudo random number in the range 1 to n if n gt 0 in the range 1 to 32767 or 32768 to n 1 if n lt 0 If n 0 returns 1
55. l operators Their priority in descending order is NOT AND OR Thus in a B 1 AND C 1 is taken first so that the computer will print OK if B AND C are 1 but will also print OK if A 1 The brackets b work in the same way as they did for arithmetic operators Thus ZX 80 will print OK if C 1 and either B 1 or 1 The operators NOT AND and OR also allow you to produce conditional expressions For example if you require X 3 ifA gt B X Q R ifA B lt obvious way is 110 IFA gt BTHEN LET X 3 120 IFA BTHEN LET X Q R 130 IFA lt BTHENLETX P However you could instead write 110 LET X A gt B AND 3 OR AND Q R OR A lt B AND P To round off this chapter on branching here is a program which throws a die The program uses a useful facility the function RND X This generates a random number in the range 1 to X It occurs in line 120 of the program in this case X 6 10 PRINT DIE THROWING 20 LET A e e eis SHIFT A 30 LET B e 40 LETC e 50 LETD e 60 LET E 120 LETX RND 6 130 PRINT YOU THREW 140 IF X 1THENGO TO 200 150 IF X 2 THEN GO TO 300 160 IFX 3 THEN GO TO 400 170 IF X 4 THEN GO TO 500 180 IF X 5THEN GO TO 600 190 IF X 6 THEN GO TO 700 195 GOTO 1000 200 PRINT E 205 PRINT E 210 PRINT B 215 PRINT E 220 PRINT E 230 GO TO 1000 300 PRINT C 305 PRINT E 310 PRINT E 315 PRINT E 320 PRINT D 330
56. ll the variables in the program Typical form 10 CLEAR RUN automatically clears the variables every time a program is run And now last but not least DIM This is of the form 10 DIM A B sets up an array A which contains B 1 variables Each variable is called an element of array A Arrays can have any single letter name and can have any number of elements providing that there is enough room to store all the elements It is possible though not a good idea to have a variable and an array with the same name example 10 LET A 4 20 DIMA A Each element is referred to by its subscript For instance B 0 is the first element of array B B 2 is the third element of array B B N is the N 1 th element of array B Because you can use any integer expression as a subscript it is possible to process array elements easily and quickly Here is an example of the use of arrays for character manipulation CHEESE NIBBLER 10 DIMA 10 20 DIMB 10 30 DIMC 10 100 FORJ 1T010 110 LETA J 1 120 LETB J 130 LETC J 1 140 NEXTJ 200 FORJ 1T010 205 IF NOTA J 1 THEN GO TO 220 210 PRINT 215 GO TO 230 220 PRINT 230 NEXTJ 240 PRINT 300 FORJ 1T010 305 IF NOT 1 THEN GO TO 320 310 PRINT 315 GO TO 330 320 PRINT 330 NEXTJ 340 PRINT 400 FORJ 1T010 410 PRINT e 415 GO TO 430 420 PRINT 430 NEXTJ 440 PRINT 445 PRINT HIT NEWLINE TO NIBBLE THE CHEESE 450 INPUTY 4
57. lly useful when you are debugging programs If the loop is completed successfully for all values of J up to 152 in this case the control variable will be left at 153 This is because the NEXT J statement not only tests the value of but also increments J by 1 Some BASIC s allow you to increment the control variable by other amounts but this isn t possible on the ZX 80 It s not a very useful feature anyway All this is quite straightforward FOR J N to M control variable starting value finishing value defines what variable you want to use what value it should start at and at what value of control variable you want to leave the loop NEXT J reminds the computer that it is in a loop and tells it what variable it should be testing and in fact where it should jump back to if the loop isn t finished You can start at any value you like try starting at J 100 The computer will go round the loop M N 1 times DON T EVER jump into a loop unless you have just jumped out of it if you do the computer will skip the FOR statement and will get very confused because it won t know what the control variable is what it should start at and what it should finish at It will do the best it can but it will probably stop when it gets to the NEXT statement The error message will be of the form 1 LINE No where 1 means no FOR statement to match this NEXT statement or 2 LINE No where 2 means variable name not found Y ou can
