← IGCSE Computer Science Paper 1

Arithmetic logic unit (ALU)

Arithmetic logic unit: the part of the CPU that does calculations (add, subtract) and logic decisions (AND, OR, NOT, comparisons). Results are held in the accumulator.

Part of: IGCSE Computer Science 0478 — 3.1 Computer Architecture — Module 6

Understanding Arithmetic logic unit (ALU) in IGCSE Computer Science Paper 1

What it does: Subtraction is an arithmetic operation, so the ALU carries it out. AND and comparison operators are logical operations, so the ALU evaluates them and gives TRUE or FALSE. The ALU (arithmetic logic unit) does the maths and the logic decisions. Storage is done by memory, not the ALU. The accumulator holds the results of calculations from the ALU. The PC holds the address of the next instruction. AND, OR and NOT are logic operations in the ALU. Fetch is a stage of the fetch-decode-execute cycle, not an ALU operation. Comparisons such as greater than, less than or equal to are logic decisions made by the ALU.

Common mistakes: The ALU should ADD 8 and 3 to give 11. Subtracting would give 5. Line 3 should be ACC ← ACC + B. The ALU must subtract 4 from 10 to get 6. Adding gives 14. It should be ACC ← ACC - 4.

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Learning
Duo
What does the ALU do in the CPU?
Carries out arithmetic calculations and logical comparisonsStores the whole program permanently
Matches
The control unit (CU) adds the numbers 4 + 3 and does the comparisons.
↓
The CU does not do calculations or comparisons. The ALU does arithmetic and logic. The CU only controls and coordinates.
Applying
Meanings
Term
ALU
Meaning
Arithmetic logic unit: the part of the CPU that does calculations (add, subtract) and logic decisions (AND, OR, NOT, comparisons). Results are held in the accumulator.
Practice
ACC ← 7
Testing
Gaps
ACC ← ACC 5
Check Answer
Spots
2B ← 3
3ACC ← ACC - B

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Quick Questions

1. What does the ALU do in the CPU?
A. Stores the whole program permanently
B. Carries out arithmetic calculations and logical comparisons ✓
The ALU (arithmetic logic unit) does the maths and the logic decisions. Storage is done by memory, not the ALU.
2. Which task is carried out by the ALU?
A. Adding two numbers together ✓
B. Sending the address to the address bus
Addition and subtraction are arithmetic operations done in the ALU. Sending signals and addresses is handled by the control unit and buses.
3. Where is the result of an ALU calculation usually held?
A. In the program counter (PC)
B. In the accumulator (ACC) ✓
The accumulator holds the results of calculations from the ALU. The PC holds the address of the next instruction.
4. Which of these is a logical operation the ALU can perform?
A. AND ✓
B. Fetch
AND, OR and NOT are logic operations in the ALU. Fetch is a stage of the fetch-decode-execute cycle, not an ALU operation.
5. The ALU compares 7 and 5 to decide if 7 > 5. What type of operation is this?
A. A logical comparison ✓
B. A memory read
Comparisons such as greater than, less than or equal to are logic decisions made by the ALU.

Match the Pairs

ALU → The part of the CPU that carries out arithmetic calculations and logical comparisons
ACC <- ACC + 5 → The ALU adds 5 to the value held in the accumulator and stores the result back in the accumulator
The ALU performs arithmetic, such as subtracting one number from another → result = 20 - 8
The ALU performs logical decisions, such as comparing two values with AND → (age >= 16) AND (hasTicket == TRUE)
The control unit (CU) adds the numbers 4 + 3 and does the comparisons. → The CU does not do calculations or comparisons. The ALU does arithmetic and logic. The CU only controls and coordinates.
The ALU can only add numbers. It cannot do logic such as AND, OR, NOT. → The ALU does arithmetic (add, subtract) and also logical operations (AND, OR, NOT, comparisons).

Key Term

ALU
Arithmetic logic unit: the part of the CPU that does calculations (add, subtract) and logic decisions (AND, OR, NOT, comparisons). Results are held in the accumulator.

Fill in the Blanks

1. ACC ← 10 ACC ← ACC + 5
Hint: Type the arithmetic operator the ALU uses to add two values.
2. ACC ← 20 ACC ← ACC - 8
Hint: Type the arithmetic operator the ALU uses for subtraction.
3. IF age >= 16 AND mark > 50 THEN
Hint: Type the logic operator that is only true when both comparisons are true.
4. IF a = 1 OR b = 1 THEN
Hint: Type the logic operator that is true when at least one comparison is true.
5. ACC ← ACC + 5
Hint: Type the name of the register that holds the ALU's result.

Spot the Error

Question 1
A ACC ← 8
B B ← 3
C ACC ← ACC - B
D OUTPUT ACC // 11
Line C — The ALU should ADD 8 and 3 to give 11. Subtracting would give 5. Line 3 should be ACC ← ACC + B.
Question 2
A ACC ← 10
B ACC ← ACC + 4
C OUTPUT ACC // 6
Line B — The ALU must subtract 4 from 10 to get 6. Adding gives 14. It should be ACC ← ACC - 4.
Question 3
A A ← 7
B R ← A > 5 AND A > 10
C OUTPUT R // TRUE
Line B — 7 is not greater than 10, so the AND comparison would be FALSE. The ALU should test A < 10 so the result is TRUE.
Question 4
A LOCKED ← TRUE
B // ALU applies NOT
C OPEN ← LOCKED
D OUTPUT OPEN // FALSE
Line C — The comment says the ALU applies NOT, which reverses TRUE to FALSE. Without NOT, OPEN would be TRUE. It should be OPEN ← NOT LOCKED.
Question 5
A ACC ← 65
B PASS ← 50
C R ← ACC <= PASS
D OUTPUT R // TRUE
Line C — 65 is greater than or equal to 50, so the comparison gives TRUE. Using <= would give FALSE. It should be R ← ACC >= PASS.

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