C64_SKILLS/.ai/skills/6510-assembly/SKILL.md

Skill: 6510 Assembly for the Commodore 64

Use this skill when

The task involves 6510/6502 machine language, addressing modes, registers, stack, flags, subroutines, speed optimization, or assembly source.

CPU model

The Commodore 64 uses the 6510, which is software-compatible with the 6502 instruction set and adds an on-chip I/O port at $0000-$0001 used heavily for memory banking and cassette control.

Registers

Register	Purpose
A	Accumulator; arithmetic, logic, data movement
X	Index register; loops, offsets, counters
Y	Index register; loops, offsets, KERNAL/BASIC calling conventions
SP	Stack pointer, stack page $0100-$01FF
PC	Program counter
P	Processor status flags: N V - B D I Z C

Addressing modes to prefer

• Immediate: lda #$01
• Zero page: lda $fb faster/smaller than absolute.
• Zero page indexed: lda $fb,x
• Absolute: lda $c000
• Absolute indexed: lda $0400,x
• Indirect indexed: lda ($fb),y for pointer walking.
• Absolute indirect only works with JMP (addr).

Subroutine discipline

Called from BASIC with SYS:

        * = $c000
start:  ; work here
        rts

If using registers for temporary work but caller expects preservation:

        pha
        txa: pha
        tya: pha
        ; work
        pla: tay
        pla: tax
        pla
        rts

Flags

• C carry: arithmetic carry/borrow; set before SBC, clear before ADC unless chaining multi-byte arithmetic.
• Z zero: many KERNAL calls and comparisons rely on it.
• N negative: mirrors bit 7 of result.
• I interrupt disable: SEI disables maskable IRQ; CLI enables.
• D decimal mode: normally keep clear on C64 unless using BCD intentionally.

Multi-byte addition pattern

        clc
        lda lo1
        adc lo2
        sta resultlo
        lda hi1
        adc hi2
        sta resulthi

Pointer walking pattern

ptr     = $fb

        lda #<source
        sta ptr
        lda #>source
        sta ptr+1
        ldy #0
loop:   lda (ptr),y
        beq done
        jsr $ffd2
        iny
        bne loop
        inc ptr+1
        bne loop
done:   rts

Avoid unless requested

• Undocumented opcodes.
• Self-modifying code without comments.
• Cycle-exact code without target PAL/NTSC note.
• Overwriting zero-page locations not reserved for your routine.

Agent checklist

For assembly answers:

1. Identify assembler syntax.
2. Give load address.
3. Show how to run it.
4. Name KERNAL calls used.
5. Declare clobbered registers.
6. Explain zero-page usage.