58. nd line There is no limit to the number of fields you may skip using commas Unless you run out of display area of course If you use a control character right at the end of a PRINT statement the next PRINT statement will obey the control character For instance 10 LETX 4 20 PRINT THE ANSWER IS 30 PRINTX will produce the output THE ANSWER IS 4 If no control character occurs at the end of a line the next PRINT statement will start printing on a new line The best way of finding out about PRINT is to experiment with it CHAPTER COPING WITH CHARACTERS COPING WITH CHARACTERS You will have noticed that in addition to the normal typewriter characters there are several other characters not usually found on a typewriter In addition there are some usable characters not written on the keyboard Before we go on to investigate what characters can be printed meet an interesting function CHR X where X is a numberJnteger variable or expression It is used mainly in conjunction with the PRINT statement CHR X means the character whose code is x Try this 10 INPUTX 20 PRINT CHRS X 30 GO TO 10 What this program does is to ask for a number and then to print out the character whose code is the number entered The codes lie between 0 and 255 A useful program for listing the symbols is given below 10 PRINT ENTER CODE VALUE 20 INPUT X 30 PRINT X CHR X 40 PRINT 50 LETX X 1 60 GO TO
59. ng an integer variable the S is not displayed after ASC but rather before it This is because ASC has already been identified as a function call but as no built in function with the name ASC is available it is faulted Having identified it as a function call however the computer does not then consider other possible parses The following keys are available to alter the input line i single character symbols letters digits punctuation etc the symbol is inserted to the left of the cursor 11 multi character tokens AND OR NOT TO THEN keywords y Each of these is stored in the computer as a single byte which as in 1 is inserted to the left ofthe cursor However they appear on the screen as more than one character Those that are alphanumeric ie all except are preceded and followed by a space the preceding space being omitted a at the beginning of the line b Where it follows another alphanumeric token This rule means that programs appear well laid out on the screen without using up scarce RAM space for explicit space characters Inserting an explicit space character before or after an alphanumeric token always inserts one extra space in the displayed form iii RUBOUT deletes the symbol or token to the left of the cursor iv cursor control keys gt and skip the cursor past the next symbol or token to the right and left respectively v EDIT replaces the inp
60. o set up the ZX 80 hardware and to use the ZX 80 BASIC language for writing your own programs For most of the book the chapters are grouped in pairs The first odd numbered chapter of a pair is purely descriptive and prepares the reader for what is covered in the second even numbered chapter of the pair The second chapter of a pair is intended to take you through the use of ZX 80 BASIC as painlessly as possible while you re actually sitting at the keyboard If you are an experienced BASIC user you won t need to read very much probably only chapter 2 the ZX 80 BASIC language summary and the Index If you re less experienced and patient it s probably best to read through the whole book chapter by chapter If you re less experienced but impatient you can skip the odd numbered chapters 3 5 7 and 9 and probably will OK NOW READ CHAPTER TWO AGAIN CHAPTER GETTING STARTED GETTING STARTED This is the most important chapter in the book It tells you how to connect up and switch on your ZX 80 The ZX 80 consists of two units 1 The computer 2 The power supply The power supply used should be 9 volts DC 600 mA unregulated terminated with a 3 5 mm jack plug The tip of the jack plug must be positive DON T PLUG IN TO THE MAINS SUPPLY YET Look at the back of the ZX 80 computer You will see four sockets Three of these are 3 5 mm jack sockets marked MIC EAR and POWER The fourth is a phono socket for the
61. riable U unsigned number in the range 0 to 65535 the BASIC will treat values in the range 32768 to 65535 as 32768 to 1 respectively These are the only variables likely to yield negative values Notes Address Contents 1 16384 less than the run time error number yields 255 normally 1 less than the error code if an error has already occurred in this statement eg PRINT 1076 AND 0 OR 16384 prints 5 because 10 6 overflows Do not POKE any value other than 255 or 0 to 8 POKE 16384 255 does nothing to 8 causes error n 1 Thus POKE 16384 8 is the same as STOP 1 16385 sundry flags which control theBASIC system 2 16386 Statement number of current statement POKE has no effect unless this is the last line in the program N2 16388 position in RAM of K or L cursor last line any input or editing was done 2 16390 Statement number of gt cursor X2 16392 VARS X2 16394 E LINE X2 16396 D LINE See section 2a of Appendix 2 X2 16398 DF EA X2 16400 DF END 16402 number of lines in lower part of screen including blank line separating the two parts 2 16403 statement number of first line on screen The LIST statement sets this and scrolling alters it 16405 address of character or token preceding the S marker 16407 statement number to which CONTINUE jumps NI 16409 sundry flags which control the syntax analysis N2 164
