; =============================================================
; irq-handler.asm — Custom IRQ Handler Skeleton for C64
; Demonstrates: raster IRQ, custom border color flash
; Assemble to $C000. Run with SYS 49152 to install, SYS 49166 to remove.
; =============================================================

        * = $C000

; ---------------------------------------------------------------
; INSTALL — Patches IRQ vector to our handler
; ---------------------------------------------------------------
INSTALL SEI                 ; disable interrupts while patching

        ; Save original IRQ vector
        LDA $0314
        STA OLDIRQ
        LDA $0315
        STA OLDIRQ+1

        ; Install our handler
        LDA #<RASTER_IRQ
        STA $0314
        LDA #>RASTER_IRQ
        STA $0315

        ; Set up VIC-II raster interrupt at line 100
        LDA $D011
        AND #$7F            ; clear raster bit 8
        STA $D011
        LDA #100            ; trigger at raster line 100
        STA $D012

        ; Enable VIC-II raster IRQ, keep CIA #1 timer IRQ enabled too
        LDA #$01
        STA $D01A           ; VIC IRQ mask: raster

        CLI                 ; re-enable interrupts
        RTS

; ---------------------------------------------------------------
; REMOVE — Restores original IRQ vector
; ---------------------------------------------------------------
REMOVE  SEI
        LDA OLDIRQ
        STA $0314
        LDA OLDIRQ+1
        STA $0315
        LDA #$00
        STA $D01A           ; disable VIC IRQs
        CLI
        RTS

; ---------------------------------------------------------------
; RASTER_IRQ — Our custom interrupt handler
; Called when VIC-II raster reaches line 100
; ---------------------------------------------------------------
RASTER_IRQ:
        ; Acknowledge VIC-II interrupt (write-back clears flags)
        LDA $D019           ; read VIC interrupt status
        STA $D019           ; acknowledge by writing back

        ; Check if this is really a raster interrupt
        AND #$01            ; bit 0 = raster IRQ
        BEQ NOT_RASTER

        ; --- RASTER EFFECT: Flash border color ---
        INC COLORIDX
        LDA COLORIDX
        AND #$0F            ; keep in 0-15 range
        STA COLORIDX
        STA $D020           ; write border color

        ; Schedule next IRQ at line 200
        LDA $D012
        CMP #200
        BEQ FLIP_RASTER
        LDA #200
        STA $D012
        JMP DONE_IRQ
FLIP_RASTER:
        LDA #100
        STA $D012

DONE_IRQ:
NOT_RASTER:
        ; Chain to original IRQ for keyboard/timer/jiffy updates
        JMP (OLDIRQ)

; ---------------------------------------------------------------
; Data
; ---------------------------------------------------------------
OLDIRQ  .WORD $EA31         ; default IRQ handler address
COLORIDX .BYTE 0

; ---------------------------------------------------------------
; End of code
; ---------------------------------------------------------------