microinstruction and microoperation

RobikShrestha

When we write machine level code of say,
add a,b
Then the CPU does many things,
I am confused, whether microinstruction is the set of all the control signals generated by the control unit i.e. is microinstruction = control unit's equivalent set of signals for add a,b as a whole? or is it just one set of control signals such that many microinstructions would be necessary to perform add a,b?

Bill Simpson

What a microinstruction consists of will depend on the architecture of the processor. Usually I would think most microcoded architectures would use several microinstructions to implement each machine instruction, say "gate a,b to the ALU" followed by "gate the sum from the ALU to the destination and update the status" and "increment program counter and gate to memory to fetch next machine instruction."

If all control signals needed to execute each machine instruction could be encoded into a single wide word and executed at once then perhaps the idea of using microinstructions would be replaced by hard wired CPU control with no microinstructions.