M. Tavera, M. Alfonseca and J. Rojas IBM Madrid Scientific Center P. Castellana, 4. Madrid-l. SPAIN.

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1 THE IBM PERSONAL COMPUTER APL SYSTEM M. Tavera, M. Alfnseca and J. Rjas IBM Madrid Scientific Center P. Castellana, 4. Madrid-l. SPAIN. INTRODUCTION AND HISTORY The Cmputer Science Department f the IBM Scientific Center in Madrid has been wrking fr several years in the field f APL-based cmpilers and APL interpreters. In 1973, we decided t enhance the sftware f ur System/7 cmputer with ne interactive time sharing system, and chse APL as the target language. System/7 is a sixteen bit minicmputer embdying a disk unit and multiple sensr based facilities, including analg and digital input/utput as well as prcess interrupts. It was ur purpse t supprt all thse facilities within ur system, in such a way that APL culd becme the main prgram develpment language fr thse prjects in the Scientific Center using this minicmputer. At that time, System/7 had a maximum strage f nly 16k 16-bit wrds. Therefre, the interpreter was divided int relcatable, 128 wrd pages. Cntrl transfer between pages was dne thrugh a 'virtual strage' supervisr, which made sure that the requested page was present in main strage. The number f pages simultaneusly active was a functin f the actual size f the System/7 strage. THE MACHINE INDEPENDENT APL INTERPRETER 2. A cmpiler is written translating IL prgrams int the assembly language f a given machine. 3. The IL interpreter is cmpiled. The generated cde is an APL interpreter written in that assembly language and, therefre, directly executable n that machine. T btain an APL interpreter fr a different machine, nly the cde generatr f the cmpiler in step 2 must be rewritten, and step 3 must be repeated. THE INTERMEDIATE LANGUAGE Once the systems prgramming apprach was chsen fr ur implementatin, we had t select an apprpriate intermediate language. Several pssibilities were cnsidered, including Macr Languages, Systems Prgramming Languages knwn at that time, as well as high level languages. Hwever, nne f these was fully adapted t ur purpses, and we finally decided t design ur wn language, which was chsen n the basis f the fllwing criteria:.it must prvide machine independence. It shuld allw easy management f different types f data, required fr the implementatin f an APL interpreter. In 1976, after the APL/7 system was cmpleted, we received certain suggestins cncerning the pssibility f starting a similar prject fr a different minicmputer system. Since the new cmputer had a cmpletely different machine language, the APL/7 system culd nt be directly migrated. Furthermre, the prblem culd recur in sme years whenever new minicmputers were available. In rder t prevent this develpment, at the same time prviding a general slutin t the prblem f the cnstructin f an APL interpreter, we decided t fllw a systems prgramming apprach alng the fllwing lines: i. An interpreter is written in an intermediate machine independent language (IL). ~rmi~in t cpy withut ~e all r part f the ma~nal is ~ant~ prv~ed t~t t~ cpes a~ nt made r distfibut~ ~r di~ct cmme~ml advantage, t~ ACM cpyfi~t nfi~ am t~ title f the 1984 ACM /84/006/0333 $ Cmpilers fr the intermediate language shuld be easy t build. T ptimize perfrmance, they shuld nt intrduce an unreasnable verhead in the cde they generate. A special systems prgramming language (IL) was designed t meet all the requirements f ur prblem. It is endwed with A high level syntax, thus making prgramming, debugging, and readability easier, at the same time prviding machine independence. O A lw level semantics, thus reducing the prgramming effrt required t build cmpilers that prduce highly efficient cde. In fact, IL can be cnsidered as a 'High Level Assembly Language'. p~ii~fin am i~ ~ appear, am ntice is given that cpying is by ~rrn~sin f the A~ciatin ~r Cmputing Machine~. T cpy ~er~se, r t republish, ~qui~s a f~ am/r spec~c ~rmissin.

