61 lines
1.2 KiB
ArmAsm
61 lines
1.2 KiB
ArmAsm
; 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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;=============================
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; random16
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;=============================
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; takes:
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; not 0, cc = 5+ = 27
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; not 0, cs = 5+12+19 = 36
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; $0000 = 5+7+19 = 31
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; $8000 = 5+6+14 = 25
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; $XX00 = 5+6+7+19 = 37
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random16:
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lda SEEDL ; 3
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beq lowZero ; $0000 and $8000 are special values ; 2
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asl SEEDL ; Do a normal shift ; 5
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lda SEEDH ; 3
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rol ; 2
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bcc noEor ; 2
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doEor:
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; high byte is in A
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eor #>XOR_MAGIC ; 2
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sta SEEDH ; 3
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lda SEEDL ; 3
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eor #<XOR_MAGIC ; 2
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sta SEEDL ; 3
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rts ; 6
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lowZero:
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; 1
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lda SEEDH ; 3
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beq doEor ; High byte is also zero ; 3
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; so apply the EOR
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; -1
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; wasn't zero, check for $8000
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asl ; 2
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beq noEor ; if $00 is left after the shift ; 2
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; then it was $80
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bcs doEor ; else, do the EOR based on the carry ; 3
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noEor:
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; 1
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sta SEEDH ; 3
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rts ; 6
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