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90 lines
2.1 KiB
Plaintext
90 lines
2.1 KiB
Plaintext
;license:MIT
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;(c) 2017-2020 by qkumba/4am/John Brooks
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;
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!cpu 6502
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!to "build/FX.INDEXED/DHGR.FIZZLE",plain
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*=$6000
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addrs=$C0 ; [$40 bytes]
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ldx #$1F ; build address lookup table
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- txa
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eor #$20
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sta addrs, x
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eor #$A0
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sta addrs+$20, x
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dex
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bpl -
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ldx #(end-start) ; copy LFSR code to zero page
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- lda start-1, x
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sta $FF, x
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dex
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bne -
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jmp copyaux
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start
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!pseudopc 0 {
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copyaux sta $C003 ; copy $4000/aux to $8000/main
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ldx #$20
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ldy #$00
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a lda $4000, y
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b sta $8000, y
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iny
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bne a
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inc a+2
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inc b+2
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dex
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bne a
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sta $C002
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sta $C001 ; 80STORE mode
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; X,Y=0 on entry to LFSR
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; in: X,Y=0
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loop txa
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loop1 eor #$35 ; LFSR form 0x3500 with period 16383
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tax
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loop2 lda addrs, x
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bmi aux
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sta $C054 ; switch $2000 access to main memory
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sta <dst+2
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eor #$60
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sta <src+2
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src lda $FD00, y
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dst sta $FD00, y
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txa
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lsr
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tax
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tya
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ror
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tay
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bcc loop2
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bne loop
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bit $C000
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bmi exit
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txa
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bne loop1
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exit lda $4000 ; last lousy byte (because LFSR never hits 0)
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sta $2000
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sta $C000 ; 80STORE mode off
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rts
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aux sta $C055 ; switch $2000 access to aux memory (read/write!)
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sta <auxsrc+2
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eor #$A0
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sta <auxdst+2
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auxsrc lda $FD00, y
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auxdst sta $FD00, y
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txa
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lsr
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tax
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tya
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ror
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tay
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bcc loop2
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bne loop
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lda $C000
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bmi exit
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txa
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bne loop1
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beq exit
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}
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end
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