Note 1 to be specific :-) "The microcode ROM forms a large region with no power connections, just ground. This is because each row in the ROM is implemented as

Note 1 to be specific :-) "The microcode ROM forms a large region with no power connections, just ground. This is because each row in the ROM is implemented as a very large NOR gate with the power pull-up on the right-hand edge. Thus, the ROM gates all have power and ground, even though it looks like the ROM lacks power connections" The outputs from the microcode go into clocked latches to the left of the array. The high-level motivation is that you want the microcode ROM to be as dense as possible, so you want to minimize the number of different signals going in there. It is constructed with just the input lines, transistors (or gaps), ground, and the output lines, so it is about as dense as possible. Even so, it takes up a large chunk of the 8086 die.