atari2600land's Blog

Kiwi · March 2, 2019

You can always go back to it if you give up. In time, solution will come to you.

Random Terrain · March 2, 2019

But drinking alcohol is the American way. So is smoking.

carlsson · March 2, 2019

Fortunately, atari2600land has a habit of returning to his abandoned projects. He just needs a nudge over the hill (in this case a random number generator of some sort) and he'll be going again.

DZ-Jay · March 4, 2019

Hey, programming doesn't come easy to me either. When I try to help is because I struggled with something for so long that it actually stuck with me, and I can then pass it on.

The LFSR (Linear Feedback Shift Register) pseudo-random number generator I posted in C was taken from Wikipedia. It should work.

#include <stdint.h>
uint32_t lfsr = 1;

/* x^32 = x^15 + x^13 + x^12 + x^9 + x^8 + x^0 */
uint32_t rand(uint32_t bits)
{
   while (bits-- != 0)
       lfsr = (lfsr >> 1) ^ (lfsr & 1u ? 0x0000B301u : 0);

   return lfsr;
}

You call that function with the number of bits you want to shift, typically the largest power of 2 that fits the range you wish. If you are looking for a random number between 0 and 255, then you call it with 8 bits:

   x = rand(;

That's it. If it doesn't work, let us know why.

Just to put your mind at ease, I didn't come up with that. I don't even understand most of the math behind it. I asked others for help and they helped me arrive at a good implementation and was recommended that something like that would work for my purposes. I translated that to Intellivision Assembly Language and then was helped a bit more to optimize it.

In the end, it's mostly the work of others, and that's OK. It is now one more tool in my kit and I don't have to agonize over it ever again.

-dZ.