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Figure 1. Three types of indirect-execution computers: (a)low-level Intermediate language; (b) medlum-level Inter-mediate language; (c) high-level Intermediate language.

Although the indirect-execution computer has lesscomplex hardware than a direct-execution computer, itscompiler can be simpler than that ofa von Neumann com-puter. The indirect-execution computer is an intermedi-ate structure, between von Neumann architecture anddirect-execution architecture.The direct-execution computer directly accepts and ex-

ecutes a high-level language program, with no need formultiple layers of conventional software. As a result,there is no compiler, no assembler, and no linkage editor.There is only one language: the high-level programminglanguage, which is the machine language recognized bythe bare hardware.The direct-execution computer is structured with an in-

terpretation cycle. A high-level language program is storedin the program memory. The lexical processor fetches thenext token from the program memory and delivers the to-ken to the language processor; the language processor thenexecutes the token accordingly. This cycle continues untilthe program ends.The direct-execution computer has several advantages.

First, there is no need for a compiler; the computer muststore only one copy of the program, the source program.More important, a direct-execution computer can givetheterminal user an interactive program execution. This is avery important advantage because an error can be cor-rected as it occurs-without waiting for the end of a com-pilation run, as is required in an indirect-execution com-puter. Furthermore, a terminal user can interact with theexecution ofthe program (but not with the absolute code)because the internal structure of the direct-executioncomputer can be displayed as the hardware executes eachtoken, clause, or statement in the source program. Adirect-execution computer is more complex in hardwarethan an indirect-execution computer, but VLSI technologypromises to balance the costs of hardware and software.

This issue presents articles on both direct-executionand indirect-execution architectures. The first, by Suss-man, Holloway, Steele, and Bell, describes the VLSI de-sign and implementation of a single-chip microcom-puter called Scheme-79 (Scheme is a dialect of the Lisplanguage).

There are two articles on the relationship of languagesand compilers to computer architecture. Chu and Abramsreveal three direct relations between programminglanguages and direct-execution computer architecture.Interactive source-program execution, programminglanguage definition, and programming language com-plexity all have direct relationships with direct-executionarchitecture. These three unique relations distinguish thedirect-execution computer from the indirect-executioncomputer and the von Neumann computer. The article byWulf presents the relation between compiler design andcomputer architecture. It is a unique and candid presenta-tion from the perspective of a compiler writer.Two articles are on the historical Symbol IIR computer

system. It is amazing that the Symbol system was designedand built 15 years ago, with modest funding and small-scale integration technology. The experience with theSymbol system inspired confidence in the ability to buildcomplex computer hardware systems. Rice, the chief ar-chitect of the system, reflects on it from his unique posi-tion. Ditzel looks at the Symbol system from a systemprogrammer's viewpoint. He argues that the Symbolsystem worked and clearly demonstrated that a high-levelcomputer, a compiler, an automatic memory manage-ment, and a major portion of a time-shared operatingsystem can be implemented successfully in hardware.The article by Yamamoto is a survey of high-level lan-

guage machines in Japan. Most have been built for experi-mental purposes, but there are a few commercial high-level language machines. Lisp and APL machines havebeen built at research laboratories and universities, whilea small number ofFortran and Cobol machines have beenbuilt by computer manufacturers.i

a Yaohan Chu, a professor of computer sci-ence and electrical engineering at theUniversity of Maryland, College Park, de-veloped the computer design languageCDL and the software design languageSDL. His current research interests are incomputer language engineering, high-levelcomputer architecture, and software engi-neering; his group is desigrning a direct-exe-

-11111 cution computer for the USAF languageJovial. He has written several books on computer design,simulation, computer organization, microprogramming, andhigh-level-language computer architecture, and is currentlydeveloping a book on software blueprints. Chu is the AFIPSrepresentive to the IFIP Technical Committee on Digital SystemDesign. He graduated from Chiao-Tung University, China, andreceived his MS and ScD degrees from MIT.

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