DESIGN OF PROGRAMMING lANGUAGE PROCESSORS-IISYMPO~UM SUMMARY

CHAIRMAN: B. RANDELL (USA)

THE INTERRELATION BETWEENPROGRAMMING LANGUAGES AND

MACHINE ORGANIZATION

R. S. BARTON

General Electric Company

Several processor organizations have been proposedbased on programming languages, notably ALGOL.Euler, a design based on a generalization ofALGOL, is. perhaps the best example of suchmachines, and is of much interest for its built-instorage management and subroutine control func-tions. Language-based processors, however, are notwidely known and understood nor do they seem tohave been properly evaluated in comparison withmore conventional machines.

Increased interest in non-numeric computer ap-plications and the demands of multi-terminaltime-sharing, have provided new motivation forgiving attention to the subject of storage manage-ment, which now seems rriore important than lan-guage characteristics in determining processor or-ganization. It thus seems appropriate to reconsiderbuilt-in functions for an information processor.

Programming languages describe the structure ofprocesses and the data upon which they operate.Such structures can be discussed or abstractly trans-formed without regard to a particular machine'scharacteristics or the manner in which the processand its data are represented in storage. Tree-likelanguage syntax leads naturally to the use of push-down stores but beyond this programming languagedevelopments seem not to have much influencedmachine organization. Of course, one may notprogress far into a realizable machine design with-out taking internal machine representation intoaccount.

The format in which information is internallyrepresented in the storage of a machine is a mostfundamental consideration. A format conventionmust exist for a program to be interpretable; thoughit is customary in today's machines to make thisquite inflexible with rigid formats for data andfixed instruction layouts. This practice prohibitsmUch versatility in making one machine imitate~nother. The elusive measurement of "efficiency"IS a weighting of storage usage and number ofaccesses: and. as every programmer has observed,minimizing both storage space and number of ac-

cesses are usually conflicting goals. To optimize theperformance of an information processor for a par-ticular application, some choice of data and pro-gram formats at machine language level should beavailable to the programmer. A general purposemachine should be able to imitate another machineeffectively by allocating its resources in logic andstorage in such fashion as to favor the most frequentoperations in the machine being imitated. Directuse of the' format of that machine is clearlyadvantageous.

Format is the description of a pattern of fieldsspecifying lengths, sequence, encodings, and in-terpretation. Each sequence and storage region hasa format associated with it and this format descrip-tion might be distributed throughout a program anddata region in storage. The two arguments againsthardware provision for variable format: (I) in-creased equipment complexity, and (2) loss of speedstemming frOID restrictions on paralleling, are validonly if the performance improvements do not justifythe cost.

Few machines have provisions for handling listsdirectly and in those that do the instruction set ismerely supplemented by a few instructions oflimited use. Lists, considered apart from list proces-sors, are a simple and useful storage allocation andsequencing device with increasing utility in applica-tions. The microprograms for link manipulationshould be incorporated into the hardware since afactor of at least 10 in performance is at stake. Onemust avoid, however, too rigid a list format.

Paging, which can be looked upon as a form ofindirect addressing, uses indirection with the highorder bits of the address only; and is another map-ping technique which allows storage to be parti-tioned into equal units of which any subset can bemade to have contiguous addresses. Paging is anatural storage management device for time-sharingand also has a role in mapping information acrossstorage levels.

Location addressing has been the rule in moststorage devices though it is common to use pro-grammed symbolic addressing for dealing with massstorage. While strict hardware symbollicallyaddressed storage has not proved economicallyattractive for computers, the counterpart in addres-sing logic (hash-addressing to list-structured equiv-alence classes) might be advantageously incorpor-ated into hardware so that the approximate mix oflocation and symbolic addressing techniques would

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618 PROCEEDINGS OF THE IFIP CONGRESS 65

be directly available to the program designer. In-stances of the application of symbolic addressing areinterpretation during program checkout, in "con-ventional mode" of communication, storage man-agement functions, and addressing of secondarystorage.

