Barber Pole Border C-64 assembly

; Commodore 64 IRQ Moving Raster Bars
; Target address: $C000 (SYS 49152 to start)

        org $c000

init_irq:
        sei             ; Disable interrupts while making changes

        ; Turn off standard CIA timer interrupts (stops cursor flash/keyboard scan from breaking timing)
        lda #$7f
        sta $dc0d
        sta $dd0d
        lda $dc0d       ; Acknowledge any pending CIA interrupts

        ; Enable VIC-II Raster Interrupts
        lda #$01
        sta $d01a

        ; Set the first raster line where our IRQ should trigger
        lda bar_y
        sta $d012
        lda $d011       ; Clear high bit of raster line (since lines go above 255)
        and #$7f
        sta $d011

        ; Point the C64's RAM interrupt vector ($0314/$0315) to our custom routine
        lda #<irq_handler
        sta $0314
        lda #>irq_handler
        sta $0315

        cli             ; Re-enable interrupts
        rts             ; Return to BASIC. The effect now runs purely in the background!

; ==========================================
; CUSTOM INTERRUPT HANDLER
; ==========================================
irq_handler:
        ; 1. Acknowledge the VIC-II interrupt flag so it can trigger again next frame
        lsr $d019

        ; 2. Draw the colored bar gradient line-by-line
        ldx #0          ; Index for our color table
color_loop:
        lda color_table,x
        sta $d020       ; Write to border color
        sta $d021       ; Write to background color

        ; Inline delay to let a color stretch across a few lines
        ldy #$0a
line_delay:
        dey
        bne line_delay

        inx
        cpx #32         ; Have we painted all 32 lines?
        bne color_loop

        ; 3. Reset colors back to black
        lda #0
        sta $d020
        sta $d021

        ; 4. Update the bar position for the next frame (Animation)
        inc frame_count
        lda frame_count
        cmp #2          ; Speed control (Higher = Slower)
        bne finish_irq
        lda #0
        sta frame_count

        inc bar_y       ; Move DOWN. (Change to 'dec bar_y' to move UP)
        lda bar_y
        cmp #$b0        ; Wrap around bottom boundary
        bne finish_irq
        lda #$30        ; Reset to top boundary
        sta bar_y

finish_irq:
        ; 5. Update the next IRQ trigger line to match the new bar position
        lda bar_y
        sta $d012

        ; 6. Exit the interrupt routine cleanly via standard KERNAL exit
        jmp $ea31

; ==========================================
; VARIABLES & DATA TABLES
; ==========================================
bar_y:       byte $30   ; Dynamic starting line for the raster interrupt
frame_count: byte 0

; 32-Byte symmetric color gradient (Dark Blue -> Light Blue -> White -> Dark Blue)
color_table:
        byte $06, $06, $06, $06, $0e, $0e, $0e, $0e
        byte $03, $03, $03, $03, $01, $01, $01, $01
        byte $01, $01, $01, $01, $03, $03, $03, $03
        byte $0e, $0e, $0e, $0e, $06, $06, $06, $06