Random Number Generator

As you could see from my title, I'm trying to get Sonic 1 to generate a random number before loading a level at the title screen or level select. It won't be completely random, I'd like to set parameters (between x number an y number.) but that's the general idea.


I understand something similar was done in RHS's Sonic 2 recreation with the ? ring box, yet I have something very different in mind with what I'd like to do with it.


If I can just get a little hint on how to do this in ASM (Hivebrain's 2005 disasm) it would greatly be appreciated.

 

Sonic 1 already comes with a pseudo-random number generator.


Which is, incidentally, the first result you get if you search for "random" in the asm file.

Code:

; ---------------------------------------------------------------------------

; Subroutine to	generate a pseudo-random number	in d0

; ---------------------------------------------------------------------------



; ||||||||||||||| S U B	R O U T	I N E |||||||||||||||||||||||||||||||||||||||





RandomNumber:

		move.l	($FFFFF636).w,d1

		bne.s	loc_29C0

		move.l	#$2A6D365A,d1



loc_29C0:

		move.l	d1,d0

		asl.l	#2,d1

		add.l	d0,d1

		asl.l	#3,d1

		add.l	d0,d1

		move.w	d1,d0

		swap	d1

		add.w	d1,d0

		move.w	d0,d1

		swap	d1

		move.l	d1,($FFFFF636).w

		rts	

; End of function RandomNumber

To use it, look at how it's called by other game subroutines (just call it with jsr or bsr and d0 will contain a random number). I'm not familiar with it, but it should return a number between 0 and 255 iirc.

 

I believe it can return huge numbers as well (size of longwords). All you do is jsr RandomNumber and then you get a random number returned in d0. To cap it just use the command and.

 

Oh wow. I've been looking at guides to try and write a subroutine myself, and here it is already in the ASM. I feel really noobish right now, thanks nineko.


I think I can take it from here.