William N. Joy**

M. Kirk McKusick***

Computer Science Division
Department of Electrical Engineering and Computer Science
University of California, Berkeley
Berkeley, California 94720

ABSTRACT

      Berkeley Pascal is designed for interactive instructional use and runs on the 11/780"" VAX 11/780. The interpreter px executes the Pascal binaries generated by the Pascal translator pi.

      The PX Implementation Notes describe the general organization of px, detail the various operations of the interpreter, and describe the file input/output structure. Conclusions are given on the viability of an interpreter based approach to language implementation for an instructional environment.

Introduction

      These PX Implementation Notes have been updated from the original 11/70"" PDP 11/70 implementation notes to reflect the interpreter that runs on the 11/780"" VAX 11/780. These notes consist of four major parts. The first part outlines the general organization of px. Section 2 describes the operations (instructions) of the interpreter while section 3 focuses on input/output related activity. A final section gives conclusions about the viability of an interpreter based approach to language implementation for instruction.

Related Berkeley Pascal documents

      The PXP Implementation Notes give details of the internals of the execution profiler pxp; parts of the interpreter related to pxp are discussed in section 2.10. A paper describing the syntactic error recovery mechanism used in pi was presented at the ACM Conference on Compiler Construction in Boulder Colorado in August, 1979.

Acknowledgements

      This version of px is a 11/70"" PDP 11/70 to 11/780"" VAX 11/780 opcode mapping of the original px that was designed and implemented by Ken Thompson, with extensive modifications and additions by William Joy and Charles Haley. Without their work, this Berkeley Pascal system would never have existed. These notes were first written by William Joy for the 11/70"" PDP 11/70 implementation. We would also like to thank our faculty advisor Susan L. Graham for her encouragement, her helpful comments and suggestions relating to Berkeley Pascal and her excellent editorial assistance.
** The financial support of the National Science Foundation under grants MCS74-07644-A03 and MCS78-07291 and of an IBM Graduate Fellowship are gratefully acknowledged.
*** The financial support of a Howard Hughes Graduate Fellowship is gratefully acknowledged.

Table of Contents

• 1. Organization
  • 1.1. Format of the object file
  • 1.2. General features of object code
  • 1.3. Stack structure of the interpreter
  • 1.4. Data types in the interpreter
  • 1.5. Runtime environment
  • 1.6. Dp, lc, loop
  • 1.7. The stack frame structure
  • 1.8. The block mark
  • 1.9. Arguments and return values
  • 1.10. The main interpreter loop
  • 1.11. Errors
• 2. Operations
  • 2.1. Naming conventions and operation summary
  • 2.2. Basic control operations
    
    
  • 2.3. If and relational operators
  • 2.4. Boolean operators
  • 2.5. Right value, constant, and assignment operators
  • 2.6. Addressing operations
  • 2.7. Arithmetic operators
  • 2.8. Range checking
  • 2.9. Case operators
  • 2.10. Operations supporting pxp
  • 2.11. Set operations
  • 2.12. Miscellaneous
  • 2.13. Mathematical Functions
  • 2.14. System functions and procedures
  • 2.15. Pascal procedures and functions
• 3. Input/output
  • 3.1. The files structure
  • 3.2. Initialization of files
  • 3.3. Block exit
  • 3.4. Flushing
  • 3.5. The active file
  • 3.6. File operations
  • 3.7. Read operations
  • 3.8. Write operations
  • 3.9. File activation and status operations
  • 3.10. File housekeeping operations
• 4. Conclusions