Difference between revisions of "Z80 Routines:Math:Random"
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Revision as of 12:48, 25 October 2009
Ion Random
This is based off the tried and true pseudorandom number generator featured in Ion by Joe Wingbermuehle
;> Generate a random number ; ouput a=answer 0<=a<=255 ; all registers are preserved except: af random: push hl push de ld hl,(randData) ld a,r ld d,a ld e,(hl) add hl,de add a,l xor h ld (randData),hl pop de pop hl ret
randData here must be a 2 byte seed located in ram. While this is a fast generator, it's generally not considered very good in terms of randomness.
Linear Feedback Shift Register
This particular prng is based on Linear feedback shift register. It uses a 64bit seed and generates 8 new bits at every call. LFSRSeed must be an 8 byte seed located in ram.
;LFSR ;James Montelongo ;optimized by Spencer Putt ;out: ; a = 8 bit random number RandLFSR: ld hl,LFSRSeed+4 ld e,(hl) inc hl ld d,(hl) inc hl ld c,(hl) inc hl ld a,(hl) ld b,a rl e \ rl d rl c \ rla rl e \ rl d rl c \ rla rl e \ rl d rl c \ rla ld h,a rl e \ rl d rl c \ rla xor b rl e \ rl d xor h xor c xor d ld hl,LFSRSeed+6 ld de,LFSRSeed+7 ld bc,7 lddr ld (de),a ret
While this may produces better numbers, it is slower, larger and requires a bigger seed than ionrandom. Assuming theres is a good seed to start, it should generate ~2^56 bytes before repeating. However if there is not a good seed(0 for example), then the numbers created will not be adequate. Unlike Ionrandom and its use of the r register, starting with the same seed the same numbers will be generated. With Ionrandom the code running may have an impact on the number generated. This means this method requires more initialization.
You can initialize with TIOS's seeds, stored at seed1 and seed2, both are tifloats but will serve the purpose.