QuickInts - Standalone Implementation of ObtainQuickVector

An independent implementation of ObtainQuickVector() and ReleaseQuickVector() for AmigaOS, providing functionality that was removed after AmigaOS V39 and adding the ability to release allocated quick interrupt vectors.

What are Zorro III Quick Interrupts?

Standard Amiga interrupts use autovectoring: when hardware asserts an interrupt line, the CPU jumps to a fixed vector based on the interrupt priority level. The OS then traverses a chain of interrupt servers to identify which device caused the interrupt and dispatch the appropriate handler.

Zorro III Quick Interrupts use the 68k's vectored interrupt capability instead. When a Zorro III card asserts INT2 or INT6, it simultaneously provides an 8-bit vector number (68-255) on the data bus. The CPU fetches the handler address directly from the exception vector table and jumps to it immediately, bypassing the interrupt server chain.

Why Quick Interrupts Matter

For hardware developers, quick interrupts provide:

• Lower latency: Direct vector dispatch eliminates interrupt server chain traversal
• Reduced CPU overhead: No need to poll multiple devices to identify the interrupt source
• Deterministic timing: No variable overhead from walking interrupt lists

The performance benefit is most noticeable for small block transfers (10-20% improvement), such as loading icons or directory listings where interrupt overhead is significant relative to transfer time. For large transfers, the advantage diminishes as DMA transfer time dominates.

Background

AmigaOS originally provided ObtainQuickVector() in exec.library to allocate quick interrupt vectors (exception vectors 68-255) for hardware that supports vectored interrupts. However:

• AmigaOS V40+ removed the ObtainQuickVector() implementation entirely
• No release mechanism ever existed - once a vector was obtained, it could never be freed

This implementation solves both problems by providing a complete, OS-independent solution that works on any 68k Amiga system.

How It Works

ObtainQuickVector()

1. Detects BadQuickInt: Scans vectors 68-255 to find the most common value (the "bad quick int" handler). This indicates unused vector slots.

2. Searches for free vector: Starting from vector 255 and working downward, finds a slot containing the BadQuickInt value.

3. Installs handler: Atomically writes the interrupt handler address to the selected vector slot.

4. VBR-aware: Properly handles the Vector Base Register on 68010+ processors.

5. Supervisor mode: All vector table modifications execute in supervisor mode for safety.

ReleaseQuickVector()

1. Validates vector number: Ensures the vector is in the valid range (68-255).

2. Restores original value: Writes the BadQuickInt value back to the vector slot, effectively marking it as unused again.

3. Atomic operation: Executes in supervisor mode with interrupts disabled.

API

ULONG ObtainQuickVector(APTR interruptCode);

Allocates a quick interrupt vector and installs the provided interrupt handler.

• Parameters: interruptCode - Pointer to the interrupt handler routine (must end with RTE)
• Returns: Vector number (68-255) on success, 0 on failure

VOID ReleaseQuickVector(ULONG vectorNum);

Releases a previously allocated quick interrupt vector.

• Parameters: vectorNum - The vector number returned by ObtainQuickVector()

Demo Program

The included demo (main.c) demonstrates usage with an Amiga 4091 SCSI host controller:

1. Locates the A4091 board via expansion.library
2. Creates a quick interrupt handler that:
   • Increments an interrupt counter
   • Flashes the screen color
   • Clears the A4091's interrupt source
3. Allocates a quick interrupt vector
4. Configures the A4091 to use the allocated vector
5. Triggers a DMA abort to generate an interrupt
6. Verifies the interrupt was received
7. Releases the vector

Running the Demo

# Compile (requires m68k-amigaos-gcc cross-compiler)
make

# Run on an Amiga with A4091
quickints

Expected output:

A4091/A4092 card at 0x40000000
Installing quick interrupt handler @0x070397d6
Installed at vector 255. qcount=0
Informed A4091 of quick interrupt vector 255.
Reset 53C710...done.
Clearing pending DMA.
Enabling DMA interrupt on Abort.
DIEN register: 0x10 (expected: 0x10)
Triggering Abort.
Waiting for Quick Interrupts (2s).
After trigger: qcount=1 (delta=1)
OK: quick interrupt fired.
Releasing quick interrupt vector 255...
Released. Final qcount=1

Building

Requirements:

• m68k-amigaos-gcc cross-compiler
• AmigaOS NDK headers
• clib2 runtime library

make clean
make

Writing an Interrupt Handler

Quick interrupt handlers must:

• Be naked assembly routines (no C function prologue/epilogue)
• Save and restore all used registers
• End with RTE instruction
• Clear the hardware interrupt source
• Execute as quickly as possible

Example:

asm volatile(
    ".globl _MyHandler         \n"
    "_MyHandler:               \n"
    "    movem.l d0/a0,-(sp)   \n"  // Save registers
    "    ... handler code ...  \n"  // Do work
    "    movem.l (sp)+,d0/a0   \n"  // Restore registers
    "    rte                   \n"  // Return from exception
);

Limitations

• Requires an Amiga capable of Zorro III Quick Interrupts (A3000/A4000)
• Only works with user vectors (68-255)
• Requires supervisor mode access
• The first call scans all vectors to find BadQuickInt (minor overhead)
• No protection against allocating the same vector twice from different code

License

This code is provided for educational and development purposes for the Amiga community.

References

• AmigaOS exec.library AutoDocs (V39)
• Amiga Hardware Reference Manual
• Zorro III Specification
• NCR 53C710 SCSI Controller Datasheet