GSCats/random.s

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;
; random.s
; GSCats
;
; Created by Quinn Dunki on 12/29/19.
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; random
; A => Random 8 bits
; Y => Less random 8 bits
;
; Taken from https://codebase64.org/doku.php?id=base:16bit_xorshift_random_generator
; This routine is not very fast, but it'll do for now. It's intended
; for 6502 so we're making due with it.
;
rng_zp_low = RANDOML
rng_zp_high = RANDOMH
random:
SAVE_AXY
EMULATION
LDA rng_zp_high
LSR
LDA rng_zp_low
ROR
EOR rng_zp_high
STA rng_zp_high ; high part of x ^= x << 7 done
ROR ; A has now x >> 9 and high bit comes from low byte
EOR rng_zp_low
STA rng_zp_low ; x ^= x >> 9 and the low part of x ^= x << 7 done
EOR rng_zp_high
STA rng_zp_high ; x ^= x << 8 done
NATIVE
RESTORE_AXY
RTS
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; seedRandom
; A = Seed
;
; Taken from https://codebase64.org/doku.php?id=base:16bit_xorshift_random_generator
;
seedRandom:
SAVE_AXY
EMULATION
STA rng_zp_low
LDA #0
STA rng_zp_high
NATIVE
RESTORE_AXY
rts