16-Bit Xorshift Pseudorandom Numbers in Z80 Assembly

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 2¹⁶-1. 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