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 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: A Galois linear feedback shift register (16-bit).
- 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:
