The problem is that you'd want a random number generator that doesn't have short loops for bad seeds. It's not too difficult to create a chaotic function that maps 32->32 bits, and that you can call repeatedly to create good random numbers. However, it's very likely that such a function will have some bad seeds where it has a short loop, or even a fixpoint where f(x) = x for some value.
There are a few ways to create a function that will go through every value before cycling, but none of these functions produce excellent random numbers. The
LCG comes closest, but the LSB is very poor (also, the wide multiply does not suit the 6502 very well). A solution is to build the random generator out of 2 parts. An iterator part that goes through entire state, and then an output function that improves the randomness.
Now, the problem on the 6502 is that you can only do arithmetic operations on the accumulator, so there's very little you can do to create the output function. You really need some temporary variables to mix things around properly. But if you add some temporary values, you might as well make them part of the state. That's how I ended up with 8 bytes of state. Note that only 5 of those bytes are guaranteed to go through all states, and the remaining 3 are used as an output function with feedback.