62. rithmetic expression 110 FOR J D 10 TO D 200 would be quite legal However the name of the control variable must be a single letter If you decide to use several FOR loops one inside the other be very careful in the way you go about it FOR FOR J c FOR I eg FORK FORK e NEXT K NEXT J NEXT J are both quite OK On the other hand FOR J FOR FOR K NEXT K NEXT J NEXT is defiantly not all right and could cause major problems Another good reason for using flowcharts One final point because the control variable is effectively tested and incremented by the NEXT statement the main part of the loop will always be executed at least once regardless of the value of the control variable even if you have jumped into the loop SUBROUTINES A subroutine is a sub program which may be used once or many times by the program or main program Example 10 FORJ 1 TO 10 20 GO SUB 1000 GO SUB is on key V 30 NEXTJ 40 PRINT END 900 GO TO 1200 1000 PRINT SUBROUTINE EXECUTED 1100 RETURN RETURN is on key B 1200 STOP Statement 20 tells the computer to GO TO the subroutine at line 1000 This prints SUBROUTINE EXECUTED Statement 1100 tells the computer that the subroutine is finished and that it should return to the main program The computer then jumps to the line immediately following the GO SUB statement and executes that in this cas
63. s we compromise at a high level by using languages such as ALGOL PL 1 PASCAL FORTRAN and BASIC These languages are much closer to English than binary The ZX 80 uses BASIC because it is easy to learn and more than adequate for most purposes BASIC stands for Beginner s All purpose Symbolic Instruction Code and was devised at Dartmouth College New Hampshire in 1964 as a simple beginners programming language Although it was intended just for beginners it has since become one of the most widespread and popular high level languages This isn t very surprising when you think about it because even scientists and engineers prefer to concentrate on science and engineering instead of trying to talk to computers in complex and difficult languages Like English BASIC has a variety of dialects depending on which computer is being used The ZX 80 BASIC differs from other BASIC S in some respects These differences are listed in the appendix Summary of ZX 80 BASIC at the end of this book As we said before computers aren t that bright We have to tell them exactly what to do by giving them a step by step list of instructions This is called a program Each instruction must be given in a clear and un ambiguous way the syntax must be right in ZX 80 BASIC the computer will tell you if it thinks that the syntax is wrong before you can run the program Some BASIC S wait until you ve tried to run the program before they say nuts Even if the
64. syntax is right the computer will be confused if you give it a badly thought out list of instruc tions Chapters 5 and 7 give some ideas for making sure that you re telling the computer to do the right things in the right order STATEMENTS AND COMMANDS ZX 80 BASIC allows you to use 22 instructions or statements These are divided into several categories 1 System commands NEW Clears ZX 80 ready for a new program RUN Runs the current program LIST Lists the current program LOAD Loads a program from tape SAVE Saves a program on tape 2 Control statements GOTO IF THEN GOSUB STOP RETURN FOR TO NEXT CONTINUE These statements allow the programmer to control the order in which instructions are carried out They are described in detail in chapters 7 10 3 Input output statements PRINT allows computer to output data INPUT allows user to enter data V O statements provide a means for getting data into and out from BASIC programs 4 Assignment statement This versatile statement is used whenever an arithmetic operation is to be performed 5 Other statements CLEAR clears the stored values of variables CLS clears the screen DIM sets the size of an array REM indicates that what follows is a remark RANDOMISE sets up random number generator POKE allows user to talk to the computer in binary code There are some normal BASIC statements which are not included in Z
