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name: compiler-design description: > Use this skill for designing and implementing compilers, assemblers, and language processors specifically for or targeting the Commodore 64 and 128. Covers lexical analysis, parsing, code generation for 6510, cross-compilation, and building assemblers in BASIC or ML. Sources: Compiler Design and Implementation (64 and 128), COMPUTE!‘s SpeedScript source.

Compiler Design and Implementation for the C64/128

Overview

Writing a compiler or assembler on (or for) the Commodore 64 requires special consideration for:

Memory constraints: 64KB total, ~38KB for BASIC programs, ~4KB free upper RAM
Speed: 1 MHz CPU — interpreter overhead is expensive; native code is essential
Target architecture: 6510 with its register-sparse, accumulator-centric ISA
Output format: C64 .prg files (2-byte load address header + raw binary)

Assembler Design on the C64

Two-Pass Assembly

Standard two-pass technique (used by all period C64 assemblers):

Pass 1 — Symbol collection:

Scan source code line by line
Record each label and its current address in a symbol table
Count instruction bytes to advance the address counter
Do NOT produce output

Pass 2 — Code generation:

Re-scan source code
Look up all labels/symbols in the symbol table
Emit object bytes to output buffer or file
Report unresolved symbols as errors

Symbol Table Implementation

For a C64 assembler written in BASIC or ML, the symbol table is typically:

A sorted array of name/value pairs (use binary search for speed)
Names limited to 6-8 characters to conserve memory
Values stored as 16-bit addresses

' Simple symbol table in BASIC (array-based)
DIM SYMNAM$(100)   ' symbol names
DIM SYMVAL%(100)   ' 16-bit values (integer)
NSYM% = 0

' Add symbol
SYMNAM$(NSYM%) = NAME$
SYMVAL%(NSYM%) = VALUE%
NSYM% = NSYM% + 1

' Find symbol (linear search)
FOR I = 0 TO NSYM%-1
  IF SYMNAM$(I) = NAME$ THEN FOUND = SYMVAL%(I) : GOTO FOUND_LABEL
NEXT I
' not found

6510 Instruction Encoding

Each instruction consists of an opcode byte followed by 0, 1, or 2 operand bytes. The assembler must map mnemonic + addressing mode → opcode byte.

Encoding pattern for most 6502 instructions:
Bits 7-5: instruction group
Bits 4-2: addressing mode
Bits 1-0: instruction select within group

Groups:
  aaa=000: BIT, JMP, JMP(), STY, LDY, CPY, CPX
  aaa=001: ORA, AND, EOR, ADC, STA, LDA, CMP, SBC
  aaa=010: ASL, ROL, LSR, ROR, STX, LDX, DEC, INC

Addressing mode encoding (for group 01):
  000: (zp,X)    001: zp      010: #imm    011: abs
  100: (zp),Y   101: zp,X    110: abs,Y   111: abs,X

Expression Evaluator

An assembler's expression evaluator handles operands like LABEL+2, $C000+OFFSET, >ADDR:

' Operators to support:
' + - * / (arithmetic)
' AND OR EOR NOT (bitwise)
' < (low byte), > (high byte)
' Precedence: NOT > */  > +- > AND > OR/EOR

Forward Reference Handling

When a label is referenced before its definition:

Emit a placeholder byte (typically $00 $00)
Record the location and label name in a fixup table
After Pass 1 completes, apply fixups using the resolved symbol table

Lexical Analysis (Tokenizer)

The first stage of any compiler — breaking source into tokens.

Token Types for an Assembler

LABEL      — identifier followed by ':'
OPCODE     — recognized mnemonic (LDA, STA, etc.)
DIRECTIVE  — assembler directive (.BYTE, .WORD, .TEXT, *= etc.)
NUMBER     — decimal ($nnn hex, %nnn binary, 'c' char literal)
STRING     — quoted text "..."
OPERATOR   — + - * / < > = ( )
COMMA      — ,
NEWLINE    — end of logical line
EOF        — end of input

Efficient Lexer in ML

; Simple character classifier for assembler lexer
; Input: A = character
; Output: A = token class (0=whitespace, 1=alpha, 2=digit, 3=operator, 4=EOL)

CLASSIFY:
        CMP #$20        ; space
        BEQ IS_SPACE
        CMP #$0D        ; CR
        BEQ IS_EOL
        CMP #$30        ; '0'
        BMI IS_OP
        CMP #$3A        ; past '9'
        BMI IS_DIGIT
        CMP #$41        ; 'A'
        BMI IS_OP
        CMP #$5B        ; past 'Z'
        BMI IS_ALPHA
        ; default: operator
IS_OP   LDA #3 : RTS
IS_SPACE LDA #0 : RTS
IS_EOL  LDA #4 : RTS
IS_DIGIT LDA #2 : RTS
IS_ALPHA LDA #1 : RTS

Parser Design (Recursive Descent)

A recursive-descent parser is ideal for the C64's memory constraints because:

Small code size (each rule is a subroutine)
No separate parse table needed
Easy to hand-code in ML

