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8bitworkshop/presets/apple2/hgrtest.a

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processor 6502
seg program
org $803
.start
sta $c050 ; set graphics
sta $c052 ; no mixed mode
sta $c057 ; set hires
ldx #0
.loop inc $2000,x ; increment hgr byte
inx
bne .loop
lda .loop+2 ; increment hi byte of instruction
clc
adc #1
sta .loop+2 ; self-modifying code
cmp #$40
bne .loop
lda #$20 ; reset to $2000
sta .loop+2
jmp .start