C64_SKILLS/examples/assembly/irq-hook-skeleton.asm

; irq-hook-skeleton.asm
; C64 IRQ hook skeleton. Educational template only.
; Installs handler at $C100 and changes border color briefly.
; Assemble/load with install at $C000. Run SYS 49152.

        * = $c000
oldirq: .word 0

install:
        sei
        lda $0314
        sta oldirq
        lda $0315
        sta oldirq+1
        lda #<irq
        sta $0314
        lda #>irq
        sta $0315
        cli
        rts

restore:
        sei
        lda oldirq
        sta $0314
        lda oldirq+1
        sta $0315
        cli
        rts

        * = $c100
irq:
        pha
        txa
        pha
        tya
        pha

        inc $d020      ; visible heartbeat; keep handler short

        pla
        tay
        pla
        tax
        pla
        jmp (oldirq)   ; chain to original IRQ handler