mirror of
https://github.com/cc65/cc65.git
synced 2024-12-24 11:31:31 +00:00
67 lines
1.5 KiB
ArmAsm
67 lines
1.5 KiB
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
|
|
|
|
.data
|
|
|
|
; The seed. When srand() is not called, the C standard says that that rand()
|
|
; should behave as if srand() was called with an argument of 1 before.
|
|
rand: .dword 1
|
|
|
|
.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
|
|
|
|
|