Pascal-S Compiler Implementation Analysis

I. About Pascal-S and Its Compiler

Pascal-S is a true subset of Pascal, developed by computer scientist N. Wirth. It retains most of Pascal’s functionality, supporting integer, real, boolean, and character types, plus arrays, records, user-defined types, functions, procedures, value/var parameters, and recursion.

The compiler is written in Pascal with ~50 functions covering lexical analysis, syntax analysis, code generation, error handling, and interpretation. Unlike standard Pascal, Pascal-S compiles to intermediate code (P-code) which is then interpreted.

II. Table Management

The compiler uses several tables: symbol table (tab), block table (btab), array table (atab), real constant table (rconst), string constant table (stab), and code table (code).

2.1 Symbol Table (tab) — stores identifier information: name, link (to previous identifier in same block), obj (kind: constant/function/variable), typ (data type), ref (index to atab/btab), normal, lev (nesting level), adr (address).

2.2 Block Table (btab) — one entry per program block (records are also blocks). Stores last, lastpar, psize, vsize.

2.3 Array Table (atab) — one entry per array. Multi-dimensional arrays are arrays of arrays. Stores inxtyp, eltyp, elref, low, high, elsize, size.

III. Lexical Analysis

51 symbols defined in a TYPE. The analyzer reads characters from a text file, recognizing them as one of the 51 symbols. Key functions: nextch (read next char), insymbol (read next symbol), readscale (read float exponent), adjustscale (adjust real). Uses line buffering and binary search for keywords (pre-sorted).

IV. Syntax Analysis

Uses syntax-directed translation with top-down recursive descent. Code generation and error handling are integrated with parsing.

The grammar is described in an ANSI-C-like notation covering Program, Block, Declaration, ConstDeclaration, TypeDeclaration, VariableDeclaration, Statement, Expression, SimpleExpression, Term, Factor, Constant, Identifier, and Figure.

V. Code Generation and Interpretation

Generates 59 types of quadruple-like intermediate instructions stored in the code array. A stack-based virtual machine executes them with a stack data segment, code segment, and registers (allocation pointer, instruction pointer, instruction register, base pointer).

For each Block, when called, five slots are allocated for: static link, dynamic link, return address, function return value, and procedure name position in the symbol table.

VI. Afterword

After half a month reading PL0 and Pascal-S source code, I finally got a basic understanding of compilers. Not knowing Pascal initially made it harder, but after learning the language, things clicked. PL0 was simpler and I traced through execution. Pascal-S was more complex — I had to guess in many places.

Despite errors in my understanding, the gains were significant. Compiler theory from class finally made sense after studying real compiler source code.