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cc65/libsrc/common/rand.s
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ArmAsm

;
; Randum number generator
;
; Written and donated by Sidney Cadot - sidney@ch.twi.tudelft.nl
;
; May be distributed with the cc65 runtime using the same license.
;
;
; int rand (void);
; void srand (unsigned seed);
;
; Uses 4-byte state.
; Multiplier must be 1 (mod 4)
; Added value must be 1 (mod 2)
; This guarantees max. period (2**32)
; Bits 8-22 are returned (positive 2-byte int)
; where 0 is LSB, 31 is MSB.
; This is better as lower bits exhibit easily
; detectable patterns.
;
.export _rand, _srand
.bss
rand: .res 4 ; Seed
.code
_rand: clc
lda rand+0 ; SEED *= $01010101
adc rand+1
sta rand+1
adc rand+2
sta rand+2
adc rand+3
sta rand+3
clc
lda rand+0 ; SEED += $31415927
adc #$27
sta rand+0
lda rand+1
adc #$59
sta rand+1
pha
lda rand+2
adc #$41
sta rand+2
and #$7f ; Suppress sign bit (make it positive)
tax
lda rand+3
adc #$31
sta rand+3
pla ; return bit 8-22 in (X,A)
rts
_srand: sta rand+0 ; Store the seed
stx rand+1
lda #0
sta rand+2 ; Set MSW to zero
sta rand+3
rts