Abstract LJritish Computer Society DATAFAIR 73 Real
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1. qC stringl string2 CHANGE the q th occurrence of string 1 to string 2 in the current line X stringl string2 Change all occurrences of string to string2 in the current line Commands that combine Searching and Editing functions LC string string2 LOCATE next line containing stringl and CHANGE it to string 2 LD string LOCATE next line containing the string and DELETE the line DF string Delete down to the next line beginning with string DL string Delete down to the next line containing the string qM stringl string2 MACRO Change all oceurr ences of string to string2 in the next q lines Commands for inserting large amounts of text 3 Enter input mode In input mode everything that is typ ed is inserted into the file upto the next ESCAPE chara cter Vn Move pointer up by n lines then enter input mode On OVERLAY Delete n lines then enter input mode Delete downto next line beginning with the string then enter input mode OF string Delete down to next line containing the string then enter input mode GL string Y filename YANK Insert whole of specified file below current line and advance pointer to bottom of inserted teat Multi file Capabilities ED can support two output streams concurrently enabling text to be diverted from the main file to an auxiliary file The user may switch the output from one stream to another at any time allowing the input file to be segmented2. AND BRIAN GAINES Department of Electrical Engineering Science University of Essex Colchester U K SRTRODUCTION This paper discusses the problems of minicomput er software development and describes experience in the development of systems software for a minicomputer using an interactive emulator embedded in an extended BASIC like language on a PDPLO timesharing system The systems devel oped for the target minicomputer Include a BASIC interpreter a floating point arithmetic package and a real time clinical patient moni toring system for the minicomputer with drum cartridge tape visual display real time clock analog digital convertor teletype fast reader and special purpose keyboards The objectives of this paper are to further elucidate some problems of software development for minicom puters and the relative merits of differing approaches and alse to demonstrate the pract ical utility of the modular BASIC like language BASYS or Essex extended BASIC for system pro gramming which was originally deseribed at DATAFAIR 1971 Gaines Gedye and Facey 1971 It is perhaps worth emphasising from the outset that the objectives of the work and the con clusions to be drawn from this paper are far more related to the ergonomics of software system development than to the computer science underlying the techniques involved We have been concerned to place the system developers in a position of power to fully and efficiently ut
3. computer emulator in the high level language with suitable facilities for calling and controlling it It so happened thgt the extended BASIC we had developed for natural language interactive text handling also featured ease of machine code linkage through its table driven command struct ure and standard subroutine calls The language was being successfully used for clinical appli cations and textual analysis and such additional commands as special terminal drivers and a speech syntheziser controller had already been added to it as linked machine code segments The emulator embedded in a high level language pre sented a further opportunity to test BASYS since the text handling and variable radix arithmetic of the language should be able to support effec tive assemblers and symbolic debugs the file handling and input output control available down to virtually direct machine level should be well suited to peripheral emulation and the ease of integrating further machine code mod ules should make it possible to embed a fast emulator for MINIC itself in a convenient form A synopsis of BASYS is given in Appendix 1 one further command was added to call the MINIC emulator MINIC array name where locations 0 through 20 of the array con tained status information and the remaining locations were regarded as a MINIC core store The status information included the state of MINIC registers the number of MINIC instruct ions to be e
4. in any desired way PUT filename Switeh output to auxiliary stream OM Switch output to main stream ED can also support two input streams enabling the output file to be built up from parts of other files GET filename Switch to auxiliary input stream IM Switch to main input stream The GET and PUT streams may be used in combin ation to perform complex block move operations This is especially useful when rearranging machine code programs for computers with limited addressing capabilities
5. INT 5 GOTO 200 K A numerical expression ne is something that can be evaluated to produce a number The following operators are allowed PREC OPERATOR MEANING 1 Unary minus 2 Pe aeb open bit shift of a by b places left if b gt O right if b lt 0 3 Exponentiation 4 Division 5 a Multiplication 6 Subtraction 7 Addition Relational operators 8 lt Less than 8 Equal 8 gt Greater than String operators 9 gt Alphabetically greater than 9 2 Identical g lt Alphabetically less than 9 p afb true if string a begins with string b 9 aeb true if string a contains string b 16 tor Quotes enclosing literal string 10 Right asseciative operator meaning string name Logical operators ll amp AND 12 Exclusive OR 13 l Inclusive OR A string expression se is anything that can be evaluated to yield a string String expressions are built up from one or more of the fields listed below The value of the expression is formed by con catenating the values of the constituent fields FIELD CORRESPONDING STRING ne The string in dollar line ne string string which may net contain string string which may not contain ne The value of ne converted to a string under control of the current output format and radix specification Carriage return and linefeed gt Has no value May be used as a separator to resolve ambiguity S nel ne2 The nel th substring of line ne2 C ne The ASCII ch
