Fill screen at $0400 with chequer pattern.

chequer.s

	;; Self-modifying code ($18 bytes in ZP, $1A bytes elsewhere)

a:	lda #$20
	ldy #$18
a_row:	ldx #$28
a_col:	sta $03E8,y
	eor #$80
	dex
	bmi a_row
	iny
	bne a_col
	inc a_col + 2
	asl a_done
	bcc a_col
a_done:	.byte %00010110	; Will be $60 = RTS after 4 asl


	;; As above, but trashing $08xx ($17 bytes in ZP, $19 bytes elsewhere)

b:	lda #$20
        tay
b_row:	ldx #$28
b_col:	sta $03E0,y
	eor #$80
	dex
	bmi b_row
	iny
	bne b_col
	inc b_col + 2
	asl b_done
	bcc b_col
b_done:	.byte %00001011	; Will be $60 = RTS after 5 asl