Re: A86: Random Number stuff

To: assembly-86@lists.ticalc.org
Subject: Re: A86: Random Number stuff
From: JayEll64@aol.com
Date: Mon, 10 Jan 2000 01:06:18 EST
Reply-To: assembly-86@lists.ticalc.org

In a message dated 1/9/00 7:39:17 PM Mountain Standard Time, MikeT843@aol.com 
writes:

> I got this off guide.iwarp.com
>  create a random number from 0 to 15
>    ld a,r            
>    and %00001111 ;a is now <16
>  
>  you could modify it so that it is within any range you want and its a lot 
>  simpler than the one used it tetris however the r register is the memory 
>  refresh register and is incremented after every each instruction executed 
by 
> 
>  the processor so if it used frequently it may not be random. Also bit 7 is 
>  never set so it can never be greater than 127

This doesn't really work as well as Jimmy's randomization...the randomization 
isn't good at all really, unless you throw in some other stuff to randomize 
the R register.  For example, to use this masking method to get a number 
between 0 and 9, you can't simply do:

 ld a,r
 and 9

...cause 9 wouldn't work as a mask in this case.  You *could* do something 
like:

Loop:
 ld a,r
 and $0F
 cp 9+1
 jr nc,Loop ;jump if resulting number isn't between 0 and 9

However, if, after the masking instruction (the < and $0F>), the result is 
10, the next randomized number will always be some other number that will 
always be the same (I think if it relooped and it was 10, it would be 
14...which would cause another reloop to 2).  So some numbers are almost 
always guarenteed to come up twice or three times as much as other numbers 
(which means bad randomization).

Of course, this method of masking off bits of the R register works great if 
you need a random number between 0 and 15, for example...but for other 
ranges, it doesn't work too well.

JayEll