The coordinated handling of several sequences ofintermixed program and data is a natural provisionin the organization of a time-shared processor.From this viewpoint sequence counters and indexregisters become instances of a more general device.Present programming languages do not usually havefacility for the expression of multi-sequence controland multi-terminal time sharing usually envisagesthe running of mixes of independent and individu-ally single-sequence programs. The expressivepower of programming languages would be in-creased by providing the needed sequencing ad-juncts; and, the processing options thus enabledwould allow for throughput improvements in time-shared processors.

The notion of substitution combines the familiarideas of assignment statement and proceduredeclaration .. A basic machine function distinguishesbetween a naine and a datum in a program. Theevaluation of a name combines the ideas of fetchand procedure call (and, incidentally, a general-ization of call-by-name).

CONCLUSIONS

The most serious obstacles to a generally ac-ceptable machine language are (I) the rigid formatsfor program and operands, (2) lack of provision forreferencing hierarchical data structures, (J)' limitedchoice of mappings, (4) the inability to expressmultisequence algorithms conveniently, (5) a toolimited concept of substitution.

If the desirability of variable program and dataformat were accepted and design emphasis directedtowards these goals, then some problems of a per-sistent and annoying nature would be alleviated oreven eliminated. The direction indicated, ratherthan language-based processor design, would seemto offer the best opportunity for major improvementin information processors.

DYNAMIC STORAGE ALLOCATION

R. L. COOK

Elliott Automation Computers Limited

This paper describes SPAN-a dynamic storageallocation scheme that has been implemented onthe Elliott 503 Computer. The system is based oncode word technique described for example byIlilfe. I

Dynamic storage allocation is used here to meanthe allocation of core storage for both data and

program at run time, i.e., during the actual opera.tion of a program.

The 503 is a medium-sized computer primarihused for scientific computation. As far as 'thispaps,is concerned it has four types of storage:

main core store (8k words)auxiliary core store (up to 132k words)magnetic disc storemagnetic tape store

The object of the SPAN system is to present t,the user what appears to be a large single levestore-the ability for the user not to have to differentiate between the two forms of core storage beinof particular importance. A secondary result of thsystem is to permit a program to be operated unchanged on two different storage configurations.

Storage is manipulated in terms of variable blockof consecutive words. Each block's current positiois completely defined by means of a single codword. By creating a block of code words which iturn is defined by a single code word, a tree cblocks of any depth can be created. A block, onecreated, is moved between the various storage d(vices either automatically by the SPAN executivsystem or, via specific macro commands issued bthe user. The size of a block is dynamically variablin size within the prescribed limits of 5 and 4,09words. Any block may be fixed, permanently ctemporarily, so that it is not manipulated by thsystem.

The system outlined above forms part of thcentral executive system of the 503 which is usewhatever programming is in operation. The systersolves, in an obvious way, the problems of ove:laying segments of program, of allocating buffareas for input and output and of allocating storagfor arrays and tables. The three principle progranming languages are an assembly language, ALGOand FORTRAN. In the case of the assembly cod,program and data segmentation is in the hands (the programmer but in the case of a high level pngramming language it is necessary to build into ttcompiler techniques for determining suitable selmentation points. For the ALGOL language dinformation contained within the block structuJ

of the program assists in this task. All blocks anprocedure bodies are compiled into distinct prgram blocks. As each block is entered, stora!blocks are found for the compiled program an~ feach of the associated data declarations contatnin the block head. If the block had previously bentered it is possible that the compiled progralllstill in main store and has not been overwritten athe system can then avoid bringing down the .prgram again. On exit from a block, the assOc1atstorage areas are marked as being no IonSrequired. f

We can see that this system avoids the use 0 •conventional dynamic run time stack by the cr.e;of independent data areas. An interesting 51 e