//-----------------------------------------------------------
// Simple IRQ Handler Demo
//
// Installs a custom IRQ handler through the kernal vector at
// $0314. The kernal has already saved the registers for us
// before it calls the vector, so our handler does NOT push or
// pull A/X/Y. When we are done we jump into the kernal so it
// can finish the IRQ and RTI properly:
//
//   jmp $ea31  - let the kernal do its full IRQ work
//                (scan keyboard, blink cursor, update jiffy
//                 clock, ...) and then RTI
//   jmp $ea81  - skip the kernal work, just restore the
//                registers the kernal saved and RTI
//
// This handler chains to $ea31 so the machine stays usable.
//-----------------------------------------------------------

#INCLUDE <c64start.c65>
#INCLUDE <c64defs.c65>

GOTO start

WORD CONST IRQ_VECTOR = $0314
WORD handler = @myIRQ          // Address of our IRQ handler

FUNC installIRQ
    ASM
        sei                    // Disable interrupts while we patch
    ENDASM

    POKEW IRQ_VECTOR, handler   // Point the vector at our handler

    ASM
        cli                    // Re-enable interrupts
    ENDASM
FEND


LABEL start
    installIRQ()
    SUBEND //exit back to basic

//-----------------------------------------------------------
// The IRQ handler.
//
// No register saving needed - the kernal did it. Do the work,
// then hand control back to the kernal at $ea31 which restores
// the registers and returns from the interrupt.
//-----------------------------------------------------------
LABEL myIRQ
    ASM
        inc $0400              // Bump the top-left screen character
        jmp $ea31              // Let the kernal finish the IRQ
    ENDASM