Creating a processor

Abstract

Creating a processor is great fun: programming down to the bare (semi)metal. At the same time your eye is on the output loads of 3000 NAND gates, your hand rests on the pulse of the market place, and something like a conscience watches to make sure that any possible instruction a compiler wants it can create. I remember the trouble I had with a literal fetch (a number is fetched from the next code address). It was a tough design because it required an increment of the instruction pointer twice in two clock cycles. I actually spent time satisfying myself that such an instruction was absolutely necessary. Increment wasn't, after all. One could always add a one to X. But where will that one come from? I am not sure what other instructions a real minimum instruction set computer might have but I am sure it will have a literal fetch. It is absolutely necessary. Nothing --- no address, no data, no jump---is possible without some form of literal fetch.