C64-dev/reference/cia-registers.md

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)

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)

' 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

; 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