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; 8-bit Xor-Shift random number generator. | |
; Created by Patrik Rak in 2008 and revised in 2011/2012. | |
; See http://www.worldofspectrum.org/forums/showthread.php?t=23070 | |
org 40000 | |
call rnd ; BASIC driver | |
ld c,a | |
ld b,0 | |
ret | |
rnd ld hl,0xA280 ; yw -> zt | |
ld de,0xC0DE ; xz -> yw | |
ld (rnd+4),hl ; x = y, z = w | |
ld a,l ; w = w ^ ( w << 3 ) | |
add a,a | |
add a,a | |
add a,a | |
xor l | |
ld l,a | |
ld a,d ; t = x ^ (x << 1) | |
add a,a | |
xor d | |
ld h,a | |
rra ; t = t ^ (t >> 1) ^ w | |
xor h | |
xor l | |
ld h,e ; y = z | |
ld l,a ; w = t | |
ld (rnd+1),hl | |
ret |
Yes, and looks like the code is correct, too, AFAICT, but I think it can be optimized further... Essentially, in xorshift, you are having n words, in our case 4 bytes, xyzw, but only the first and last (x and w) are used in the computation, the rest is merely moved/shifted (y to x, z to y, and w to z). So I think the last store of the C register which you do at the end could be actually moved to the opening sequence, as you really need just to store the result from A to w at the end (the result also serves as the feedback for the next round). This would have the advantage that the result remains in A once the routine returns, which was the idea in the first place (puting it to BC is done just for Sinclair Basic).
Thank you very much for taking the time to check this! I think I get what you mean about the last store of the C register and I think I now understand this code enough that I've been able to optimise it accordingly by loading out of RAM specifically, rather than using HLI/HLD commands:
rnd:
; ld bc,(seed) ; xz -> yw
; ld de,(seed+2) ; yw -> zt
; ld (seed),de ; x = y, z = w
ld a,[random+1]
ld b,a ; b = random+1
ld a,[random+3]
ld [random+1],a ; random+3 -> random+1
ld a,[random]
ld [random+3],a ; random -> random+3
ld a,[random+2]
ld [random],a ; random+2 -> random
ld e,a ; e = random+2
; ld a,e ; w = w ^ ( w << 3 )
add a,a
add a,a
add a,a
xor e
ld e,a
ld a,b ; t = x ^ (x << 1)
add a,a
xor b
ld d,a
rra ; t = t ^ (t >> 1) ^ w
xor d
xor e
; ld b,c ; y = z
; ld c,a ; w = t
; ld (seed+2),bc
ld [random+2],a
; ld b,0
ret
The complication that I didn't mention before is that I am trying to improve the random number generator in a hack of an existing game and calling the function involves bank switching, so the result in the A register is lost by the time the code returns to the point that it called for a random number in the first place anyway. I suppose I could possibly move the function, but I am opting for retrieving the result for now. This does mean spending another byte of code on the retrieval process, but this revision is six bytes less than the original adaption (bringing the total to twenty-four bytes once the random value has been retrieved).
Thanks for sharing this! I am also hoping to adapt this code for use in a Game Boy game. I am relatively new to programming and was wondering whether you would be willing to proofread my attempt to adapt your code for gbZ80. It is based on the 16th post of the World of Spectrum thread that you linked:
My understanding is that the label "random" should point to a 4-byte range and that at least one of these bytes should be initialised to a non-zero value before calling the function for the first time. It will then generate a random 8-bit number which I can retrieve from random+2 after the function has returned. Is this correct?