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What's good for RANDOM?
http://www.retrosoftware.co.uk/forum/viewtopic.php?f=73&t=523		 Page 1 of 1
Author:  PitfallJ [ Thu Oct 21, 2010 11:34 am ]
Post subject:  What's good for RANDOM?
Hi,

Anyone know a good hardware address or/and routine to use for random ?

I am trying LDA $FE44 but it doesn't seem very random. (saw that in some game)

What I ideally need is a random number between 0 and 'N' as well.

- PJ
Author:  RichTW [ Thu Oct 21, 2010 2:28 pm ]
Post subject:  Re: What's good for RANDOM?
You can try combining a load from &FE44 with a circular table of random numbers, e.g.
Code:
.random
; increment and fetch random number table index
LDX randindex
INX
CPX #16
BCC wrapindex
LDX #0
.wrapindex
STX randindex
; fetch random number
LDA randtable,X
EOR &FE44
STA randtable,X
RTS

.randtable
; put 16 random numbers here


If you want to limit the number to between 0 and 2^n-1, that's easily done with an AND. If it's a non-power-of-two, e.g. between 0 and 23, unfortunately I can't think of an easier way than:
Code:
.try
JSR random
AND #31
CMP #24
BCS try
Author:  PitfallJ [ Wed Dec 01, 2010 9:08 pm ]
Post subject:  Re: What's good for RANDOM?
Thanks for that - the AND trick is pretty neat - I used it in my recent BeebSID game.
Author:  recycled [ Thu Dec 02, 2010 9:30 am ]
Post subject:  Re: What's good for RANDOM?
I recently used the LSB of the countdown interval timer (wow, I've actually read a manual for this) to pull a random seed for one of my proggies.

lda &02A0

Lets not get too involved in how non-random this is going to be 'cause my program is started at exactly the same nano-second after the computer is turned on each time... ;)

and then for a fast '8 bit' random number;

lsr A
bcs skip1
eor #&B2
.skip1
; random number in accumulator

Courtesy of some Commodore programming website I ran into.
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