c64bgbars.s

  * = $1000
;;; disable interrupts
LDA #%01111111
STA $DC0D

;;;  disable text screen
LDA #11
STA $D011

newline:
  NOP                           ; 2 cycles
  NOP

  LDX #2                        ; 2 cycles, load in red
  STX $D020                     ; 4 cycles, write to graphics mem

  INC $D020                     ; 6 cycles, next colour
  INC $D020
  INC $D020
  INC $D020
  INC $D020
  INC $D020

  LDX #0                        ; 2 cycles, back to black
  STX $D020



  LDA $d012                     ; 4 cycles, load current line into A

waitforline:                    ; 9 cycles when looping, 7 when hit
  CMP $d012                     ; 4 cycles
  BNE newline                   ; 2 cycles when missed
  JMP waitforline               ; 3 cycles