65. terative programs can get stuck in endless loops One way to avoid this is to allow the computer to go round only a set number of times One way of doing this would be to use IF and GO TO statements Consider the following program which prints t 152 times 10 LETJ 1 20 PRINTS 30 IF J 152 THEN GO TO 60 40 LETJ J 1 50 GO TO 20 60 STOP First the program sets J the loop control variable to 1 Then it prints Then it tests to see 1f J 152 It isn t and so the computer goes on to line 40 which increments the value of J by 1 to 2 The computer passes on to 50 which tells it to jump back to 20 This process goes on until J 152 when the IF statement is satisfied and the computer jumps out of the loop to line 60 The arrows on the program help you to see where the program is going for short programs this is quicker than drawing a flow diagram and does make it easier to see what the program is doing The program took 5 lines if you don t count the STOP statement which doesn t actually do anything it just serves as a target for the GOTO in line 30 Loops are so useful that you d have thought that there was an easier way of writing programs using them Well there is Turn to chapter 10 CHAPTER LOOPING THE LOOP LOOPING THE LOOP Because loops are so useful some special statements have been devised to make it easy to use them Program for printing 10 FORJ 1 TO 152 use SHIFT 4 for TO 20 PRINT S 60
66. the string has been shortened to the null string There is a further function which can be useful This 15 STR variable or number This allows an integer number or variable to be treated as a string variable 10 LET G STR 1234 setsG 1234 101 5 5 does the same thing for an integer variable if X 4852 then G 4852 So far we ve not said very much about the graphics symbols These have been designed so as to double the effective resolution of the display which gives 23 lines of 32 characters each Here is a program which plots TWO bar charts on the same display IOLETX 0 Defines 20 PRINT YZ AXIS variables 30 PRINT X and 40 FOR I 1 TO 21 prints 50 LET Y X titles 60 LET Z 24 X Generates Y 70 PRINTX and X 80 FOR J 1 TO 20 85 IF J gt YANDJ ZTHEN PRINT CHRS 3 90 IF J gt Y AND J gt Z THEN GO TO 135 95 IF JY AND J gt Z THEN PRINT CHRS 11 100 IF J lt Y AND J lt Z THEN PRINT CHRS 139 105 IF J lt Y AND J Z THEN PRINT CHRS 139 110 IF J lt Y AND J gt Z THEN PRINT CHRS 11 115 IF J Y AND J lt Z THEN PRINT CHRS 139 120 IF J gt Y AND J lt Z THEN PRINT CHRS 3 125 IF J Y AND J Z THEN PRINT CHRS 139 130 NEXTJ 135 PRINT 140 LETX X 1 150 NEXTI The program prints Z as black bars Y as grey bars Statements 80 to 130 determine what graphic symbol is to be used imi or nothing at all This program calculates the grap
67. through the procedure mentioned above There is a simpler way to delete lines though Type the line number then NEWLINE and you ll see the line vanish from the listing at the top of the screen The cursor gt will also vanish It will not reappear until you type a new line use the control keys f and U or use the LIST command THE LIST COMMAND As you get more experienced with the ZX 80 you ll want to write programs longer than the 24 lines which will fit onto the screen This could pose a problem or could it We can find out by writing a really long program Type NEW then NEWLINE Enter the following program which prints a large number of blank lines 10 PRINT 20 PRINT 30 PRINT 40 PRINT and so on to 230 PRINT keep an eye on the listing in the upper part of the screen 240 PRINT 250 PRINT 260 PRINT We seem to have lost the first few lines Or have we Hit LIST key A followed by NEWLINE the program is now displayed from line 10 down Now try LIST 200 the program is displayed from line 200 You can list the program starting from any line you want in this way Notice that the current line cursor is altered to the line you selected Try using the up down cursor control keys go on hitting the key and see what happens In this way you can display any part of the program you want By now you will have realised that the ZX 80 has some powerful and very convenient facilit
68. tive numbers when A is less than B All fairly straightforward so far Now let s try division Alter line 60 to 60 LET C A B Run the program for A first number 24 B second number 12 The computer will come up with THE ANSWER IS 2 Run the program again this time for A 18 and B 12 THE ANSWER IS 1 Shouldn t that be 1 5 Well yes but the ZX 80 uses integer arithmetic only whole numbers can be expressed What the computer does is to do a division normally and the truncate the result towards ZERO As examples ANSWER 2 58 would become gt 0 01 would become 0 212 1 would become 212 5941 98 would become 5941 This means that you may have to be careful when you use division it s always fairly accurate when you re dividing a large number by a small number but may be less accurate when the two numbers are closer together in size Luckily there are ways of getting round this problem For instance here is a program which will give quotients to 3 decimal places using integer arithmetic only 10 PRINT DIVISION PROGRAM 20 PRINT DIVIDEND 30INPUT X 40 PRINT DIVISOR 50 INPUT Y 60 LET Z X Y Divides X by Y 70 LET X Z Y Calculates Remainder 80 LET D1 10 R1 Y Divides 10 remainder by Y to give first decimal 90 LET R2 10 R1 D1 Y Calculates 2 remainder 100 LET D2 10 R2 D2 Y Divides to give 2 decimal place 110 LET R3 10 R2 D2 Y Calculates 3 remaind