Grammar for a Simple BASIC-like Language

program     → statement*
statement   → LET var '=' expr NEWLINE
            | PRINT expr NEWLINE  
            | IF expr THEN statement
            | GOTO number
            | FOR var '=' expr TO expr [STEP expr]
            | NEXT [var]
            | END

expr        → term (('+' | '-') term)*
term        → factor (('*' | '/') factor)*
factor      → NUMBER | STRING | VAR | '(' expr ')' | '-' factor | NOT factor

Parser Subroutine Template (ML)

; Parse an expression; result in FAC1 (using BASIC math)
; Returns with carry set on error

PARSE_EXPR:
        JSR PARSE_TERM      ; parse first term
        BCS PERR
EXPR_LOOP:
        JSR PEEK_TOKEN      ; look at next token
        CMP #TOK_PLUS
        BEQ EXPR_ADD
        CMP #TOK_MINUS
        BEQ EXPR_SUB
        RTS                 ; done: no more + or -
EXPR_ADD:
        JSR NEXT_TOKEN      ; consume '+'
        JSR SAVE_FAC1       ; save left side
        JSR PARSE_TERM
        BCS PERR
        JSR FADD            ; FAC1 = left + FAC1
        BCC EXPR_LOOP
PERR    SEC : RTS

Code Generation for 6510

Register Allocation Strategy

The 6510 has only 3 registers (A, X, Y) and no general-purpose registers. Effective code generation strategies:

Accumulator-primary: Keep the most recent value in A; use X/Y for indices and loop counters
Zero-page variables: Allocate frequently used compiler temporaries in zero page ($FB-$FE free)
Stack-based expression evaluation: For complex expressions, use the hardware stack (PHA/PLA)
Inline vs. subroutine: For short sequences (≤8 bytes), inline is faster; longer sequences justify JSR

Expression Code Generation Example

For A + B * C (where A, B, C are zero-page variables):

; Generated code for: A + B * C
        LDA B           ; load B
        STA TEMP        ; save
        LDA C           ; load C
        ; multiply TEMP * A (need ML multiply routine)
        JSR MULTIPLY    ; result in A (low byte)
        CLC
        ADC A_VAR       ; add A
        STA RESULT

Peephole Optimization

Common optimizations for 6510 code generation:

Pattern	Optimized
`STA $xx; LDA $xx`	`STA $xx` (remove redundant load)
`LDA #0`	`LDA #0` → prefer `AND #0` when flags needed
`TAX; TXA`	Remove both (no-op)
`PHA; PLA`	Remove both if A not changed
Branch over branch	Convert to opposite-condition branch

Building the C64 `.PRG` File Format

A C64 program file begins with a 2-byte load address (little-endian), followed by raw binary:

; Assembler output format for a file that loads at $C000:
.BYTE $00, $C0      ; load address: $C000 (low byte first)
.BYTE <code>        ; raw binary from $C000 onward

To generate from BASIC:

' Write PRG file with load address $C000 (49152)
OPEN 1,8,1,"MYPRG,P,W"   ' SA=1 for raw PRG output
PRINT#1, CHR$(0) CHR$(192)  ' load address low, high
' ... write code bytes ...
CLOSE 1

SpeedScript Architecture (Real-World Example)

COMPUTE!‘s SpeedScript is a complete word processor written in assembly — a model for structured C64 application design:

Memory layout:

$0200–$02FF: I/O buffer and workspace
$033C–$03FB: Cassette buffer (used for printer spooling)
$C000–$CFFF: Main application code (4KB upper RAM)
$0400–$07FF: Screen display buffer
$D800–$DBFF: Color RAM (controlled directly)

Key architectural patterns:

IRQ for keyboard — custom IRQ handler for responsive input
Modular subroutines — each function (cursor move, insert, delete) is a separate JSR
Kernal for I/O — uses standard Kernal OPEN/CLOSE/BASIN/BSOUT for file operations
Direct VIC-II control — manipulates screen and color RAM directly for speed
Zero-page workspace — all frequently used pointers and counters in zero page

Cross-Compilation Considerations

When building a compiler that targets the C64 (runs on a modern machine, produces C64 code):

Little-endian 16-bit values throughout
Memory model: Assume program loads at $0801 (BASIC stub) or $C000 (pure ML)
Calling conventions (for generated subroutines):
- Parameters: A (1 byte), X/Y (2 bytes combined), or zero page
- Return values: A (byte), A/Y (16-bit, A=low), FAC1 (float)
Stack depth: Max recursion ~20 levels (stack is only 256 bytes)
ROM routines: Can JSR to Kernal at $FFxx from generated code — document dependencies

BASIC Line Header for Machine Code Stubs

; BASIC stub that does SYS 49152 when RUN
$0801: $0B,$08  ; link to next line = $080B
       $0A,$00  ; line 10
       $9E      ; SYS token
       $20,"49152",$00  ; " 49152" as text
$080B: $00,$00  ; end of BASIC program
; Code follows at $080D or $C000

Minimal BASIC stub loader (5-line BASIC):

2018 SYS 2074  ' must be at line 2018 for standard stub

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