6. T nel ne2 gives formatted listing on channel 1 default TTY of program lines nel through ne2 CLEAR nel ne deletes program lines nel through ne2 CODE se reacts to the string se as if it had been typed in during the keyboard edit phase enables program to compile additional lines at run time for example dynamic arrays X number look for replace by changes look for to replace by in programline number and prints changed line primarily for swift editing CORE ne adjusts user s core allocation until free space is at least ne characters GARB collects garbage to maximize free space normally done automatically when free space goes below limit Peripheral Transfers BASYS input output is largely device independ ent through numbered channels To minimize programmer effort standard device assignments are set up when a user logs in but these may be changed ad lib INIT channel devicename initializes the channel and attaches the named device RELEASE channel closes the channel and releases the device CREATE channel mode filename creates a new file for use in a certain mode and opens on a channel OPEN channel mode filename opens an existent file for transfers in given mode on a channel DELETE channel filename deletes a file from the device assigned to a channel RENAME channel newname oldname renames a file newname and oldname are strin
7. aracter formed by taking the value of n modulo 128 nel ne2 The value of nel converted to a string in format ne2 nel ne3 The value of nel converted to a string using radix ne3 nel ne2 ne3 Both as above zk amp amp A A Synopsis of BASYS Commands Storage null effect on execution ARRAY name number sets up an array in the program line with number the highest subscript The value of the line number is assigned to the sim ple variable name when the RUN command is executed enabling array elements to be referenced symbolically with the usual syn tax characters sets up a character string in the program iine to be used as a string variable REM characters sets up a character string for comments Assignsent LET namel nel name2 me2 etc assigns the value nel to the numerical variable namel ete LET array name list of variables packs or unpacks the listed variables into or from the array the list may consist of variable precision numbers and variable length strings in conjunction with READ and WRITE commands this enables random access mixed record structures to be set up on the backing store tg a se search mode destination look for replace with generates the string se and decomposes it as specified the last four fields may be iterated many times and execution of the command only continues as long as the search string l
8. ays This system was then used to generate two packages previously written for MINIC a floating point arithmetic package and a BASYS interpreter At the same time well before a hardware configuration was available work commenced on writing a clinical monitoring system for a real time MINIC with a variety of peripherals These were not formal experiments and could not be so since the control develop ments differed in detail and had not been ade quately monitored The results however indic ate clearly the tremendous potential of an effective interactive system to aid software development the software packages were devel oped in about one tenth of the time and took up about sixty percent of the space of comparable previous developments For the real time system the back of the software development was broken well before the hardware was ready and the transfer to the real system was fairly painless This is an ongoing experiment and the results of this first phase do little more than indic ate the value of further research on the opti mization of the software development environ ment However the following conclusions are so far worthy of note a A major benefit accrues solely from the use of a powerful high speed interactive text edi tor In particular the block move capability made it easy to re arrange programs for ease of reading and to annotate them sensibly b The majority of program bugs may be detected tbro
9. g expressions PRINT channel se writes the string se onto the prescribed channel default is channel 1 usually TTY INPUT channel ne PUT command decompos ition syntax if ne is present PRINTS line ne as a cve otherwise PRINTS asterisk as cue reads a line of text from device and decomposes it as for PUT command Note that command defaults to simple list of numeric variables or character string input TAR channel ne se print the string se repeatedly until the column ne is reached used to tabulate and produce charts on TTY WRITE channel array name write contents of array in line ne as binary block on channel used for record structures in random access files READ channel array name reads block of binary data from channel into array SAVE channel filename nel ne2 saves program lines nel through ne2 of current program as a text file CALL start channel filename calls the specified program file and starts program at prescribed line values of variables previously defined are re tained DUMP channel dumps the current program in core imare form LOAD start channel nel ne2 loads a core image created by DUMP and start at prescribed line values of variables retained lines nel through ne2 of old program also retained allowing arrays and strings to be passed This command gives rapid overlaying The channel specification may be om
10. gh level language is desirable This conflict has been resolved in the Essex system by adding a command MINIC to thelanguage BASYS Essex Extended BASIC on the PDP1O which has the same syntactical structure as other BASIC commands but takes as its argument the name of an array to be regarded as the core store of a MINIC minicomputer When the instruction is executed it takes the first 20 locations of the array as the MINIC hardware registersand as status information controlling execution and data traps time quantum for execution etc and emulates the MINIC CPU Control returns back tthe BASYS program when either a emulation level time quantum has elapsed a certain number of instructions has been executed certain instruct ions have been executed or certain data accessed or an input output or transfer to variable microprogram instruction has been executed Status information is set up in the array argument to indicate the reason for the return to main program and the internal status of the MINIC CPU This project has demonstrated in an actual real time system development the advantages of working under an interactive emulator embedded in a high level language The experiments have also emphasized the importance of other facilities such as a high speed contextual editor Summaries of BASYS and a suitable editor are given as appendices to the paper REAL TIME SYSTEM DESIGN UNDER AN EMULATOR EMBEDDED IN A HIGH LEVEL LANGUAGE PETER FACEY
