I second the suggestion that every software engineer should at least be able to imagine how they would go about building computer from discrete logic. When com

I second the suggestion that every software engineer should at least be able to imagine how they would go about building computer from discrete logic. When commercial computers were built from discrete logic in this manner, there was distinction between "horizontal" and "vertical" microcode. With horizontal being what most people would today call microcode, ie. implementing most of the control logic as one somewhat wide memory array with datapath being mostly directly controlled by bits coming form such memory. The vertical approach essentially involved designing simplistic RISC-like CPU with fixed width instructions which then ran interpreter for the actual instruction stream of the machine (ie. what the author's CPU does). And obviously there are various middle-ground approaches. The reason for this is that control-stores for purely horizontal microcode are not that dense and involve memory with weird and large word lengths (which is somewhat inconvenient for implementation from commercially available (P)ROM chips)