Why 21 bits? The first factor is there is absolutely no need for an instruction to be 8, 16, or 32 bits. Modern processors use these sizes to be compatible with

Why 21 bits? The first factor is there is absolutely no need for an instruction to be 8, 16, or 32 bits. Modern processors use these sizes to be compatible with the 8-bit byte, but older processors used whatever word size was convenient: 12 bits, 13 bits, 17 bits, 27 bits, or pretty much anything. The second factor is that (as you suspect) there isn't any connection between the size of the machine instructions and the size of the micro-instructions. Finally, the width of a micro-instruction depends on how much you want to control at once. You can have a micro-instruction that is 100 bits wide, letting the processor do different things in parallel (e.g. IBM 360 mainframe), or a very short micro-instruction that doesn't do very much. The choice of 21 bits is enough for each micro-instruction to do two things (a data transfer and an operation). Having a 100-bit wide micro-instruction would improve performance but make the ROM too large, so it is a tradeoff. (Lookup vertical and horizontal microcode for more information.) For your second question, the photos are all looking downward onto the chip through a microscope. In the first die photo, you're looking at the metal layer. But in the closeup photo, I've removed the metal and polysilicon layers with acid, so you see the silicon layer. It's tricky to show how all the overlapping layers are connected, so I didn't do that in this post. But I have another post that shows how the layers work together in the 6502: http://www.righto.com/2013/01/a-small-part-of-6502-chip-explained.html http://www.righto.com/2013/01/a-small-part-of-6502-chip-expl...