11. ilize their own capacities and have not hesitated to be lavish with the computer facil ities required to support this objective The original thesis has been that problems of system development and implementation on computers generally reflect poor man machine communication This applies not only to the implementation of defined systems but also to the generally most difficult and unmanageable phase of system definition the capability to rapidly develop demonstration systems to enhance man man commun ications during system definition seems to us to be one of the most important facilities that should be made available through the use of computers MINICOMPUTER SOFTWARE DEVELOPMENT The availability of small computers of ever increasing power and ever decreasing cost has made it attractive to design systems in which the minicomputer is just another component albeit the most complex and powerful one It is probable that already more programming effort is being devoted to these minicomputer components than to the larger computer installations This will certainly be so when the costs fall even further with the exploitation of complex semi conductor production techniques Whilst the use of minicomputers in this way has opened up new areas for system implementation on a scale previously impossible it has also gene rated new problems or at least regenerated old problems in a new guise The fabrication of a system by programm
12. ing rather than hy more conventional and concrete engineering techniques gives the system a unique flexibility and capah ility for modification both during development and in the Field However the shift of imple mentation technology requires a concomitant shift in design and development technology the development of software equivalants to the drawing office prototype preduction explorat ory test beds etc This is all the more impor tant because many perhaps the majority of minicomputer buyers are first time users with out previous experience of computers and without computer based support facilities In time the ready availability of the machines which is causing this flood into new territory will also have the compensatory effect of saturating most potential applications areas however for several years at least the growth of applic ations is likely to exceed the growth of the necessary backup technology It is not trivial to note that systems heavily dependent on computer software development are those most ripe for the application of computer aided design This may be obvious but the fact that editors assemblers and compilers are CAD tools for software development itself tends to be overlooked in that they are seen as an essential part of the computer system not as tools largely independent of the computer which should be tailored to the requirements of the system designer It is true that most mini compute
13. ingham Sime M E Green T R G Guest D J Psycho logical Evaluation of Two Conditional Construct ions Used in Computer Languages Int J Man Machine Studies 5 1 January 1973 APPENDIX 1 SUMMARY OF BASYS BASYS is a systems and interactive applications programming language with a syntax based on Dartmouth College BASIC It is an ongoing development and versions of the language have been used on various configurations of the PDP8 PDP9 PDP1O POPI1 PDP15 MINIC MODULAR ONE and MICRO 16 The main extensions are 1 Improved string handling modelled on SNOBOL handles arbitrary length dynamically changing strings with full ASCII 7 bit character set string relational operators allow for anchored and embedded searches and tests for string equality inequality or telephone diractory comparisons 2 A new PUT command provides keyword search ing pattern matching and string decomposition facilities These give simple and natural gyntax analysis capabilities and allow convers ational programs to be written which can commun icate with untrained users in an approach to natural language 3 A new CODE command enables a running BASYS program to modify itself dynamically generating arrays or new code 4 All error conditions may be trapped by a BASYS program enabling supervisory programs to be constructed that give the user access to the language but retain control 5 Numerical expressions are accep
14. itted normally when the standard default channels are used Additional input output and supervisory commands will exist for most systems since it has been the policy to interface most operating system facilities directly to BASYS with suitable protection where necessary APPENDIX 2 SUMMARY OF ED ED is a line by line string oriented editor suitable for implementation on any computer having a block addressable storage device At any moment during an editing session one line of the user s file is accessible for editing We may think of a symbolic pointer the current line pointer which designates a particular line of the file Editing consists of moving the pointer to the required line and then making a change The pointer may be moved freely up and down the file and any amount of text may be deleted or inserted at any point The necessary file handling and buffering are done automatic ally by ED in a way that is transparent to the user File Control Commands Opens an existing file for editing IN filename OUT filename Creates a new file If used inconjunction with IN defines the name of the edited file LIST filename Opens an existing file for examination only CLOSE Finishes editing and restarts the editor EXIT Finishes editing and returns to the monitor File Backup Measures It is important that the file being edited should be adequately backed up both to protect the user from his own mistakes and as a
