Microinstructions

Microcode

The distinction between custom logic and microcode may seem small, one uses a pattern of diodes and gates to decode the instruction and produce a sequence of signals, whereas the other encodes the signals as microinstructions that are read in sequence to produce the same results. The critical difference is that in a custom logic design, changes to the individual steps require the hardware to be redesigned. Using microcode, all that changes is the code stored in the memory containing the microcode or the storage device from which the microcode is loaded into the control store. This makes it much easier to fix problems in a microcode system. It also means that there is no effective limit to the complexity of the instructions, it is only limited by the amount of memory one is willing to use.