There are a few ways to do pseudo RNGs, but nothing that I'd use for
cryptography :) Erik Mooney posted something back in March 1997 about this
and it's in the archives here:
http://www.biglist.com/lists/stella/archives/199703/msg00296.html
I used the routine in SCSIcide to determine the starting positions and
colors of the data bits. In is here:
http://www.pixelspast.com/games/scsicide/index.php?ID=games&subID=scsicide
and code is here: http://www.pixelspast.com/games/scsicide/scsi131.s.txt
I had to use 4 bytes of RAM:
rand1 ds 1 ; Variables for random bit/byte
generation routine
rand2 ds 1
rand3 ds 1
rand4 ds 1
Maybe it could be optimized in some way, but it worked well for me.
I had to seed the RNG at the beginning of my game calculations:
JSR RandomBit ; Seed the PRNG
And the actual routines are:
;
; rand1, rand2, rand3, rand4 are RAM locations, initialized to any non-zero
; value at program start
;
; RandomBit generates one random bit.
; RandomByte generates one random byte and returns it in the accumulator.
;
RandomBit
LDA rand4
ASL
ASL
ASL
EOR rand4 ; New bit is now in bit 6 of A
ASL
ASL ; New bit is now in carry
ROL rand1 ; Shift new bit into bit 0 of register, bit 7 goes
into carry
ROL rand2 ; Shift old bit 7 into bit 8, etc.
ROL rand3
ROL rand4
RTS
RandomByte
LDX #8
RandomByte1
JSR RandomBit
DEX
BNE RandomByte1
LDA rand1
RTS
Have fun,
Joe
---- Original Message ----
From: "David Davies" <davidwyn.davies@xxxxxxxxxxxx>
To: <stella@xxxxxxxxxxx>
Sent: Tuesday, January 27, 2004 12:16 PM
Subject: [stella] Random Numbers
> Just started on a game for the 2600 based on the 1977 arcade game,
> Depthcharge (well, okay, it's a straight port with added game
> variations). It's my first attempt at taming the 2600, so please be
> gentle :-)
>
> I've been doing some thinking as to what needs doing on the game. I
> definately need a random number generator. Is there a way of doing
> that on the 2600?
>
> David Wyn Davies
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