15. mulated etc On executing this command the MINIC emulator was called and the specified numbers of instructions emulated with appropriate changes to the array core and registers and status information As usual command then passed to the next instruction in the program line unless the emulation could not be completed because MINIC input output or extended microprogram instructions had been encountered in these cases control was trans ferred to the following line with appropriate status information set up to indicate the cause of interruption It was convenient to pack 4 MINIC 8 bit words into one 36 bit word on the PDP1O and use the residual bits to tag each MINIC word and cause interruption when it was accessed by the emu lator hence break points were simply realized under the emulater Running under interrupts was simulated simply by setting the number of instructions to be executed and real time interrupts from the PDP1O timesharing tele printer were generated through a buffer flag test MINIC peripherals were simulated by BASYS program segments entered after interruption of the MINIC command which adjusted the array core and registers to emulate the appropriate peripheral transfers EXPERIENCE WITH THE EMULATOR SYSTEM An assembler for the normal MINIC assembly language was written in BASYS in about two man days A symbolic debug and environment for the MINIC emulator was written in BASYS in about three man d
16. ook for is found hence there is an implied conditional in the command and action may be taken if the decomposition rule cannot be applied Conditionals IF ne continues execution of the line if the value ne is non zero UNLESS ne continues execution of the line if the value ne is zero Many other commands have implied condition als and continue execution of the line only if they have been performed satisfactorily Transfer of Control DO ne executes program line ne control returns to line following DO commands GOTO ne transfers control to line ne of program GOSUB ne stacks the current line number and trans fers control RETURN ne unstacks the line number stacked by last GQSUB and assigns value to system vari able QA transfers control to next line with number greater than or equal to QA ne if me is absent a value of 1 is used if a command follows the GOSUB only the unstacking is performed and the trans fer of control is not made Hence variable returns and non return transfers may be made from gosubs RUN ne deletes simple variables clears system stacks gives system variables default values and transfers control to next line with number greater than or equal to ne STOP stops execution and returns to keyboard edit mode EXIT stops execution and CALLS standard system program BYE stops execution and logs user off system Editing and Housekeeping LIS
17. r to turn such an emulator into an effective design tool presents problems Generally an emulator of the computer alone is inadequate for all but pure software development What is required is a complete configuration emulator including peripherals such as discs analog digital convertors printers and displays and the timing problems associated with real time control of physical devices At the design stage moreover many of these peripheral systems May not be available their characteristics may be unknown or it may be part of the design to actually develop them If the emulator is written as a machine code program it is compar atively simple to run the target computer main frame diagnostics on it to check its operation However if the ill defined peripherals are algo emulated with machine language programs the process of system development involves joint writing and debugging of machine code programs on both the design machine and the target mach ine a situation fraught with error This problem could be largely overcome by writing the emulators in a suitable high level language However in emulating the target computer speed is of great importance in order to provide proc essing power on a comparable time scale general ly about 10 20 times slower Since the computer will remain constant whilst peripherals change with configuration it seems reasonable to com bine the advantages of both approaches by em bedding a machine code
18. r manufacturers supply the design tools for their computer software in such a form that some configurations of their own computer can actually implement them However this is technically irrelevant and introduces some practical confusion in that it is highly un likely that the minicomputer configuration required for the final system is able to support an adequate software generation system the two specifications are generally far apart Cross7assemblers and compilers enabling the programs for one machine to be created on another have been in use for some time However they represent only one comparatively minor step in the design process the translation from program specification to machine acceptable code On one side of them are the text creation system such as interactive editors and on the other side are the code testing systems such as emul ators Both editors and emulators are systems which cannot be handled well under batch facil ities and require a good interactive system to support them This in itself is a positive reason for having design software on a mini computer it is easy to provide an interactive editor on a suitable minicomputer configuration and the machine can generally be programmed as an efficient emulator of itself under a symbolic debug However the editor requires backup stove preferably drum or disc and the debug requires additional mainframe memory The logical conclusion is that for minicomputer
19. safe guard against system failures and telephone dis connections In addition to preserving the initial state of the user s file ED also keeps a second up to date backup copy SAVE Saves the current state of the edited file BACK Cancels all editing done since the last SAVE command Editing Commands Some commands take numeric arguments If these are omitted a default value of 1 is used Commands that search for strings or move the current line pointer normally cause the pointer to step towards the bottom of the file If the command is prefixed with the letter U the move ment will occur in the upwards direction towards the top of the file Commands for Printing Pn PRINT n lines PF string PRINT FIND Print down to line beginning with the string PL string PRINT LOCATE Print down to line containing the string PB PRINT down to the BOTTOM of the file Commands for Positioning the Pointer T Move pointer to the TOP of the file B Move pointer to the BOTTOM of the file No NEXT Move pointer down by n lines Un Move pointer UP by n lines F string FIND the next line beginning with the string L string LOCATE the next line contain ing the string Commands for Editing the Current Line R string REPLACE the current Line with the string A string APPEND the string to the end of the current line I string INSERT the string below the current line UI string Insert the string above the current line