2 The primitive peratins f the language are thse mst cmmnly encuntered in current assembly languages. But they have been given a high level syntax. The nly assumptin abut the target machine is that its memry is cnsidered t be a vectr f units f fixed size, cnsecutively numbered. The number f bits in a memry unit shuld be a pwer f tw, but is therwise undefined. The language supprts several types f data bjects: numeric and literal cnstants, parameters, integers f different sizes, flating pint bjects, pinters, as well as internal and external label names. Sme f these bjects may be vectrs f numbers, addresses r literals. Others (parameters and labels) are nly allwed t be scalars. An IL prgram cnsists f tw different parts: declaratins and executable statements. All variables and parameters appearing in a prgram must be declared. Their type is implicit in the first letter f the name. Tw different declaratin statements are supprted. Assignment f initial values t static variables. Assignment f synnyms t a previusly defined variable name. Different cmbinatins f data types within this instructin make it pssible t define pinter based variables; vectr variables f variable length; and variables f different types sharing the same address. THE APL/IL INTERPRETER A cmplete APL system cannt be made fully machine independent. There remain several perating system dependent functins, t clse t the machine level, which must be left utside ur prcedure. We call the set f all these functins an APL supervisr. They include System initializatin. Time sharing cntrl, if needed. Sign n/ff prcedures. Terminal input/utput. Supprt f wrkspace libraries. Timer rutines. The APL supervisr amunts t 5% t 10% f the ttal prgramming effrt. The remaining functins in the APL system, equivalent t 90% t 95% f the prgramming effrt, make up what we call the Interpreter. This has been prgrammed in IL, and debugged n a System/370 cmputer. THE LANGUAGE Our machine independent APL interpreter fllws the APL standard, with the fllwing specificatins, differences and enhancements: The executable sectin f an IL prgram cnsists f a number f executable IL statements. These are analyzed frm right t left. There are n precedence rules, and parentheses are nt allwed. The fllwing primitive peratins are supprted: Assignment, increment, decrement, and pint t. Arithmetic peratins: Additin, substractin, multiplicatin, divisin, and residue. Bit shift t the right, r t the left. Bit t bit lgical peratins: OR, AND, NOT, exclusive OR. The nly cntrl statements are the fllwing: Uncnditinal branch. Implementatin dependent limits I. The maximum number f characters in the name f an APL bject is The maximum rank f an APL variable is The maximum number f lines in a functin is Restrictins. i. The value f Quad HT has n actin n terminal input/utput. 2. Quad TT always gives a result f zer. 3. The fllwing system cmmands are nt supprted: )COPY and )PCOPY. They are replaced by )IN and )OUT. Cnditinal branch. Cmputed g t. 'On verflw' cnditin. Subrutine call and return. - )GROUP, )GRP and )CRPS - )CONTINUE and )CONTINUE IIOLD 4. )FNS and )VARS list the crrespnding names in Quad NL rder. A first letter cannt be mentined. Tavera, Alfnseca, Rjas 334 The IBM Persnal Cmputer APL System

3 Enhancements. I. Multiple llne deleting is supprted in functin definitin. 2. Ambivalent defined functins (dyadic-deflned functins that can be used mnadically). The functin itself may discver the valence f the call by applying system functin Quad NC t its left argument. 3. New APL primitives: Dyadic grade-up and grade-dwn. Picture frmat, a pwerful utput generating facility. 4. New APL system functins: 5. Execute alternative (Quad EA), the errr trapping system functin. Peek and Pke f memry cntents, plus the executin f machine-cde subrutines, thrugh a new ambivalent system functin (Quad PK). - Standard data transfer frm (Quad TF), which prvides cnversin between the internal frm and the transfer frm f APL bjects. New APL system cmmands: )RESET clears the state indicatr and is equivalent t as many right arrws as asterisks are there in the indicatr. )OUT prduces a file f APL bjects in transfer frm. )IN cpies int the active wrkspace sme r all f the APL bjects included in a transfer file. 6. Fur data types are supprted: blean (packed ne bit per element), integer (tw bytes per element), real (in flating pint frmat, eight bytes per element), and literal (ne byte per element). The system autmatically perfrms cnversins f numeric data-types t minimize strage space. The APL active wrkspace is cnsidered t be divided int tw parts: ne (the main wrkspace, MWS) where nrmal cmputatins take place. The ther (the elastic extensin EWS) where APL bjects nt currently in use may bestred. This extensin may be lcated'either in main strage r in a fast direct access device, depending n the actual implementatin. Under nrmal circumstances, the system frgets abut the existence f the elastic extensin, and wrks nly n the main sectin. Whenever a wrkspace full cnditin arises, the fllwing prcedure is fllwed: A garbage cllectin is perfrmed. If the wrkspace full cnditin has subsided, the peratin is reattempted, and the system ges n as befre. Otherwise, the user defined APL bjects (functins r variables) in the MWS nt currently needed, are cpied int the elastic extensin, and erased frm the MWS. The lcatin f the APL bject in the elastic extensin is stred in the symbl table f the main wrkspace. A new garbage Cllectin is then perfrmed and the perati n is again reattempted. If ne f the bjects in the EWS is again needed, it is cpied int the main wrkspace. Finally, whenever ne f these bjects is changed, redefined r erased, the new value, if any, is generated in the MWS, and any ld cpy in the EWS is deleted. The elastic wrkspace is managed dynamically, with autmatic reusability f any free space it may cntain. When an bject is deleted there, the space it ccupied becmes available fr ther purpses. APPLICATION TO REAL ENVIRONMENTS Our interpreter writing system has been applied t generate APL interpreters in three different envirnments: i. IBM System/370, which has been used as a test case. 2. IBM Series/l. THE ELASTIC WORKSPACE EXTENSION 3. IBM Persnal Cmputer. Since the machine independent APL interpreter can be applied t many different cmputers, including thse with an address space limited t nly sixteen bits, smething had t be dne t increase the size f the active wrkspaces in such machines. Therefre, we devised the cncept f an elastic wrkspace. THE IBM PERSONAL COMPUTER APL SYSTEM 'Cnversatins with Entry Systems Bca Ratn twards the applicatin f ur prcedure t the IBM Persnal Cmputer began in September The authrs began Tavera, Alfnseca, Rjas The IBM Persnal Cmputer APL System

