Questions: Processor Control Unit Design

Question 1

A processor designer wants to add ten new instructions to an existing design. Which statement best describes how this affects hardwired versus microprogrammed control?

• A. Both approaches require rewriting the entire control logic, so neither has an advantage
• B. Hardwired control requires redesigning the combinational gate network; microprogrammed control requires writing new microcode sequences in the control store
• C. Microprogrammed control requires rebuilding the ROM from scratch; hardwired control only needs minor updates
• D. Hardwired control is easier to extend because signals propagate through gates without any lookup overhead

Answer: (B)

This is the central tradeoff between the two approaches. In microprogrammed control, adding a new instruction means writing a new microprogram sequence — a software-like change to the ROM contents. In hardwired control, the entire combinational logic network must be redesigned to handle the new opcode, which grows in complexity nonlinearly. This is why microprogrammed control dominated during the CISC era with large, irregular instruction sets.

Question 2

In a microprogrammed control unit, what is the role of the control store?

• A. It stores the architectural registers used by the programmer
• B. It is a fast ROM that holds microinstruction sequences specifying which control signals to assert each cycle
• C. It is the instruction cache that buffers recently fetched machine instructions
• D. It is the combinational logic that decodes instruction opcodes into control signals

Answer: (B)

The control store is a small, fast ROM where each address contains a microinstruction — a bit pattern specifying exactly which control signals should be asserted during one clock cycle. Each machine instruction maps to a starting address in the control store, and the control unit steps through the microprogram sequence to execute it. This is distinct from the instruction cache (which holds machine instructions before decoding) and from the combinational decoder used in hardwired control.

Question 3

Hardwired control units use a ROM to store control signal patterns, making them faster than microprogrammed units.

• T. True
• F. False

Answer: False

This reverses the two approaches. It is microprogrammed control that uses a ROM (the control store) to store control signal patterns. Hardwired control generates signals directly from instruction bits through combinational logic — decoders, gates, and multiplexers — with no ROM lookup. This is precisely why hardwired control is faster: signals propagate at gate speed without any memory access latency.

Question 4

Modern x86 processors use a microcode layer internally even though they present a hardwired interface to software.

• T. True
• F. False

Answer: True

Modern x86 chips internally translate complex CISC instructions into simpler micro-operations using a microcode layer, a legacy of the x86 architecture's historically large and irregular instruction set. The processor exposes the x86 ISA externally while using a RISC-like microarchitecture internally. This is a hybrid: the front end uses microcode for flexibility and compatibility; the back end executes micro-ops efficiently like a hardwired RISC core.

Question 5

Why do modern RISC processors favor hardwired control while legacy CISC architectures like x86 relied on microprogrammed control?

Model answer: RISC instructions are simple, uniform, and typically execute in one cycle, making the control logic tractable as combinational hardware. CISC instructions are numerous, irregular, and may take many cycles, making a hardwired implementation prohibitively complex — microprogramming handles the variety by treating each instruction as a small program. As RISC's simplicity made hardwired control feasible and fast, it became the preferred approach for high-performance designs.

The key is matching the control strategy to the instruction set's complexity. RISC (Reduced Instruction Set Computer) was in part motivated by the observation that hardwired control for a small, regular instruction set is fast and manageable. CISC instruction sets grew large enough that microprogramming was the only practical way to implement them — but the ROM lookup overhead is a real cost. Modern RISC processors achieve high clock rates partly because their hardwired control has no microcode latency.