69. try this by adding to the program 5 GO TO 20 and running and then adding 4LETJ 4 and running you must say RUN not GO TO 4 This will jump the ZX 80 into the major loop and it will stop when it gets to statement 60 NEXT J giving the error message 2 60 or 1 60 In chapter 6 there was a program to divide one number by another to give a quotient to 3 decimal places Here is a program to give any number of decimal places Statement 10 is a remark The computer disregards REM statements when it executes programs It s often useful to put comments and remarks into programs so that you can remind yourself what the program does when you look at it often months later 10 REM HIGH PRECISION DIVISION 20 PRINT HOW MANY DECIMAL PLACES 30 INPUTD 40 PRINT DIVIDEND 50 INPUTR 60 PRINT DIVISOR 70 INPUT Y 80 LETZ R Y 90 LETR R Z Y 100 PRINT QUOTIENT IS Z 110 FOR J 1 TOD 110 to 150 evaluate 120 LET Z 10 R Y successive decimal 130 LETR 10 R Z Y places until the program 140 PRINT Z has produced D 150 NEXTJ decimal places Try running this first of all with dividend 1 divisor 3 D 100 say Note line 110 110 FOR J 1 to D The starting and finishing values can be variables This means that you can control the starting value and finishing value of the loop variable as you wish The starting and finishing values can also be expressions of the form D 2 D 1 5 or indeed any other a
70. ts number when the program is run such lines are ignored A line which has no line number or which has line number zero is a command and is obeyed immediately For as long as it takes to obey the command which for most commands is very brief the screen is blank then on completion the upper part of the display contains any output generated and the lower part contains a display of the form m n where m is a single digit and n is 2 for most commands If m 0 execution was successful if m 9 a STOP command was executed otherwise m is an error code see Appendix I Where a command RUN GO TO GOSUB CONTINUE has caused the program to be entered n is the line number of the offending instruction if m is an error code exception if the error is in a GO TO or GOSUB then n may be the target of the jump the tine number of the STOP if m 9 land the line number of the last line in the program if m 0 Except in the case of m 0 or m 9 CONTINUE is a jump toline number n butsee3 c If m 9 CONTINUE is a jump to line number n 1 Sometimes only the first digit of n is displayed because there is no room in the RAM for any more display file For example beware confusing line number 240 of which only the first digit is displayed with tine number 2 A jump to a line number which is beyond the end of the program or greater than 9999 or negative gives m 0 n the line number jumped to The commands are described indi
71. used 90 Unused 91 Unused 92 Unused 93 Unused 94 Unused 95 Unused 96 Unused 97 Unused 98 Unused 99 Unused 100 Unused 101 Unused 102 Unused 103 Unused 104 Unused 105 Unused 106 Unused 107 Unused 108 Unused 109 Unused 110 Unused 111 Unused 112 Unused 113 Unused 114 Unused 115 Unused 205 Unused 206 Unused 207 Unused 208 Unused 209 Unused 210 211 Unused 212 213 214 215 216 217 218 219 NOT 220 221 222 223 224 225 226 227 228 gt 229 lt 230 LIST 231 RETURN 232 CLS 233 DIM 234 ISAVE 235 FOR 236 GOTO 237 238 INPUT 239 RANDOMISE 240 LET 241 242 243 NEXT 116 Unused 117 Unused 118 Unused 119 Unused 120 Unused 121 Unused 122 Unused 123 Unused 124 Unused 125 Unused 126 Unused 127 Unused 244 PRINT 245 7 246 NEW 247 248 STOP 249 CONTINUE 250 251 GO SUB 252 LOAD 253 CLEAR 254 255 Inverse means that the character appears white on a black background Inverse space is a black square for instance The graphics characters are shown on the next page CHR X allows us to print any character we want SYMBOL CODE SYMBOL CODE SYMBOL CODE SYMBOL CODE A 2 pm 7 130 p 135 SOHO He 6 11 134 as 139 Table of Graphic S
