ALGOL-W (Subset) Compiler

ALGOL-W (SUBSET) COMPILER

Carl Clifford Brewer

Monterey, California

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ALGOL-W (Subset) Compiler

Carl Clifford Brewer

Thesis Advisor: Alan B. Roberts

June 19 72

Appiovnd ^oh. pubtic KQ.X.

ALGOL-W (Subset) Compiler

Carl Clifford Brewer Lieutenant, United States, Navy

B.E.E. , Auburn University, 1964

Submitted in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE IN COMPUTER SCIENCE

from the

NAVAL POSTGRADUATE SCHOOL June 19 72

ABSTRACT

This paper describes the ALGOL-W (subset) compiler and language. The primary objective of this project was to develop a compiler that would accept a modified integer version of ALGOL-W that could be used as a programming system to aid in the study of the compilation and execution of ALGOL-like languages. Included is a brief discussion of the ALGOL-E machine options that complement the compiler's capabilities. This compiler was written in the XPL language

TABLE OF CONTENTS

I. INTRODUCTION 4

II. THE LANGUAGE 5

III. THE COMPILER — 7

A. IMPLEMENTATION ~ 7

1. Operator REM - 7

2. READ and READON 8

B. ADVANTAGES —— 8

1. Instructional Capabilities 8

2. Compiler and Machine Options 8

3. Bachus-Naur Form (BNF) 9

IV. TESTING 9

A. TESTING THE BNF 9

B. TESTING DURING PROGRAMMING 9

C. FINAL TESTING 10

V. RECOMMENDATIONS 10

A. DOCUMENTATION 11

B. EXTENSIONS 11

VI. CONCLUSIONS--- 11

APPENDIX A - GRAMMAR FOR ALGOL-W (SUBSET) 12

APPENDIX B - ALGOL-W (SUBSET) COMPILER 17

APPENDIX C - SAMPLE PROGRAM 59

LIST OF REFERENCES 61

INITIAL DISTRIBUTION LIST 62

FORM DD 1473-- 63

I. INTRODUCTION

The programming languages course at the Naval Postgradu- ate School currently makes use of the language ALGOL-E

[Ref . 1] , an integer-only ALGOL-like language which has many desirable instructional features. In the latter part of the course the language ALGOL-W (revised) [Ref. 2] is used to investigate some of the more sophisticated programming language features, such as list processing and character string manipulation. The language ALGOL-W

(revised) is then used by students in a number of courses which follow in the curricula. The two languages, ALGOL-E and ALGOL-W (revised) are quite similar; however they do have a number of syntactic differences that make the mid- course transition more difficult and time-consuming than is desirable.

The advantage of the ALGOL-E language is in the instructional features included in its implementation; hence the obvious solution to the transition problem seemed to be the implementation of a form of ALGOL-W (revised) with the inclusion of these same teaching aids. A significant portion of the advantage gained in the ALGOL-E programming system is due to features included in the ALGOL-E machine, a simu- lated stack processing machine. In view of these facts, the decision was made to implement an integer-only subset of

ALGOL-W (revised) which would be compatible with the ALGOL-E machine. It was further decided that care would be taken

. ,

throughout the project to produce a proper subset of ALGOL-W

(revised) so that programs written under the instructional system could also be run on the ALGOL-W compiler without change. Thus, the process of transition from the instructional system to the full ALGOL-W language would be purely one of expansion.

The ALGOL-E programming system was implemented under the

XPL compiler generator system discussed in detail by McKeeman

Horning and Wortman [Ref. 3], The high degree of similarity that exists between ALGOL-E and ALGOL-W and the fact that the ALGOL-E machine was to be used as the object machine of the compilation process made it very desirable to use the

XPL system in the generation of the ALGOL-W (subset) compiler.

The ALGOL-E compiler served as a model for the development of this compiler.

The objective of this project was to produce the ALGOL-W

(subset) compiler to execute on the ALGOL-E machine and to provide the basic material so that the proper instructional documentation may be produced if it is desired that this system be implemented for student access.

II. THE LANGUAGE

The ALGOL-W (subset) language is, as previously mentioned, a proper subset and an integer version of ALGOL-W (revised)

Records and references have not been included nor have any string manipulation capabilities.

The syntax provides for: normal integer arithmetic, in- cluding exponentiation; ASSIGNMENT statements; GOTO statements;

WHILE statements; FOR statements; IF statements; READ statements; and WRITE statements.

Procedures, both function and proper, are included and have a recursive capability as evidenced in the sample program in Appendix C. All procedure parameters are "call by value;" in fact, the words "INTEGER VALUE" are required in the parameter list in keeping with the ALGOL-W conventions.

The syntax provides indirectly for the null statement

that appears in ALGOL-W (revised) . This capability is provided within the block head and block end structures.

The complete syntax is listed in Appendix A.

Although a GOTO statement is included, it is not implemented in the traditional manner. It can only be used in one direction. The label for the GOTO statement has to occur before the GOTO statement itself in the program. The reason for implementing it in this manner was to avoid the teaching problem that could arise when trying to teach students a block structured language if the only languages

they have previously known were FORTRAN or FORTRAN- like . If the GOTO statement had been implemented in the usual manner it would be easy for students to rely on this capability and

never learn the block structure programming advantages . Im- plementing the GOTO statement in the backward direction only makes it impossible to avoid the use of the block structured facility.

