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optional · CSS study notes

Control Unit and Instruction Processing

CSS Optional — Paper-II, Computer Organization & Architecture

1. Definition

The Control Unit (CU) is the CPU component that generates the timing and control signals needed to fetch, decode, and execute instructions — it does not process data itself.

2. Classification — Control Unit Design

  • Hardwired control — control signals generated by fixed combinational logic circuits.
    • ✅ Fast.
    • ❌ Inflexible; hard to modify or extend once built.
    • Favoured by RISC designs.
  • Micro-programmed control — control signals generated by a sequence of microinstructions stored in a control memory (effectively "a program that runs the CPU").
    • ✅ Easy to modify or debug — updating behaviour means updating microcode, not rewiring hardware.
    • ❌ Slower than hardwired control due to the microcode lookup step.
    • Favoured by CISC designs.

3. Classification — Instruction Representation

  • Opcode — specifies the operation to perform.
  • Operand(s) / addressing information — specifies where the data involved in the operation is located.

🗺️ Architecture Diagram — Control Unit Types

graph TD
    IR["Instruction Register
opcode"] --> CU{"Control Unit Design"}
    CU -->|Hardwired| HW["Fixed Logic Circuits
Fast, inflexible - RISC"]
    CU -->|Micro-programmed| MP["Control Memory
microinstruction sequence
Flexible, slower - CISC"]
    HW --> SIG["Control Signals"]
    MP --> SIG
    SIG --> DP["Datapath
ALU, Registers, Buses"]

🎯 CSS Exam Angle

Hardwired vs micro-programmed control is a classic trade-off question, often paired with RISC (hardwired) vs CISC (micro-programmed) reasoning. See Machine Instructions for addressing-mode numericals.

🔁 Repeated CSS Questions

  1. Differentiate between hardwired and micro-programmed control units.
  2. Why do RISC processors favor hardwired control while CISC processors favor micro-programmed control?
  3. Explain the structure of a machine instruction (opcode + operand).

⚠️ MCQ Trap

Hardwired control is faster but less flexible; micro-programmed control is slower but easier to modify.