72. ut line with a copy of the current line from the program If no line has the current line number the first line after it is used If the current line is after the last line in the program the last line is used If there are no lines of program at all then an empty line is used Note that any existing input line is lost EDIT followed by NEWLINE is in fact the quickest way to get rid of an unwanted line but beware typing EDIT in mistake for NEWLINE vi NEWLINE is ignored if the inverse video S symbol is present Otherwise it marks the end of input and the line or command is submitted to the system see next section Note that the whole line is submitted not just the part to the left of the cursor c After input When a line with a nonzero line number is submitted to the system it is inserted into the program any existing line with the same number being first deleted The input area is then cleared A special case is where the new line consists only of a line number possibly preceded by spaces the existing line 1f any is deleted but nothing replaces it and it therefore simply disappears from the listing The current line number is still set to its number however so the inverse video gt disappears also see a above If the line being inserted has one or more spaces after the line number but no other symbols or tokens it is still inserted in the program and appears in the listing as a line which is blank except for i
73. video cable The connection diagram shows how to connect up the ZX 80 to the power supply and to a domestic television which will be used to display the output for the ZX 80 A video cable with a coaxial plug at each end is provided to connect the TV to the computer A twin cable with 3 5 mm jack plugs at both ends is provided to connect the ZX 80 to a cassette recorder Don t worry about this for the moment Connect the power supply to the ZX 80 by plugging the 3 5 mm jack plug into the socket marked 9V DC IN if you do get it in the wrong jack socket you won t damage your ZX 80 even if you switch on the power It won t work until you get the plug in the right socket though SEE DIAGRAM Connect the TV using the coax lead provided You will have to tune the TV to the ZX 80 frequency approximately channel 36 on UHF so check that you can do this most TV sets either have a continuously variable tuning control or if they select channels with push buttons have separate tuning controls for each channel If your set is a push button set select an unused channel ITV 2 perhaps Turn down the volume control OK we can t put off the moment of truth any longer Switch on the TV When it has warmed up SWITCH ON THE ZX 80 POWER SUPPLY UNIT T V SET AERIAL SOCKET CASET TE RECORDER Connecting up the ZX80 10 You will be disappointed to see a horrible grey mess on the screen Try tun
74. vidually in section 3 2 Computer s view 35h means Hexadecimal 35 35h 3 16 5 53 0Ah to OFh are the decimal numbers 10 to 15 aJRAM The contents ofthe RAM are Byte containing 80h space Display file i E RAMBOT VARS E_Line D_File DF EA The first area is fixed in size and contains various system variables which store various items of information such as the current line number the line number to which CONTINUE jumps the seed for the random number generator etc etc Those that could possibly be useful with PEEK etc have been documented elsewhere Appendix 3 An important subset of the system variables are the five contiguous words labelled VARS to DP END which hold pointers into the RAM and define the extent of the remaining areas apart from the stack The program consists of zero or more lines each of the form Text of line Line Is Byte new line 76h Line ms byte ie beginning with the line number stored ms byte first contary to the usual practice on Z80 s and ending with a newline The line number is in the range 1 to 9999 so that the ms 2 bits of the first byte are zeroes The ms 2 bits of the byte pointed to by VARS will not both be zeroes this gives a simple test for end of program The program lines are stored in ascending order of line number The text consists of ordinary characters codes 0 to 3Fh
75. vision may give rise to funny answers which don t give error codes If you think that you re going to run into truncation then do multiplications first on the other hand it may be better to do divisions first to avoid overflow That s probably enough arithmetic for now Let s move on to more interesting things CHAPTER DECISIONS DECISIONS DECISIONS DECISIONS So far we have only considered problems which can be solved by carrying out a list of instructions starting at the beginning of the list and working steadily down the list until the last instruction is carried out The next four chapters deal with ways in which you can make the ZX 80 or any computer using BASIC carry out much more complex programs which can perform many tedious tasks In chapter 5 we talked briefly of the use of flowcharts for checking that the program steps were in a logical sequence This can be useful even for simple programs When it comes to complex programs flowcharts are vital Let s consider a problem we face every day getting up in the morning There is a flowchart for getting up on the next page The program is said to branch at each decision diamond As you can see one feature of branches is that they allow some parts of the program to be skipped if they are not neccessary Programs can get difficult to understand if there are a lot of branches and this is where flow diagrams help It is a good idea to keep a p
