Z80 Routines:Math:Random - Revision history

Einar: Added links to Patrik Rak's excellent random number generators

2020-10-05T12:44:51Z

Added links to Patrik Rak's excellent random number generators

John metcalf: Added 16-bit Xorshift

2019-08-02T12:10:22Z

Added 16-bit Xorshift

Zeda: Added two PRNGs and fixed a typo

2018-06-14T00:18:17Z

Added two PRNGs and fixed a typo

Galandros: Routines:Math:Random moved to Z80 Routines:Math:Random: unite z80 routines

2009-10-25T19:48:42Z

Routines:Math:Random moved to Z80 Routines:Math:Random: unite z80 routines

Jim e: Minor correction, Why isn't the table of contents showing up?

2007-03-29T06:37:47Z

Minor correction, Why isn't the table of contents showing up?

Jim e: Added some random routines, some please expand on usage and what not.

2007-03-29T06:29:23Z

Added some random routines, some please expand on usage and what not.

New page

[[Category:Z80 Routines:Math|Random]]
[[Category:Z80 Routines|Random]]

==Ion Random==
This is based off the tried and true [http://en.wikipedia.org/wiki/PRNG pseudorandom number generator] featured in Ion by Joe Wingbermuehle

<nowiki>;-----> 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</nowiki>

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 [http://en.wikipedia.org/wiki/LFSR 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.

<nowiki>;------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 (LFSRSeed),a
ret</nowiki>

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.

← Older revision		Revision as of 12:44, 5 October 2020
Line 203:		Line 203:
	ret		ret
	</nowiki>		</nowiki>
		+
		+	== Related Topics ==
		+	Better random algorithms are available at:
		+	* [https://gist.github.com/raxoft/c074743ea3f926db0037 Patrik Rak's Xor-Shift random number generator]
		+	* [https://gist.github.com/raxoft/2275716fea577b48f7f0 Patrik Rak's CMWC random number generator]

@@ Line 156: / Line 156: @@
      add hl,bc
      ret
 </nowiki>

@@ Line 6: / Line 6: @@
   <nowiki>;-----> Generate a random number
-; ouput a=answer 0<=a<=255
+; output a=answer 0<=a<=255
 ; all registers are preserved except: af
 random:
@@ Line 69: / Line 69: @@
 You can initialize with TI-OS's seeds, stored at seed1 and seed2, both are ti-floats but will serve the purpose.

← Older revision	Revision as of 19:48, 25 October 2009
(No difference)

@@ Line 63: / Line 63: @@
          ld bc,7
          lddr
-         ld (LFSRSeed),a
+         ld (de),a
          ret</nowiki>