4 wrking in the interpreter in December 1981, when we received the first Persnal Cmputer at the Madrid Scientific Center. Since the IBM Persnal Cmputer is based n the INTEL 8088 micrprcessr, we wrte a cmpiler t translate IL cde int 8088 assembly language. Hwever, we had t take a decisin cncerning the pssibility f supprting the mathematical cprcessr (the 8087 chip) t enhance the perfrmance f flating pint peratins. Althugh the chip was nt fully available at that time, we decided t run the risk. Therefre, we wrte the cmpiler t translate IL cde int bth 8088 and 8087 instructins. Ne wrte the cmpiler in APL and executed it under VSAPL at an IBM/370 cmputer. The ttal cst f this step in the translatin prcess was abut ne persn-mnth. Once the cmpiler was available, we translated all mdules f the APL interpreter int assembly cde fr the Persnal Cmputer and sent them t ur Persnal Cmputers via a cmmunicatin link. Then we assembled and link-edited all thse mdules, tgether with fur assembly written rutines making up the machine independent supervisr. We finished the first versin f the prduct in May It was then that we decided t include several enhancements, such as picture frmat, dyadic grades, and several auxiliary prcessrs. During the remainder f 1982 and the first quarter f 1983, we implemented the additins, perfrmed a large testing and debugging cycle, and dcumented the system, all f this in Madrid. The final versin f the IBM Persnal Cmputer APL bk was cmpleted by February 1983 in Bca Ratn. nd, the users can design new auxiliary prcessrs t answer needs that may nt be currently supprted by the system. At lad time APL can be invked with a list f auxiliary prcessrs t be used during the sessin. The shared variable prcessr is autmatically laded nly if at least ne auxiliary prcessr has been requested. An applicatin can als be started by specifying an APL system cmmand at lad time. A number f auxiliary prcessrs have been prvided with the system: AP80 supprts the IBM Graphics Printer t prduce APL characters, and can be used either as a system lg t prvide a recrd f the wrk sessin, r t selectively print a desired APL bject r result. API00 manages DOS and BIOS interrupts. AP205 gives full-screen input and utput capability, and supprts a full-screen, defined functin editr. AP210 supprts DOS file management frm APL functins. AP232 permits asyncrnus cmmunicatin with VM/370, and supprts the uplad and dwnlad f files. AP440 can be used t generate music. It is pssible t wrk with bth graphics and mnchrme mnitrs, (althugh the APL characters can nly be displayed in the frmer), and switch autmatically between them. SYSTEM FEATURES The IBM Persnal Cmputer APL system wrks n bth the IBM Persnal Cmputer and the IBM Persnal Cmputer XT and supprts the APL keybard, the US keybard and all the Eurpean Natinal keybards. Persnal Cmputer APL wrkspaces can have variable size accrding t the actual size f the available memry in the machine. The addressing capability f the 8088 micrprcessr is 20 bits, but nly 64 K bytes are directly accessible at a given time. The use f the elastic wrkspace cncept has been applied t vercme this drawback thus prviding access t wrkspaces ver half a megabyte f size. The APL system has been designed in such a way that the APL interpreter, the auxiliary prcessrs and the interface between the frmer and the latter (the shared variable prcessr), are cmpletely separated. This feature presents tw advantages. First, the auxiliary prcessrs can be individually laded, and sec- The IBM Persnal Cmputer APL wrks either n the IBM Persnal Cmputer r n the IBM Persnal Cmputer XT, under the DOS i.i r 2.0 perating systems, and needs a minimum f 128K f randm access memry, ne diskette drive, the IBM Persnal Cmputer Math C-Prcessr, and the clr/graphics mnitr. Optinally, it supprts als the IBM 80 CPS Graphics printer, the mnchrme mnitr, the IBM Persnal Cmputer Expansin Unit, and the IBM Asynchrnus Cmmunicatin Adapter. CONCLUSIONS The efficiency f the systems prgramming apprach has been tested and fund acceptable. The prcedure makes it pssible t develp full APL interpreters at a cst f a few persn-mnths, as cmpared t the several years required t build them frm scratch. Tavera, Alfnseca, Rjas 336 The IBM Persnal Cmputer APL System

5 REFERENCES (i) M. Alfnseca, M. Tavera, R. Casajuana, 'An APL Interpreter and System fr a Small Cmputer', IBM Syst. J., 16, 18 (1977). (2) M. Alfnseca, M. Tavera, 'A Machine Independent APL Interpreter', IBM J. Res. Develp., 22, 413 (1978). (3) M. Tavera, M. Alfnseca, 'IL. An Intermediate Systems Prgramming Language', SCR.01.78, Centr de Inv. UAM-IBM, Madrid (1978). Tavera, Alfnseca, Rjas 337 The IBM Persnal Cmputer APL System

It has hardware. It has application software.

It has hardware. It has application software. Q.1 What is System? Explain with an example A system is an arrangement in which all its unit assemble wrk tgether accrding t a set f rules. It can als be defined as a way f wrking, rganizing r ding ne