76. y re copying from the program stored in the area RAMBOT to VARS again converting tokens into characters as part of this process and the lower part is then output afresh When a line is to be inserted into the program its line number is converted into binary space is made at the appropriate place by copying everything else up and the text of the line from which the cursor has already been deleted is copied in The working space and display file are then re made the former now containing just the cursor and a newline When a command is executed it is interpreted in situ in the working space area Program lines are of course interpreted in their place in the program 3 Statements a expressions nin Throughout section 3 n will be used to represent any space integer expression and s to represent any string expression String expressions are 1 string variable 11 literal string iii CHRS n iv TLS s v STRS n vi s where is any letter no intervening space allowed Example A May be used as dest for INPUT see3 b delimited by quotes any symbols or tokens other than quote permitted inside the string Examples ABC the character with code n or the null string if n 1 Note code 212 is a token which is represented by the quote character and this code may be supplied as the parameter to CHR to get something which prints as a quote the strind s minus
77. yed on the TV since initialisation CLEAR CLS GOSUB n RETURN NEXT a modulo 256 delete all variables clear the upper part of the screen eg to get rid of a message appropriate to an INPUT as in 20 PRINT TYPE THE VALUE FORA 30 INPUT A 40 CLS as GO TO n but also push a GOSUB block on the stack so that RETURN will jump to the line following this one pop a GOSUB block off the stack and jump to the line number contained in it equivalent to LETa a 1 IF NOT a gt value supplied with TO THEN GO TO line following FOR a The combined effect of and is of and m FOR a n TO n2 m is the line number NEXT a m LET a n LET LIMITa n2 assuming LIMITa isn t used elsewhere in the program 1 IF NOT a gt LIMITa THEN GO TOm 1 Note that this does not preclude 1 assignment to 8 ii several NEXTs matching one FOR or one NEXT matching several FORs Note also that FOR GOSUB NEXT RETURN and FOR I FOR J NEXT NEXT J are possible though not very useful Ifthe BREAK key is found to be pressed at the end of execution line execution does not Follow on to the next line but stops showing 0 1 where n is the line number of the next line that would have been executed but for the break in space 64 print as 128 Inverse space 192 print as 65 print as 129 Inverse 193 print as GRAPHICS 66 print as 130 Inverse GRAPHICS
78. ymbols There is a further group of facilities which enables us to handle characters These are a TLS string this gives the string minus its first character The string can be a literal string inside quotes or a string variable 10 PRINT TL ABC would give BC 10 PRINT TL G will cut off the first character of string G b CODE string this gives the code corresponding to the first character in a string either a string variable or a literal string 10 PRINT CODE ABC would print 38 which is the code for A An example of the way in which these can be used is the following program which accepts a string and prints it out in inverse video 10 PRINT ENTER YOUR STRING 20 INPUT G 30 PRINT G 40 LET X CODE G 50 LETX X 128 60 IF G CHRS 1 THEN GO TO 100 70 PRINT CHR X 80 LETG TL G 90 GO TO 40 100 STOP Statement 40 sets X code of the first character of G 50 adds 128 to the code this gives inverse of the letters digits and graphics 60 test the string to see if it is a null string i e has no characters in it If it is a null string either the input string was a null string to begin with or all the characters have been converted to inverse video and printed 70 prints the inverse video character 80 chops off the character which has just been printed and the program then jumps back to 40 and the code for the next character is extracted The program goes on until

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