20. software development an interactive system not necessarily related to the target system should be set up with suitable computer aided design tools for minicomputer software development a powerful interactive editor an assembler pref erably a meta assembler and an interactive emulator This has been our approach to develo pments on a number of systems for example clin ical timesharing systems on the PDP8 and micro 16 and the effort involved in writing effective editor assembler debug systems has been well worth the resultant saving in development time The software development aids in these cases were written on the actual target machines because in all cases the configuration pad drum or disc backup stores and sufficient main mem ory When faced with a situation however in which we did not possess a suitable target machine configuration we went further than this and set up cross assemblers and cross emulators for the target system a Micro computer systems MINIC on available open shop systems a Digital Equipment PDP9 and PDP1O In partic ular the PDP1O MINIC emulator was written as an additional module for the BASYS extended and modified BASIC interpretor we had written for general use on the PDP1O The advantages of doing this and the form of implementation are described in the next section A_MINIC EMULATOR UNDER BASYS To write an instruction set emulator for one machine upon another is generally simple How eve
21. ted wherever a number could occur and these may contain logical and relational operators Hence all transfers of control may be computed 6 Several commands may follow on one line and any statement may follow a conditional Many commands have implied conditionals such that execution along the line continues only if the command is successfully executed 7 Extensive file handling facilities allow BASYS programs to create delete rename read and write files either in ASCII or in binary form A single LET command allows a list of numeric and string variables to be packed or unpacked to or from an array and arrays them selves may be written into arbitrary locations in random access files hence complex record structures are readily set up 8 Simple overlaying is possible through com mands which load and run a filed BASYS program passing to it the current simple variables and selected strings and arrays 9 A variety of priviledged modes allows the construction of system level programs protected from examination or interference by the user Such system level programs are automatically called when the user logs in or out or presses certain control keys hence the system may be trailored in detail to the exact requirements of each class of user Expressions in BASYS A BASYS program consists of a set of numbered lines each containing one or more commands separated by colons eg 100 INPUT 5 UNLESS 5 YES PR
22. ugh a combination of assembler diagnostics and the examination of well laid out and form atted listings In particular the lack of hard ware for the real time system diverted effort into examination of the program texts and elimi nated most faults even before tests on the emu lator c Ease and speed of operation are of para mount importance The magtape based editor on the PDP9 was substantially less satisfactory than the disc based PDP10 and this is strongly reflected in the program quality of the parts of the system produced on the PDP9 editor emulator system d An emulator embedded in a high level lang uage does combine speed with ease of peripheral simulation and debugging support in a powerful combination In some respects we are only re discovering well known if not well kept programming adages However as Sime Green and Guest 1973 have suggested it seems eminently sensible to invest igate the psychological technical foundations for the production of good software and to use the knowledge gained to design more effective program design and development aids REFERENCES Facey P V ED A Contextual Editor Depart ment of Electrical Engineering Science University of Essex Facey P V BASYS User s Manual Department of Electrical Engineering Science University of Essex Gaines B R Gedye J L Facey P V A Versa tile Multi User Interactive Language System for a Minicomputer B C S Datafair 71 Nott
23. uritish Computer Society DATAFAIR 73 Real Time System Design Under an Emulator Embedded in a High Level Language Peter Facey and Brian Gaines Department of Electrical Engineering Science University of Essex Colchester U K Abstract The paper discusses the problems of minicomputer software development and describes experience in the development of a small real time computer system under an inter active emulator embedded in extended BASIC on the PDP1O Major problems with minicomputer software development are a Even if the minicomputer is capable of supporting adequate software develop Ment tools the configuration for development may not do so lack of core backup storage etc b The operation of the system may not be possible except on site or in conjunction with specialist peripheral devices c The system development may involve the definition of specialist hardware inter faces and peripherals which will not be available during the software develop Ment phase These consideraitons suggest that system development under an emulator is attractive particularly one on an interactive time sharing system where the hands on advantages of the minicomputer will not be lost To emulate at a reasonable speed it is necessary to code an order code interpreter for the minicomputer in machine language or preferably as a microprogram but to maintain maximum flexibility in emulating peripheral and associated systems a hi
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