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diff --git a/rand/README.md b/rand/README.md new file mode 100644 index 0000000..12f8902 --- /dev/null +++ b/rand/README.md @@ -0,0 +1,33 @@ +## Generating pseudo-random numbers + +Usually there is a requirement from NES/Famicom games to have access to random +numbers. This is not entirely possible on the NES/Famicom since there is no +hardware-specific implementation for any of this (i.e. as it happens on modern +CPUs). But there's this common saying: if you can't make it, fake it! That's a +common mentality when programming on the NES/Famicom due to its shortcomings in +comparison to modern hardware. + +In this case, there are quite clever algorithms for generating **pseudo** random +numbers. That is, numbers which are not absolutely random, but they are quite +close to it. + +First of all, you need to generate a **seed**: from which number you start +generating new random numbers. The approach taken here is the same as in many +other games: the player is presented with a `Start` screen. On `nmi` code we +count the frames until the player actually presses the `Start` button. +Obviously, this is not really random, but unless you are on a TAS or you are an +amazing player with frame-perfect input skills, it's good enough. + +All of this is implemented on the [rand.s](./rand.s) file. After the "Start" +screen the player is presented with the algorithm being used and the random +value that we got. The player can then press `Select` to change the algorithm +being used, or press `A` to select a new number. There are a total of two PRNG +algorithms being tested here: + +- [linear.s](./linear.s): A Galois linear feedback shift register (16-bit). +- [precalc.s](./precalc.s): Indexing a pre-computed set of random numbers. + +These algorithms are better explained at the top comment from their respective +files. All in all, we get the following result: + + |