amiga-bootcamp/06_exec_os/interrupts.md

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Interrupts — Levels, INTENA, AddIntServer, CIA Interrupts

Overview

AmigaOS supports 7 hardware interrupt levels (68k IPL0–IPL6) plus a software interrupt mechanism. Custom chip interrupts are filtered through the INTENA / INTREQ registers; CIA-generated interrupts arrive on level 2 (CIA-A) and level 6 (CIA-B).

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Interrupt Priority Levels

IPL	Source	AmigaOS Use
1	TBE, DSKBLK, SOFTINT	Software interrupts (SoftInt)
2	PORTS (CIA-A)	Keyboard, timer, parallel, floppy motor
3	COPER, VERTB, BLIT	Copper, vertical blank, blitter
4	AUD0–AUD3	Audio DMA completion
5	RBF, DSKSYNC	Serial receive, disk sync
6	EXTER (CIA-B)	External interrupts, CIA-B timers, TOD
7	NMI	Non-maskable (unused on stock Amiga)

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Custom Chip Interrupt Registers

Register	Address	Description
INTENAR	$DFF01C	Interrupt enable status (read)
INTENA	$DFF09A	Interrupt enable set/clear (write)
INTREQR	$DFF01E	Interrupt request status (read)
INTREQ	$DFF09C	Interrupt request clear/set (write)

INTENA / INTREQ Bit Map

Bit	Constant	Source
0	INTF_TBE	Serial transmit buffer empty
1	INTF_DSKBLK	Disk DMA block complete
2	INTF_SOFTINT	Software interrupt
3	INTF_PORTS	CIA-A interrupt (level 2)
4	INTF_COPER	Copper interrupt
5	INTF_VERTB	Vertical blank
6	INTF_BLIT	Blitter interrupt
7	INTF_AUD0	Audio channel 0
8	INTF_AUD1	Audio channel 1
9	INTF_AUD2	Audio channel 2
10	INTF_AUD3	Audio channel 3
11	INTF_RBF	Serial receive buffer full
12	INTF_DSKSYNC	Disk sync word match
13	INTF_EXTER	CIA-B / external interrupt
14	INTF_INTEN	Master interrupt enable bit

To enable vertical blank: write $C005 to INTENA (bit 14 set = enable, bit 5 = VERTB). To clear vertical blank request: write $0020 to INTREQ.

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Adding an Interrupt Server

struct Interrupt myVBL = {
    { NULL, NULL, NT_INTERRUPT, 0, "My VBL" },
    NULL,        /* is_Data (passed to handler) */
    myVBLHandler /* is_Code */
};

AddIntServer(INTB_VERTB, &myVBL);  /* INTB_VERTB = 5 */
/* ... run ... */
RemIntServer(INTB_VERTB, &myVBL);  /* always remove before exit */

Interrupt Handler Rules

• Handler is called at interrupt level — no OS calls that Wait()
• D0–D1, A0–A1 may be trashed; all others preserved
• Return D0 = 0 if you did not handle it (pass to next server)
• Return D0 ≠ 0 if you handled it (stop server chain)

Handler signature:

myVBLHandler:
    ; A1 = is_Data pointer (from struct Interrupt)
    ; do fast work only
    MOVEQ  #1, D0      ; handled — stop chain
    RTS

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CIA Interrupts

CIA-A (at $BFEC01) generates level 2 interrupts. CIA-B (at $BFD000) generates level 6. Each CIA has an ICR (Interrupt Control Register) with 5 sources:

Bit	Source
0	Timer A underflow
1	Timer B underflow
2	TOD alarm
3	Serial register full
4	Flag pin / FLG

CIA interrupts are serviced via AddIntServer on INTB_PORTS (level 2, CIA-A) or INTB_EXTER (level 6, CIA-B).

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Disable / Enable vs Forbid / Permit

Function	Effect	Scope
Forbid()	Disables task switching	Task-level (interrupts still run)
Permit()	Re-enables task switching	Reverses Forbid()
Disable()	Masks all hardware interrupts	Hardware + task switching
Enable()	Unmasks hardware interrupts	Reverses Disable()

Caution

Disable() / Enable() can be held for only a few microseconds — never do I/O or complex operations inside a Disable() section.

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References

• NDK39: hardware/intbits.h, hardware/cia.h
• ADCD 2.1: AddIntServer, RemIntServer, SetIntVector, Disable, Forbid
• cia_chips.md — CIA timer and ICR details
• custom_registers.md — INTENA/INTREQ register listing