6502-assembly

6502 assembly for the PET

Assemble with ca65/ld65 through pet build FILE.s (produces a .prg plus a VICE label file) or run in one step with pet run FILE.s (assembles, loads, and RUNs, registering the labels on the session for symbolic debugging). The machine-level reference for addresses and ROM routines is the pet-development skill's reference files (memory map, ROM routines, zero page, PETSCII).

The program skeleton

A PET program loads at $0401 and needs a tiny BASIC stub so that RUN transfers control to your machine code. This skeleton assembles as-is (it is the project’s tests/programs/hello-asm example):

; print a message via the ROM CHROUT routine, then return to BASIC.
; Layout: 2-byte load address ($0401), then a BASIC stub "10 SYS 1037",
; then code at $040D (= 1037).

CHROUT = $FFD2

        .segment "LOADADDR"
        .word   $0401

        .segment "EXEHDR"
        .word   nextln          ; pointer to next BASIC line
        .word   10              ; line number 10
        .byte   $9E, "1037", $00 ; SYS 1037
nextln: .word   $0000           ; end of BASIC program

        .segment "CODE"
start:  ldx     #0
loop:   lda     msg,x
        beq     done
        jsr     CHROUT
        inx
        bne     loop
done:   rts

msg:    .byte   "HELLO FROM ASM", $0D, $00

Why SYS 1037

The load address $0401 is emitted by the LOADADDR segment (not loaded into RAM as data — it is the PRG header). Starting at $0401 the EXEHDR segment lays down a single BASIC line — next-line pointer, line number 10, the SYS token $9E, the digits "1037", and a $00 terminator — followed by the $0000 end-of-program marker. That stub occupies 12 bytes ($0401–$040C), so your CODE segment begins at $040D, which is decimal 1037. Hence SYS 1037 jumps to start. Change the message and the code, not the stub.

Segments available in the linker config: CODE, RODATA, DATA, BSS (and ZEROPAGE). Put executable code and mutable data in CODE/DATA, constants in RODATA, and uninitialized storage in BSS.