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46 lines
793 B
ArmAsm
46 lines
793 B
ArmAsm
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; 16-bit 6502 Random Number Generator
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; Linear feedback shift register PRNG by White Flame
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; http://codebase64.org/doku.php?id=base:small_fast_16-bit_prng
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; The Apple II KEYIN routine increments this field
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; while waiting for keypress
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SEEDL = $4E
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SEEDH = $4F
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XOR_MAGIC = $7657 ; "vW"
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random16:
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lda SEEDL
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beq lowZero ; $0000 and $8000 are special values // ; Do a normal shift
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asl SEEDL
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lda SEEDH
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rol
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bcc noEor
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doEor:
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; high byte is in .A
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eor #>XOR_MAGIC
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sta SEEDH
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lda SEEDL
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eor #<XOR_MAGIC
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sta SEEDL
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rts
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lowZero:
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lda SEEDH
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beq doEor ; High byte is also zero, so apply the EOR
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; wasn't zero, check for $8000
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asl
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beq noEor ; if $00 is left after the shift
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; then it was $80
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bcs doEor ; else, do the EOR based on the carry bit
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noEor:
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sta SEEDH
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rts
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