The Z-80 has an interesting way of executing instructions, but it's different from microcode. The first step is the PLA, which basically pattern-matches instruc

The Z-80 has an interesting way of executing instructions, but it's different from microcode. The first step is the PLA, which basically pattern-matches instructions into related categories, e.g. generating a signal for bit pattern 01XX100X, which might be instructions that read a value from the accumulator for instance. There are two ways this is different from microcode. First, one instruction can trigger multiple "categories". Second, time is not an input here, so there is no stepping through operations. The next part of the Z-80 instruction decoding is a big pile of AND-OR gates that generate the control signals based on the PLA outputs and the timing signals. For example, a gate might trigger the read-from-A-register signal for PLA-output-1 and time-1 or PLA-output-3 and time-1 or PLA output-1 and time-4. (These gates are all complex and were hand-drawn.) The point of this is that both microcode and discrete logic generate particular control signals based on the instruction and time step, but they use very different ways to do this. With microcode, you can point to each micro-instruction as it executed, but in the Z-80, things are happening all over the place seemingly randomly.