# Z80 Routines:Math:Random

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## Ion Random

This is based off the tried and true pseudorandom number generator featured in Ion by Joe Wingbermuehle

```;-----> Generate a random number
; all registers are preserved except: af
random:
push    hl
push    de
ld      hl,(randData)
ld      a,r
ld      d,a
ld      e,(hl)
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 TI-OS's seeds, stored at seed1 and seed2, both are ti-floats but will serve the purpose.