Generating "Random" numbers.

[Wickeycolumbus]

I was wondering, what is a good, efficient way to generate "random" numbers in assembly?

Edited April 19, 2008 by Wickeycolumbus

[+karri]

I was wondering, what is a good, efficient way to generate "random" numbers in assembly?

 

From the old cc65 archives this is my favourite routine for generating starfields.

* RANDOM.M65
*
* Offered by Rob Nicholson.
* It works great _and_ is fast !
* ported :) by 42BS
*
* C: int random()
* Added routine to set the seed to be able to create pseudorandom
* sequences by Karri Kaksonen
* C: void srandom(int seed)

			xref tstax
			xref popax


			global _random
			global _srandom
_srandom::
			jsr popax
			sta random_seed
			stx random_seed+1
			eor #$55
			sta random_seed+2
			rts
_random::
			lda random_seed+1			  ; havn't a clue what this does (
me neither 42BS;-)
			tax
			eor #$80
			rol A
			rol random_seed+2
			ror random_seed+1
			lda random_seed
			sta random_seed+1

			ror A
			eor random_seed+2
			stx random_seed+2
			sta random_seed
;
;
; 16-Bit random in AX
;
			pha
			txa
			lsr A
			tax
			pla
			ror A
;
; shift right once => unsigned random-number
;
			jmp tstax	  ; set cc

random_seed	 dc.w 0
last_random	 dc.b 35

 

This is the best implementation I have seen for practical game programming.

 

--

Karri

[vdub_bobby]

Do a search, there are several threads here on AA devoted to that very subject.

 

You want a linear feedback shift register (LFSR). Do a search.

 

I will say that for the 2600 anything more than an 8-bit random number is almost always overkill and a waste of your precious RAM.

Edited April 21, 2008 by vdub_bobby