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From: eeta@yoda.byu.edu
Newsgroups: comp.sys.apple2
Subject: Re: random no. in assembly?
Message-ID: <382eeta@yoda.byu.edu>
Date: 31 Jan 91 00:14:49 GMT
Lines: 52

>>Hi all.  I am looking for a way to generate random numbers in an assembly
>>language program.  The entry point to the applesoft RND function would
>>be a start, but I really just need an algorithm.  I'm using one that
>>shifts a 2-byte seed left once and adds it back to the seed. that doesn't
>>give very good results after the first few numbers.
>
>I wrote a simple one that basically does a CRC-16 (I can send you a
>6502 CRC-16 calculator) on values grabbed from somewhere on page $C0
>(something like $C022 or something). These softswitches usually have
>random changing garbage in them, so they supply numbers about as
>random as you can get.
>
The problem with this (at least on older II's) is that these registers'
so called "random numbers" are actually the current video line being scanned
and therefore have a limited range.  (This fact was used in some of the
higher quality games of the past to avoid flicker by waiting for the vertical
blanking time.  With the //e, a specific register was set up to do this.)
So, it seems to me that these numbers would make a good seed, but then some
code like this would make a good random algorithm:

RND      EPZ   $xx         ; Random number base address -- some location
                           ; in zero page, you need 6 contiguous bytes
RANDOM   CLD
         SEC
         LDA RND+1
         ADC RND+4
         ADC RND+5
         STA RND
         LDX #$04
^1       LDA RND,X
         STA RND+1,X
         DEX
         BPL <1
         RTS

I used that code chunk in a game I wrote about 6 years ago called "Rat Patrol".
It returns a random number between 0 and 255 in the accumulator.  If you need
more bits precision than 8, you can take the bytes in RND+x.  (I've never had
a use for more than 8 bits of precision, and I find this code to be pretty good-
it doesn't lock up and has what appears to be a good distribution.)  To seed
the random numbers, put a seed in RND at least.  One good way to seed it that I
use is to increment RND, RND+, .... , RND+5 as if they were a 48-bit counter
while waiting for a user to press a key.  Or you could get your seed from one
of the unused I/O registers as mentioned above...

Well, anyway, that's my $0.02.  (Take it or leave it, etc., etc.)  Happy random
number generating!

(Oh, BTW, I'm looking for a 3.5" disk drive and controller for a //e, if anyone
wants to unload one...)

--  Don Yacktman
eeta@yoda.byu.edu