amiga-bootcamp/01_hardware/common/cia_chips.md

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CIA Chips — 8520 MOS Technology

Overview

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

CIA-A: $BFE001

CIA-A handles:

Bit	Port A (PRA, read $BFE001)
7	/FIR1 — joystick port 1 button
6	/FIR0 — joystick port 0 button
5	/RDY — floppy ready
4	/TK0 — track 0 sensor
3	/WPRO — write-protect
2	/CHNG — disk change
1	/LED — power LED (write: 0=bright)
0	/OVL — Chip RAM overlay (write during boot)

Port B (PRB, $BFE101): Parallel port data lines D0–D7.

CIA-A interrupts appear on CPU IPL level 2 via INTENA bit INTB_EXTER — actually CIA is on IPL 6.

CIA-B: $BFD000

CIA-B handles floppy drive motor/selection and disk DMA sync:

Bit	Port A (PRA, $BFD000)
7	/MTR — motor on/off
6	/SEL3 — drive 3 select
5	/SEL2 — drive 2 select
4	/SEL1 — drive 1 select
3	/SEL0 — drive 0 select
2	/SIDE — head side (0=upper)
1	/DIR — step direction
0	/STEP — step pulse

Port B (PRB, $BFD100): Parallel port shadow (less commonly used on B).

CIA-B interrupts appear on CPU IPL level 6.

Timers

Each CIA has two 16-bit countdown timers (Timer A and Timer B):

• Count from a loaded latch value down to zero
• Can be one-shot or continuous
• Clock sources: system clock (709 kHz PAL / 715 kHz NTSC), or Timer A output (for Timer B)
• Timer A can generate SDR baud rate for serial output

Control Register A (CRA) bits:

bit 0: START  — 1 = timer running
bit 1: PBON   — 1 = timer output on Port B bit 6
bit 2: OUTMODE — 0=pulse, 1=toggle
bit 3: RUNMODE — 0=continuous, 1=one-shot
bit 4: LOAD   — 1 = force load latch into counter
bit 5: INMODE — 0=clock, 1=count rising edges on CNT pin
bit 6: SPMODE — 0=SDR input, 1=SDR output
bit 7: TODIN  — 0=60 Hz TOD, 1=50 Hz TOD (PAL)

Time-of-Day (TOD) Clock

24-bit counter, clocked at 60 Hz (NTSC) or 50 Hz (PAL):

• CIA-A TOD: used by OS as software clock
• TOD registers latch on read of TODHI — must read TODHI first, then TODMID, then TODLO
• ciaa.ciatodhi → ciaa.ciatodmid → ciaa.ciatodlo
• Set by writing TODHI → TODMID → TODLO (halts during write)

Interrupt Control Register (ICR)

Write to enable interrupts, read to see which fired:

/* Enable Timer A interrupt */
ciaa.ciaicr = CIAICRF_SETCLR | CIAICRF_TA;

/* On read: bits indicate which sources fired */
UBYTE icr = ciaa.ciaicr;
if (icr & CIAICRF_TA) { /* Timer A fired */ }
if (icr & CIAICRF_TB) { /* Timer B fired */ }
if (icr & CIAICRF_ALRM) { /* TOD alarm fired */ }
if (icr & CIAICRF_SP) { /* Serial register full/empty */ }
if (icr & CIAICRF_FLG) { /* /FLAG pin (index pulse on CIA-B) */ }

Write bit 7 (CIAICRF_SETCLR): 1 = set enable bits, 0 = clear enable bits.

AmigaOS Timer Device Integration

AmigaOS's timer.device uses CIA timers internally:

• UNIT_MICROHZ — uses CIA-A Timer A for microsecond delays
• UNIT_VBLANK — uses vertical blank interrupt (not CIA)
• UNIT_ECLOCK — uses the E clock (709/715 kHz, same as CIA clock)

Direct CIA programming should be done with ciaa/ciab resource claims via OpenResource("ciaa.resource") — not by poking CIA registers directly.

C Access via NDK Headers

#include <hardware/cia.h>
#include <resources/cia.h>

/* CIA-A is at fixed address */
#define ciaa (*((volatile struct CIA *)0xBFE001))
#define ciab (*((volatile struct CIA *)0xBFD000))

/* struct CIA fields (hardware/cia.h): */
/* ciaa.ciapra, ciaa.ciaprb, ciaa.ciaicr, ciaa.ciacra, ... */

References

• MOS Technology 6526/8520 datasheet
• ADCD 2.1 Hardware Manual — CIA chapter: http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/
• NDK39: hardware/cia.h, resources/cia.h
• Autodocs: cia resource — http://amigadev.elowar.com/read/ADCD_2.1/Includes_and_Autodocs_3._guide/node00C7.html