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107 lines
2.0 KiB
Plaintext
107 lines
2.0 KiB
Plaintext
;license:MIT
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;(c) 2017-2019 by qkumba and 4am
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!cpu 6502
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!to "build/FX/CHECKER.FIZZLE",plain
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*=$6000
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copymask_even = $f0
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copymask_odd = $f1
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counter = $f2
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!source "src/fx/macros.a"
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;init masks
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lda #%10000000
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sta copymask_even
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lda #%11111111
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sta copymask_odd
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lda #2
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sta counter
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@outerloop
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;init RNG
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ldx #1
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stx @rnd1+1
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dex
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stx @rnd2+1
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;iterate
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@loop
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ldy @rnd1+1
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ldx @rnd2+1
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lsr @rnd2+1
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ror @rnd1+1
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bcc +
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;feedback polynomial forms #$100D for period of 8191
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lda @rnd1+1
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eor #$0d
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sta @rnd1+1
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lda @rnd2+1
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eor #$10
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sta @rnd2+1
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;little hack to avoid missing offset zero
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;screen hole at $xxFF is missed instead
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+ tya
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eor #$ff
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sta $26
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sta $3c
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txa
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;target page 1
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ora #$20
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sta $27
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eor #$60
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sta $3d
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;copy pixel from other page to this page
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ldy #0
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lda $26
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and #1
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sta $00
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lda $26
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rol
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rol
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and #1
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eor $00
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bne @odd
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lda ($26),y
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eor ($3c),y
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and copymask_even
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eor ($26),y
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sta ($26),y
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jmp @next
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@odd
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lda ($26),y
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eor ($3c),y
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and copymask_odd
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eor ($26),y
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sta ($26),y
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@next
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lda $c000
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bmi @exit
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;and exit condition
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@rnd2 lda #0
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bne @loop
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@rnd1 lda #0
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cmp #1
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bne @loop
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lda #$FF
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sta copymask_even
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sta copymask_odd
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dec counter
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+LBNE @outerloop
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@exit rts
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