# CIA 6526 Register Reference

Both CIA chips use the same register layout. CIA #1 = $DC00, CIA #2 = $DD00.

## Register Map (offset from chip base)

| Offset | CIA#1 Addr | CIA#2 Addr | Name | Description |
|--------|------------|------------|------|-------------|
| +0 | $DC00 | $DD00 | PRA | Port A data register |
| +1 | $DC01 | $DD01 | PRB | Port B data register |
| +2 | $DC02 | $DD02 | DDRA | Port A direction (1=output) |
| +3 | $DC03 | $DD03 | DDRB | Port B direction (1=output) |
| +4 | $DC04 | $DD04 | TIMALO | Timer A low byte (latch/counter) |
| +5 | $DC05 | $DD05 | TIMAHI | Timer A high byte |
| +6 | $DC06 | $DD06 | TIMBLO | Timer B low byte |
| +7 | $DC07 | $DD07 | TIMBHI | Timer B high byte |
| +8 | $DC08 | $DD08 | TODTEN | TOD tenths of seconds (BCD) |
| +9 | $DC09 | $DD09 | TODSEC | TOD seconds (BCD) |
| +A | $DC0A | $DD0A | TODMIN | TOD minutes (BCD) |
| +B | $DC0B | $DD0B | TODHR | TOD hours + AM/PM (bit 7) |
| +C | $DC0C | $DD0C | SDR | Serial data register |
| +D | $DC0D | $DD0D | ICR | Interrupt control register |
| +E | $DC0E | $DD0E | CRA | Control register A |
| +F | $DC0F | $DD0F | CRB | Control register B |

## CIA #1 ($DC00) — Keyboard and Joystick

### Port A ($DC00) — Keyboard Column Select / Joystick 2
- Writing to $DC00 selects keyboard columns to scan (0=select)
- Reading $DC00 gives joystick 2 state (bits 0-4, active low)

### Port B ($DC01) — Keyboard Row Read / Joystick 1
- Reading $DC01 returns keyboard row data for selected column
- Also joystick 1 state (bits 0-4, active low)

### Keyboard Matrix
The keyboard is a 8×8 matrix. To read a key:
1. Write column mask to $DC00 (0 selects that column)
2. Read rows from $DC01

| Bit set in $DC00 column | Keys in that column |
|------------------------|---------------------|
| $FE (col 0) | DEL, RETURN, ←→, F7, F1, F3, F5, ↑↓ |
| $FD (col 1) | 3, W, A, 4, Z, S, E, Left Shift |
| $FB (col 2) | 5, R, D, 6, C, F, T, X |
| $F7 (col 3) | 7, Y, G, 8, B, H, U, V |
| $EF (col 4) | 9, I, J, 0, M, K, O, N |
| $DF (col 5) | +, P, L, -, ., :, @, , |
| $BF (col 6) | £, *, ;, Home, Right Shift, =, ↑, / |
| $7F (col 7) | 1, ←, CTRL, 2, Space, C=, Q, Stop |

### Joystick Bits ($DC00 joystick 2 / $DC01 joystick 1)
```
Bit 0: Up        (0=pressed)
Bit 1: Down      (0=pressed)
Bit 2: Left      (0=pressed)
Bit 3: Right     (0=pressed)
Bit 4: Fire      (0=pressed)
```

```basic
J = PEEK(56321)          ' Read joystick 1 ($DC01)
IF (J AND 16) = 0 THEN PRINT "FIRE!"
IF (J AND 1)  = 0 THEN PRINT "UP!"
```

## CIA #2 ($DD00) — Serial Bus and VIC-II Bank

### Port A ($DD00) — Serial Bus / VIC-II Bank
```
Bit 7: Serial bus data line (input)
Bit 6: Serial bus clock line (input)
Bit 5: Serial bus data line (output)
Bit 4: Serial bus attention ACK (output)
Bit 3: Serial bus clock (output)
Bit 2: RS-232 data output (User Port)
Bit 1: VIC-II bank select bit 1  \  Together: 00=bank3, 01=bank2
Bit 0: VIC-II bank select bit 0  /            10=bank1, 11=bank0(default)
```

```basic
' Switch VIC-II to Bank 1 ($4000-$7FFF)
POKE 56576, (PEEK(56576) AND 252) OR 2
' Bank 0 (default): POKE 56576, (PEEK(56576) AND 252) OR 3
```

## Control Register A (CRA — $DC0E / $DD0E)

```
Bit 7: TODIN  — TOD clock: 1=60Hz, 0=50Hz
Bit 6: SPMODE — Serial port: 1=output, 0=input
Bit 5: INMODE — Timer A input: 1=CNT pulses, 0=system clock
Bit 4: LOAD   — Force load timer (write 1 to reload latch into counter)
Bit 3: RUNMODE — 0=continuous, 1=one-shot
Bit 2: OUTMODE — Timer A output to PB6: 1=pulse, 0=toggle
Bit 1: PBON  — Port B output enable (1=Timer A appears on PB6)
Bit 0: START — 1=start timer, 0=stop timer
```

## Control Register B (CRB — $DC0F / $DD0F)

```
Bit 7: ALARM — 1=write TOD alarm, 0=write TOD clock
Bit 6: INMODE (high bit) — with bit 5: timer B source
Bit 5: INMODE (low bit)  — 00=φ2 clock, 01=CNT, 10=Timer A, 11=TimA/CNT
Bit 4: LOAD   — Force load
Bit 3: RUNMODE — 0=continuous, 1=one-shot
Bit 2: OUTMODE — Timer B output to PB7
Bit 1: PBON   — Port B output enable
Bit 0: START  — 1=start, 0=stop
```

## Interrupt Control Register ($DC0D / $DD0D)

**Reading**: Returns interrupt flags (1=occurred), clears all flags on read
**Writing**: Sets/clears interrupt enables
```
Bit 7: (read) IRQ occurred (any source) / (write) 1=enable, 0=disable bits 0-4
Bit 4: FLG  — FLAG pin interrupt
Bit 3: SP   — Serial port interrupt
Bit 2: ALRM — TOD alarm interrupt
Bit 1: TB   — Timer B interrupt
Bit 0: TA   — Timer A interrupt
```

## Timer Calculations

Timer counts down from latch value to 0, then triggers interrupt and reloads.

```
Timer Period = (latch value + 1) / clock_frequency

NTSC: clock = 1,022,727 Hz
PAL:  clock = 985,248 Hz

For 1/60 second at NTSC:
  latch = 1,022,727 / 60 - 1 = 17,044 = $4294

For 1/50 second at PAL:
  latch = 985,248 / 50 - 1 = 19,704 = $4CB8
```

```asm
; Set CIA #1 Timer A for ~1/60 second, continuous IRQ
        LDA #$94        ; $4294 low byte
        STA $DC04
        LDA #$42        ; high byte
        STA $DC05
        LDA #$81        ; enable Timer A IRQ
        STA $DC0D
        LDA #$11        ; start timer, continuous, system clock
        STA $DC0E
```