← IGCSE Computer Science Paper 1

Control unit (CU)

The part of the CPU that decodes instructions and sends control signals to coordinate the ALU, registers, memory and other parts during the fetch-decode-execute cycle.

Part of: IGCSE Computer Science 0478 — 3.1 Computer Architecture — Module 7

Understanding Control unit (CU) in IGCSE Computer Science Paper 1

What it does: During the decode stage the instruction in the CIR is passed to the control unit, which works out what it means. The CU does not calculate itself. It sends a control signal to the ALU, telling it which operation to carry out. The CU sends control signals to the ALU, registers, memory and other components so they know what to do and when. The CU keeps the fetch-decode-execute cycle in the right order. Long-term storage is done by secondary storage, not the CU. The CU does not calculate. It coordinates by sending signals, and the ALU does the adding. The CU is one of the main components of the CPU, along with the ALU and the registers.

Common mistakes: Decoding instructions is the job of the control unit (CU), not the ALU. The line should start 'CU decodes the CIR contents'. The CU coordinates all the parts of the CPU and the computer, including registers, memory and other devices, not just the ALU. It should say that the CU sends control signals.

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This module's key terms are also in a printable crossword (Modules 204–217) and word search (Modules 189–217).

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Learning
Duo
Which part of the CPU decodes instructions?
The arithmetic logic unit (ALU)The control unit (CU)
Matches
The CU adds 5 + 3 to get 8.
↓
Arithmetic is done by the ALU, not the CU. The CU only decodes the instruction and tells the ALU to carry out the addition.
Applying
Meanings
Term
Control Unit (CU)
Meaning
The part of the CPU that decodes instructions and sends control signals to coordinate the ALU, registers, memory and other parts during the fetch-decode-execute cycle.
Practice
REPEAT
Testing
Gaps
decodes the instruction in the CIR
Check Answer
Spots
1MAR <- PC
2ALU decodes the CIR contents

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Quick Questions

1. Which part of the CPU decodes instructions?
A. The arithmetic logic unit (ALU)
B. The control unit (CU) ✓
The CU decodes each instruction to work out what it means. The ALU only carries out calculations and logic.
2. What does the control unit send to coordinate the other parts of the computer?
A. Control signals ✓
B. Pixels
The CU sends control signals to the ALU, registers, memory and other components so they know what to do and when.
3. Which part performs the calculation 5 + 3 during the execute stage?
A. The ALU ✓
B. The CU
The ALU does arithmetic and logic. The CU only tells the ALU to carry out the operation, using a control signal.
4. Which statement about the control unit is correct?
A. It stores all the programs when the computer is switched off.
B. It manages the order of the fetch-decode-execute cycle. ✓
The CU keeps the fetch-decode-execute cycle in the right order. Long-term storage is done by secondary storage, not the CU.
5. The CU decodes an instruction that says to add two numbers. What does it do next?
A. Sends a control signal to the ALU ✓
B. Adds the numbers itself
The CU does not calculate. It coordinates by sending signals, and the ALU does the adding.

Match the Pairs

CU → The part of the CPU that decodes instructions and sends control signals to coordinate the other parts of the computer.
ALU → The part of the CPU that carries out arithmetic and logic operations, such as adding numbers or comparing values, under the direction of the CU.
The CU decodes the instruction held in the CIR to work out what the instruction means and what must happen next. → CIR -> CU (decode)
The CU sends control signals to other parts, such as telling the ALU which operation to do or telling RAM to read data. → CU -> ALU: ADD
The CU adds 5 + 3 to get 8. → Arithmetic is done by the ALU, not the CU. The CU only decodes the instruction and tells the ALU to carry out the addition.
The CU stores the data that the CPU is working on in RAM permanently. → The CU does not store data. It sends control signals to coordinate the fetch-decode-execute cycle, and data is held in registers or RAM.

Key Term

Control Unit (CU)
The part of the CPU that decodes instructions and sends control signals to coordinate the ALU, registers, memory and other parts during the fetch-decode-execute cycle.

Fill in the Blanks

1. CU decodes the instruction in the CIR
Hint: Type the abbreviation for the part of the CPU that decodes instructions.
2. CU sends control signals to the ALU
Hint: Type the word for the kind of signal the CU sends to coordinate other parts.
3. FETCH -> DECODE -> EXECUTE
Hint: Type the middle stage of the cycle, the one the CU carries out.
4. ALU: calculations and logic CU: coordinates the other parts
Hint: Type the verb that describes the CU's job, as opposed to the ALU's calculating.
5. CU sends signals to RAM, ALU and registers
Hint: Type the CPU components that hold data temporarily and receive CU signals.

Spot the Error

Question 1
A MAR <- PC
B ALU decodes the CIR contents
C CU sends control signals
Line B — Decoding instructions is the job of the control unit (CU), not the ALU. The line should start 'CU decodes the CIR contents'.
Question 2
A CIR <- MDR
B CU only controls the ALU
C ALU adds the two numbers
Line B — The CU coordinates all the parts of the CPU and the computer, including registers, memory and other devices, not just the ALU. It should say that the CU sends control signals.
Question 3
A CU encodes the instruction
B CU sends control signals
C ALU performs the calculation
Line A — The CU decodes instructions, which means it works out what the instruction means. It does not encode them. It should be 'CU decodes the instruction'.
Question 4
A CIR <- MDR
B CU decodes the CIR
C ALU tells the CU to add
Line C — The CU is in charge. It sends control signals telling the ALU what to do, not the other way round. It should be 'CU tells the ALU to add'.
Question 5
A CU starts the fetch stage
B CU starts the decode stage
C CU starts the store stage
Line C — The CU manages the order of the fetch-decode-execute cycle. After decode comes the execute stage. There is no store stage in the cycle. It should be 'CU starts the execute stage'.

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