# Skill: CIA I/O, Timers, Keyboard, Joystick, Serial Bus, and User Port

## Use this skill when

The task involves CIA chips, keyboard scanning, joysticks, timers, real-time clock, serial bus, disk/printer handshaking, RS-232, or user port I/O.

## CIA bases

| Chip | Decimal | Hex | Common role |
|---|---:|---:|---|
| CIA #1 | 56320 | `$DC00` | Keyboard matrix, joystick ports, timers, TOD, IRQ |
| CIA #2 | 56576 | `$DD00` | Serial bus, RS-232/user port, VIC bank select, NMI |

## CIA register layout

Each CIA exposes 16 registers mirrored across its 256-byte block.

| Offset | Purpose |
|---:|---|
| 0 | Port A data |
| 1 | Port B data |
| 2 | Port A data direction |
| 3 | Port B data direction |
| 4-5 | Timer A latch/counter low/high |
| 6-7 | Timer B latch/counter low/high |
| 8-11 | Time-of-day clock |
| 12 | Serial data register |
| 13 | Interrupt control/status |
| 14 | Control register A |
| 15 | Control register B |

## Joystick reading

Joystick values are active low.

- Port 2 commonly read at `$DC00` / `56320`.
- Port 1 commonly read at `$DC01` / `56321`.

Typical bits:

| Bit | Mask | Direction/button |
|---:|---:|---|
| 0 | 1 | Up |
| 1 | 2 | Down |
| 2 | 4 | Left |
| 3 | 8 | Right |
| 4 | 16 | Fire |

Pressed means bit is `0`.

BASIC example:

```basic
10 J=PEEK(56320)
20 IF (J AND 16)=0 THEN PRINT "FIRE"
```

## Serial bus concepts

- Device numbers distinguish serial devices.
- The C64 sends ATN, then TALK or LISTEN, plus optional secondary address.
- Data is transferred one byte at a time using handshaking.
- Prefer KERNAL routines (`SETLFS`, `SETNAM`, `OPEN`, `CHRIN`, `CHROUT`, `CLOSE`) unless bit-banging is the task.

## Timer principles

- CIA timers can count CPU cycles or external CNT pulses.
- Timer underflow can generate interrupts.
- For reliable timing, acknowledge CIA interrupt source and avoid leaving stale flags.

## Agent checklist

1. Identify CIA #1 vs CIA #2.
2. Preserve data direction registers unless intentionally changing port direction.
3. Remember joystick and keyboard are active-low.
4. For serial/disk/printer, prefer KERNAL I/O.
5. For timers, show latch, control, interrupt enable, and acknowledge steps.