# Skill: C64 Memory Map and Address Discipline

## Use this skill when

The task involves memory addresses, `PEEK`, `POKE`, page zero, screen/color memory, ROM/RAM/I/O banking, vectors, or safe placement of machine-language code.

## Core concepts

- The 6510 can address 64K: `$0000-$FFFF` / `0-65535`.
- Most two-byte addresses are stored little-endian: low byte first, high byte second.
- Page zero `$0000-$00FF` is fast and heavily used by BASIC/KERNAL.
- Page one `$0100-$01FF` is the processor stack.
- Screen RAM normally starts at `$0400` / `1024`.
- Color RAM normally starts at `$D800` / `55296`; only the low nybble is meaningful.
- I/O registers normally occupy `$D000-$DFFF`; character ROM or RAM can be banked into the same range.
- BASIC ROM is normally at `$A000-$BFFF`; KERNAL ROM at `$E000-$FFFF`.
- RAM exists under ROM/I/O areas, but using it requires banking and interrupt care.

## Key addresses

| Decimal | Hex | Meaning |
|---:|---:|---|
| 0 | `$0000` | 6510 data-direction register |
| 1 | `$0001` | 6510 processor port; controls ROM/I/O banking and cassette lines |
| 43-44 | `$002B-$002C` | Start of BASIC program text pointer |
| 45-46 | `$002D-$002E` | End of BASIC program text pointer |
| 55-56 | `$0037-$0038` | Top of BASIC memory pointer |
| 198 | `$00C6` | Keyboard buffer length |
| 631-640 | `$0277-$0280` | Keyboard buffer |
| 646 | `$0286` | Current text color |
| 780-783 | `$030C-$030F` | `SYS` register save/pass area: A, X, Y, status |
| 788-789 | `$0314-$0315` | IRQ vector |
| 790-791 | `$0316-$0317` | BRK vector |
| 792-793 | `$0318-$0319` | NMI vector |
| 1024-2023 | `$0400-$07E7` | Default 40x25 screen RAM |
| 2040-2047 | `$07F8-$07FF` | Default sprite pointers |
| 49152 | `$C000` | Common safe ML area if protected from BASIC use |
| 53248 | `$D000` | VIC-II register base |
| 54272 | `$D400` | SID register base |
| 55296 | `$D800` | Color RAM base |
| 56320 | `$DC00` | CIA #1 base |
| 56576 | `$DD00` | CIA #2 base |

## Safe machine-language placement

Common choices:

- `$C000` / `49152`: often used for ML routines because BASIC normally has about 38911 bytes free and does not use this area directly.
- Above a lowered BASIC top: set top-of-memory pointer and issue `NEW` before loading BASIC program.
- In a cartridge/ROM area only when explicitly building cartridge code.
- Avoid `$033C-$03FB` unless you know which cassette buffer or workspace is being used.

## Banking rules

The 6510 processor port `$0001` controls what the CPU sees in ROM/I/O ranges. Use cautious read-modify-write:

```asm
lda $01
pha
; change bits only as needed
; ... do banked access quickly ...
pla
sta $01
```

When banking out I/O or banking in character ROM, interrupts that use I/O can fail. Disable IRQ briefly with `SEI`, restore `$01`, then `CLI`.

## BASIC bit manipulation patterns

Set a bit:

```basic
POKE A,PEEK(A) OR MASK
```

Clear a bit:

```basic
POKE A,PEEK(A) AND (255-MASK)
```

Toggle a bit:

```basic
POKE A,PEEK(A) XOR MASK
```

## Agent checklist

Before suggesting memory writes:

1. Identify whether the address is RAM, ROM, I/O, color RAM, vector, or register.
2. State decimal and hex.
3. Warn about global side effects.
4. Preserve existing bits unless replacing the whole register is intentional.
5. Explain how to restore the original value.