What is the primary architectural advantage of a Microprogrammed Control Unit over a Hardwired one?

Correct — B. A hardwired control unit makes its control signals with fixed combinational logic, so changing the instruction set means changing the wiring.

A microprogrammed unit keeps each instruction's steps as microinstructions in a control memory. Changing or adding an instruction means changing that stored microprogram, which can be debugged and replaced much like software.

Easier instruction-set updates → option (b).

Why the others are wrong

• (a)
  Lower power consumption — Lower power consumption is not what the design offers. A microprogrammed unit reads a microinstruction from control memory at every step of every instruction, a step the hardwired unit does not have.
• (c)
  Faster execution speed — Speed is the hardwired unit's strength: hardwired control units are generally faster. A microprogrammed unit usually spends one clock cycle on each step of an instruction's microprogram.
• (d)
  Reduced physical gate count — Reduced physical gate count is not what microprogramming promises. It swaps custom decoding logic for a control memory and sequencing logic, which are hardware too. The dependable gain is that the stored code can change.

The control unit turns each machine instruction into the control signals that drive the rest of the CPU. It can be built two ways.

Hardwired: combinational logic produces the signals directly. It is fast but has little flexibility.

Microprogrammed: each instruction runs a short program of microinstructions held in a control memory, an idea Maurice Wilkes introduced in 1951. It is slower, usually taking a clock cycle per step, but the microprogram can be changed without redesigning the circuit.

Key facts

• Hardwired control: fixed combinational logic, generally faster, changed only by rewiring.
• Microprogrammed control: microinstructions in a control memory, easier to change, slower.
• A microprogrammed unit usually spends one clock cycle per step of the microprogram.
• Maurice Wilkes introduced microprogramming in 1951.

Common traps

• Choosing faster execution because microprogramming sounds more advanced, when hardwired control is the faster design.
• Reading simpler structure as fewer gates, when the control memory is hardware too.

No directly related past PYQ was found.