// A minimal working raster IRQ
  // Commodore 64 PRG executable file
.file [name="irq-raster.prg", type="prg", segments="Program"]
.segmentdef Program [segments="Basic, Code, Data"]
.segmentdef Basic [start=$0801]
.segmentdef Code [start=$80d]
.segmentdef Data [startAfter="Code"]
.segment Basic
:BasicUpstart(main)
  .const IRQ_RASTER = 1
  .const WHITE = 1
  .const BLACK = 0
  .const CIA_INTERRUPT_CLEAR = $7f
  .label KERNEL_IRQ = $314
  .label RASTER = $d012
  .label VICII_CONTROL1 = $d011
  .label IRQ_STATUS = $d019
  .label IRQ_ENABLE = $d01a
  .label BG_COLOR = $d020
  .label CIA1_INTERRUPT = $dc0d
.segment Code
// Interrupt Routine
irq: {
    // *BG_COLOR = WHITE
    lda #WHITE
    sta BG_COLOR
    // *BG_COLOR = BLACK
    lda #BLACK
    sta BG_COLOR
    // *IRQ_STATUS = IRQ_RASTER
    // Acknowledge the IRQ
    lda #IRQ_RASTER
    sta IRQ_STATUS
    // }
    jmp $ea81
}
main: {
    // asm
    sei
    // *CIA1_INTERRUPT = CIA_INTERRUPT_CLEAR
    // Disable CIA 1 Timer IRQ
    lda #CIA_INTERRUPT_CLEAR
    sta CIA1_INTERRUPT
    // *VICII_CONTROL1 |=$80
    // Set raster line to $100
    lda #$80
    ora VICII_CONTROL1
    sta VICII_CONTROL1
    // *RASTER = $00
    lda #0
    sta RASTER
    // *IRQ_ENABLE = IRQ_RASTER
    // Enable Raster Interrupt
    lda #IRQ_RASTER
    sta IRQ_ENABLE
    // *KERNEL_IRQ = &irq
    // Set the IRQ routine
    lda #<irq
    sta KERNEL_IRQ
    lda #>irq
    sta KERNEL_IRQ+1
    // asm
    cli
    // }
    rts
}