# Skill: Interrupts, Reset, IRQ, NMI, BRK, and Vectors

## Use this skill when

The task involves IRQ/NMI handlers, raster interrupts, STOP+RESTORE, reset behavior, BRK, vector patching, or safe interrupt disabling.

## Core concepts

- RESET loads the CPU program counter from vector `$FFFC-$FFFD`.
- IRQ/BRK loads from `$FFFE-$FFFF` after pushing PC/status to stack.
- NMI loads from `$FFFA-$FFFB` and cannot be masked by the I flag.
- C64 RAM vectors allow patching system behavior without changing ROM.

## Important RAM vectors

| Decimal | Hex | Meaning |
|---:|---:|---|
| 788-789 | `$0314-$0315` | IRQ vector |
| 790-791 | `$0316-$0317` | BRK vector |
| 792-793 | `$0318-$0319` | NMI vector |
| 768-779 | `$0300-$030B` | BASIC vectors: error, main loop, tokenize, list, execute, evaluate |

## Safe IRQ hook pattern

1. `SEI`.
2. Save old vector.
3. Store new low/high bytes to `$0314/$0315`.
4. `CLI`.
5. Handler saves registers.
6. Handler acknowledges its interrupt source.
7. Handler restores registers.
8. Handler jumps to old IRQ or normal KERNAL IRQ path.

## Minimal handler skeleton

```asm
irq:    pha
        txa
        pha
        tya
        pha

        ; acknowledge source here, e.g. VIC raster IRQ:
        lda #$01
        sta $d019

        ; do short, deterministic work

        pla
        tay
        pla
        tax
        pla
        jmp $ea31      ; normal KERNAL IRQ continuation on C64
```

## Warnings

- Do not leave IRQ disabled unless building a controlled loader/demo effect.
- NMI is used by RESTORE and CIA #2; avoid breaking STOP+RESTORE recovery unless intentional.
- Raster IRQ code must be short and cycle-aware.
- Always acknowledge the hardware source or the interrupt will immediately retrigger.

## Agent checklist

1. State interrupt source: VIC-II, CIA #1, CIA #2, BRK, RESTORE/NMI.
2. Show vector write low byte then high byte while IRQ disabled.
3. Save/restore A, X, Y unless deliberately not needed.
4. Acknowledge interrupt flags.
5. Chain to old handler or document full replacement.