For comodor 64

screen_color_flash.asm

    processor 6502
    org $1000

init_screen
    lda #$00
    sta $d020
    sta $d021
    lda #$20     ; #$20 is the spacebar Screen Code
clear   
    sta $0400,x  ; fill four areas with 256 spacebar characters
    sta $0500,x 
    sta $0600,x 
    sta $06e8,x 
    inx           ; increment X
    bne clear     ; did X turn to zero yet? if not, continue with the loop
;   rts           ; return from this subroutine

loop:   lda $d012
    cmp #$ff
    bne    loop
    inc $d021
    jmp loop