The Amiga uses **two MOS 8520 CIA** (Complex Interface Adapter) chips, providing timers, parallel/serial I/O ports, a time-of-day clock, and interrupt generation. They are the primary source of hardware timing outside the vertical blank and audio DMA.
- **CIA-A** at `$BFE001` (even byte addresses)
- **CIA-B** at `$BFD000` (odd byte addresses)
Both CIAs are accessed via byte reads/writes; the 68000 byte-lane placement means CIA-A uses even offsets and CIA-B uses odd offsets on the 16-bit bus.
## Register Map
Each CIA has 16 registers, spaced 256 bytes apart in the Amiga address space:
| Offset | Register | CIA-A Function | CIA-B Function |
|---|---|---|---|
| $000 | PRA | Parallel port data (input) | Disk control outputs |
| $100 | PRB | Parallel port data (output) | Disk status inputs |
| $200 | DDRA | Port A direction (1=output) | Port A direction |
| $300 | DDRB | Port B direction | Port B direction |
| $400 | TALO | Timer A low byte | Timer A low byte |
| $500 | TAHI | Timer A high byte | Timer A high byte |
| $600 | TBLO | Timer B low byte | Timer B low byte |
| $700 | TBHI | Timer B high byte | Timer B high byte |
| $800 | TODLO | TOD clock low (1/60 s) | Disk position (latched) |
| $900 | TODMID | TOD clock mid | |
| $A00 | TODHI | TOD clock high | |
| $B00 | (unused) | — | — |
| $C00 | SDR | Serial data register | Serial data register |
| $D00 | ICR | Interrupt control | Interrupt control |
| $E00 | CRA | Control register A | Control register A |
| $F00 | CRB | Control register B | Control register B |
> On CDTV and CD32, the CIA-B floppy control bits are electrically disconnected from any drive hardware. Writing to them has no effect. If an external floppy is connected (CDTV with external drive, or CD32 with SX-1), only `/SEL0` and related signals are active.
### CD32 Gamepad — CIA Shift Register Protocol
The CD32 gamepad uses CIA-A's **Serial Data Register (SDR)** for button reads. The controller contains a shift register that is clocked via the joystick port pin 5:
```asm
; Read CD32 gamepad buttons
; Returns 7 button bits in d0
move.b #$FF, $BFE301 ; CIA-A DDRA: all outputs (temporarily)
bset #6, $BFE001 ; Pin 5 high (clock start)
bclr #6, $BFE001 ; Pin 5 low → latch button state
; Clock in 7 bits via pin toggling
moveq #6, d1
.read_pad:
bset #6, $BFE001 ; clock pulse high
btst #0, $BFE001 ; read data bit
bclr #6, $BFE001 ; clock pulse low
roxl.b #1, d0 ; shift into result
dbf d1, .read_pad
```
Standard 2-button Atari joysticks ignore the clock signal and remain fully compatible.
- See also: [Kickstart Boot Diagnostics](../../02_boot_sequence/kickstart-boot-diagnostics.md) — CIA-A LED blink pattern (coldCrash handler), overlay bit in PRA
