Control unit (CU)

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Quick Questions

Shade in the A or B circle for each question.

1. Which part of the CPU decodes instructions?

A The arithmetic logic unit (ALU)

B The control unit (CU)

2. What does the control unit send to coordinate the other parts of the computer?

A Control signals

B Pixels

3. Which part performs the calculation 5 + 3 during the execute stage?

A The ALU

B The CU

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.

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

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1CU		AThe CU does not store data. It sends control signals to coordinate the fetch-decode-execute cycle, and data is held in registers or RAM.
2ALU		BArithmetic is done by the ALU, not the CU. The CU only decodes the instruction and tells the ALU to carry out the addition.
3The CU decodes the instruction held in the CIR to work out what the instruction means and what must happen next.		CCU -> ALU: ADD
4The CU sends control signals to other parts, such as telling the ALU which operation to do or telling RAM to read data.		DThe part of the CPU that carries out arithmetic and logic operations, such as adding numbers or comparing values, under the direction of the CU.
5The CU adds 5 + 3 to get 8.		ECIR -> CU (decode)
6The CU stores the data that the CPU is working on in RAM permanently.		FThe part of the CPU that decodes instructions and sends control signals to coordinate the other parts of the computer.

Definitions

Control Unit (CU)

Fill in the Blanks

Complete each line of code by filling in the missing parts.

__ decodes the instruction in the CIR

CU sends _______ signals to the ALU

FETCH -> ______ -> EXECUTE

ALU: calculations and logic CU: ___________ the other parts

CU sends signals to RAM, ALU and _________

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A MAR <- PC

B ALU decodes the CIR contents

C CU sends control signals

Question 2

A CIR <- MDR

B CU only controls the ALU

C ALU adds the two numbers

Question 3

A CU encodes the instruction

B CU sends control signals

C ALU performs the calculation

Question 4

A CIR <- MDR

B CU decodes the CIR

C ALU tells the CU to add

Question 5

A CU starts the fetch stage

B CU starts the decode stage

C CU starts the store stage

Answer Key

Control unit (CU)

Section A: Quick Questions

B — The control unit (CU)

A — Control signals

A — The ALU

B — It manages the order of the fetch-decode-execute cycle.

A — Sends a control signal to the ALU

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

CU decodes the instruction in the CIR

CU sends control signals to the ALU

FETCH -> DECODE -> EXECUTE

ALU: calculations and logic
CU: coordinates the other parts

CU sends signals to RAM, ALU and registers

Section E: Spot the Error

B — Decoding instructions is the job of the control unit (CU), not the ALU. The line should start 'CU decodes the CIR contents'.

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.

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'.

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'.

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'.