# 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`:

```asm
        * = $c000
start:  ; work here
        rts
```

If using registers for temporary work but caller expects preservation:

```asm
        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

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

## Pointer walking pattern

```asm
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.