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67 lines
1.3 KiB
ArmAsm
67 lines
1.3 KiB
ArmAsm
;
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; Randum number generator
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;
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; Written and donated by Sidney Cadot - sidney@ch.twi.tudelft.nl
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;
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; May be distributed with the cc65 runtime using the same license.
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;
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;
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; int rand (void);
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; void srand (unsigned seed);
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;
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; Uses 4-byte state.
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; Multiplier must be 1 (mod 4)
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; Added value must be 1 (mod 2)
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; This guarantees max. period (2**32)
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; Bits 8-22 are returned (positive 2-byte int)
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; where 0 is LSB, 31 is MSB.
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; This is better as lower bits exhibit easily
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; detectable patterns.
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;
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.export _rand, _srand
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.data
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; The seed. When srand() is not called, the C standard says that that rand()
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; should behave as if srand() was called with an argument of 1 before.
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rand: .dword 1
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.code
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_rand: clc
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lda rand+0 ; SEED *= $01010101
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adc rand+1
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sta rand+1
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adc rand+2
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sta rand+2
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adc rand+3
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sta rand+3
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clc
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lda rand+0 ; SEED += $31415927
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adc #$27
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sta rand+0
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lda rand+1
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adc #$59
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sta rand+1
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pha
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lda rand+2
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adc #$41
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sta rand+2
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and #$7f ; Suppress sign bit (make it positive)
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tax
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lda rand+3
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adc #$31
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sta rand+3
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pla ; return bit 8-22 in (X,A)
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rts
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_srand: sta rand+0 ; Store the seed
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stx rand+1
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lda #0
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sta rand+2 ; Set MSW to zero
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sta rand+3
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rts
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