**Xorshift** is a simple, fast pseudorandom number generator developed by George Marsaglia. The generator combines three xorshift operations where a number is exclusive-ored with a shifted copy of itself:

```
/* 16-bit xorshift PRNG */
unsigned xs = 1;
unsigned xorshift( )
{
xs ^= xs << 7;
xs ^= xs >> 9;
xs ^= xs << 8;
return xs;
}
```

There are 60 shift triplets with the maximum period 2^{16}-1. Four triplets pass a series of lightweight randomness tests including randomly plotting various *n* × *n* matrices using the high bits, low bits, reversed bits, etc. These are: 6, 7, 13; 7, 9, 8; 7, 9, 13; 9, 7, 13.

7, 9, 8 is the most efficient when implemented in Z80, generating a number in 86 cycles. For comparison the example in C takes approx ~1200 cycles when compiled with HiSoft C v1.3.

```
; 16-bit xorshift pseudorandom number generator
; 20 bytes, 86 cycles (excluding ret)
; returns hl = pseudorandom number
; corrupts a
xrnd:
ld hl,1 ; seed must not be 0
ld a,h
rra
ld a,l
rra
xor h
ld h,a
ld a,l
rra
ld a,h
rra
xor l
ld l,a
xor h
ld h,a
ld (xrnd+1),hl
ret
```

Oh my gosh, my new favorite site! I tried implementing this a while back with no luck. I did however find that combining a simple 16-bit LFSR and a 16-bit LCG works well. It's not as fast (148cc), but it does pass CACert labs' testing. Not sure how to post code boxes, but:

ReplyDeleteprng16:

;collab with Runer112

;;Output:

;; HL is a pseudo-random int

;; A and BC are also, but much weaker and smaller cycles

;; Preserves DE

;;148cc, super fast

;;26 bytes

;;period length: 4,294,901,760

seed1=$+1

ld hl,9999

ld b,h

ld c,l

add hl,hl

add hl,hl

inc l

add hl,bc

ld (seed1),hl

seed2=$+1

ld hl,987

add hl,hl

sbc a,a

and 101101

xor l

ld l,a

ld (seed2),hl

add hl,bc

ret

Great stuff! I ported this to C64, 30 cycles without the RTS. I didn't need what is equivalent to the second lda a,l / rra because 6502 EOR does not touch carry:

ReplyDeleterng_zp_low = $02

rng_zp_high = $03

; seeding

LDA #1 ; seed, can be anything except 0

STA rng_zp_low

LDA #0

STA rng_zp_high

...

random

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

RTS

Thanks SO much for this. This is AWESOME!

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