Electronic – Computer architecture why is MemRead used

Not all of the CPU instructions have the same complexity. When instructions are particularly complex (like some x86 instructions), it is more convenient to implement them as a series of elementary instructions that are contained in a very fast internal memory. When a microcoded instruction is executed the microprocessor split it into a series of simple hardware coded instructions and execute them in series.

So microcode is used in more complex instructions, while simple instructions are directly executed. For example today Intel microprocessors uses microcode in order to execute some old legacy instructions. The x86 microprocessors, that on the paper are CISC, internally uses an architecture that is more like RISC in order to boost the performance and they offer support to the legacy instructions through microcode.

You can find many RISC CPUs that don't use microcode. This is essentially due to their reduced instruction set, that makes more convenient implement all the instructions in hardware. An example of microprocessor that doesn't use microcode is the MOS 6502.

The timing of the "Second ALU Input" depends on three things:

• the delay through the sign extender to the "1" input of the mux
• the delay through the control logic to the select input of the mux
• the worst-case propagation delay through the mux from either the "1" or select inputs

Yes, the labeling on the timing diagram is a bit unclear, but this is all they're saying. Either the path through the control logic or the path through the sign extender will be the long path that determines the final settling time for the second ALU input.

In static timing analysis, you need to take the worst case combination, and they seem to be implying that the worst case is the path through the control logic.