All other portions of the language have been implemented in the traditional manner.

III. THE COMPILER

The compiler was written using portions of McKeeman's

proto-compiler and Kildall's ALGOL-E compiler as guides.

The Backus-Naur Form (BNF) syntax productions were input

to McKeeman's syntax analyzing program ANALYZER which pro-

duced the parsing tables needed in the compiler. The

compiler was written in XPL. The compiler program listing

is shown in Appendix B.

A. IMPLEMENTATION

In order to implement the compiler, certain minor

changes would have to be made to the ALGOL-E machine and to

the ALGOL-W (subset) compiler.

1. Operator REM

The operator REM in ALGOL-W signifies the remainder

after integer division. This operator does not exist in the

ALGOL-E machine as it is not used in the ALGOL-E language. It

could have been implemented in the compiler instead, and

initially it was , but it would not be in keeping with the

normal scheme for emitting assembly code in the compiler be-

cause it is a terminal operand that should have a counterpart

in the associated assembly language. This operator could

easily be added to the ALGOL-E machine. The only other change

needed for this operator would be to initialize the value of

the operator and add a call to the code emitting procedure from

the approproate case statement in SYNTHESIZE.

. . .

2. READ and READON

In ALGOL-W (revised) the READ command always begins with the next data; while the command READON is used to read further from the same card. The READ command, as it is implemented in the ALGOL-E machine, is RDV which reads free- form and a change would be required in the machine in order to properly implement READ and READON in the desired fashion.

B. ADVANTAGES

As previously stated, the main advantage of having this compiler available for the programming languages course is that it is an easy transition from this language to ALGOL-W

(revised)

There are other advantages that exist in this compiler, along with its machine, that also exist in the ALGOL-E system,

1 Instructional Capabilities

This system would provide the user with a means of investigating the data structures of one particular system that would enable him to gain insight into the internal workings of compiler systems in general,

2 Compiler and Machine Options

There are compiler options available that allow the user to trace the parsing and code generation processes at compilation- time and machine options that allow traces of execution and dumps of memory at execution- time

The output formats of traces and dumps are in such a form as to be easily interpreted by the user. All of the options output the information in mnemonic form that relates to the

ALGOL-E assembly language.

3. Backus-Naur Form (BNF.)

The entire language is formally defined in the

Backus-Naur Form (BNF) . The tree-like structure of the

BNF makes the syntax easy to teach and understand.

IV. TESTING

The testing of the compiler was accomplished in three separate phases. The first phase of testing was the testing of the completed BNF. The second phase was the testing that was accomplished during the actual writing of the compiler program. The third phase consisted of running test programs on the completed compiler.

A. TESTING THE BNF

After the proposed BNF had been successfully run in the syntax analyzing program ANALYZE and the produced tables had been included in the framework of the compiler, null case statements were inserted into the code synthesizing procedure SYNTHESIZE, which is a case statement on the production number, in order to be able to run sample programs that would just exercise the parsing routines. These null statements were to later become the code emitting portion of the compiler

A number of sample programs were run in this manner and they demonstrated that the compiler would accept programs written in the desired syntax.

B. TESTING DURING PROGRAMMING

The code generation routines were written in a modular fashion based on groups of BNF productions which were closely

related. As each such group was completed, tests were made

by running programs that relied on those productions. Errors

that became evident during this phase of testing were cor-

rected before proceeding to program further productions

This modular type of programming and testing made the cre-

ation and debugging of the compiler significantly easier.

c. FINAL TESTING

After the compiler was completed it was necessary to run

programs that would exercise the entire system. These pro-

grams would have to test for conflicts between production

groups and also check for conflicts resulting from the em-

bedding of structures tested individually. Two such programs were run and both executed properly in the completed compiler,

An example of one of these programs is shown in Appendix C.

Time did not permit further testing. As would be the

case with any new compiler, adequate testing would have to

include running test programs written by a number of users

or user groups prior to utilizing it in an instructional

capacity.

V. RECOMMENDATIONS

There are tasks that would have to be preformed if the

ALGOL-W (subset) compiler is to become the active instruction-

al compiler for the programming languages course in addition

to the further testing previously mentioned.

A. DOCUMENTATION

Instructional documentation should be prepared using the ALGOL--E documentation as a basic guideline.

B. EXTENSIONS

Additional ALGOL-W features could be added to the language if desired, and as desired, with comparative ease.

Due to the modularity of the compiler structure and the cookbook straightforwardness of the XPL compiler generating system, the process of modifying the language is made easy.

VI. CONCLUSIONS

It has to be concluded that automatic compiler generation is a worthwhile endeavor, as evidenced by the fact that is was possible to produce an operational compiler in the short time alloted for this project. In the past, the creation of a compiler of this size generally took much greater time and' effort.

It is felt that the compiler produced during this project could be successfully utilized in the programming languages course. The accomplishments thus far leave a fairly small amount of work to complete the task of providing a complete system ready for use in the classroom.

The writing of the instructional manual would even be a relatively minor task in view of the available ALGOL-E documentation that could be used as a model due to the inherent similarities of the two systems.

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