Recently, we looked at a simple LFSR and how it works. Shift the bits XOR some of the bits together; we call these the taps Replace a bit with the XORed value. Repeat Easy peasy. I’ve long been fascinated by the Sound Interface Device (SID) in the Commodore 64, mostly because you can use it to get random numbers. But how does it work? The Commodore 64 SID LFSR is 23-bits.