← IGCSE Computer Science Paper 1

Address bus

A one-way bus that carries the address of a memory location from the CPU (sent from the MAR) to RAM. It carries addresses, not data.

Part of: IGCSE Computer Science 0478 — 3.1 Computer Architecture — Module 15

Understanding Address bus in IGCSE Computer Science Paper 1

What it does: The arrow shows a single direction, from the CPU to RAM. RAM never sends addresses back to the CPU along this bus. The MAR holds the address of the location to be accessed, and this is what the address bus carries to RAM. The address bus carries the address, not the data stored there. Only the address 100 goes along the address bus. The instruction comes back to the CPU on the data bus. The address bus links the CPU to RAM so the CPU can choose which memory location to use.

Common mistakes: The address bus is one-way, from the CPU to RAM. RAM receives the address, so line 4 should be 'RAM receives the address'. The address sent on the address bus comes from the MAR. The MDR holds data being transferred, so it should be 'Sender: MAR'.

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This module's key terms are also in a printable crossword (Modules 218–227) and word search (Modules 218–239).

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Learning

Duo

What does the address bus carry?

The data or instruction being readThe address of a memory location

Matches

The address bus carries data from RAM to the CPU

↓

The address bus carries addresses, not data. Data travels along the data bus.

Applying

Meanings

Term

address bus

Meaning

A one-way bus that carries the address of a memory location from the CPU (sent from the MAR) to RAM. It carries addresses, not data.

Practice

MAR ← 4096

Testing

Gaps

Address on the bus comes from the

Check Answer

Spots

3Address bus carries it one way

4RAM sends the address to CPU

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Quick Questions

1. What does the address bus carry?

A. The address of a memory location ✓

B. The data or instruction being read

The address bus only carries addresses. The data bus carries the actual data and instructions.

2. In which direction does the address bus send signals?

A. Both ways: CPU to memory and memory to CPU

B. One way: from the CPU to memory ✓

The address bus is one-way. The CPU tells RAM which location it wants, and RAM never sends addresses back.

3. Which register supplies the address that is placed on the address bus?

A. Memory data register (MDR)

B. Memory address register (MAR) ✓

The MAR holds the address of the location to be accessed. The MDR holds the data that was read or is to be written.

4. Which statement about the address bus is correct?

A. It carries addresses, not data ✓

B. It carries both addresses and data

Addresses and data travel on separate buses. The address bus only ever carries addresses.

5. The CPU wants to read the instruction stored at address 100. What is sent along the address bus?

A. The address 100 ✓

B. The instruction stored at address 100

Only the address 100 goes along the address bus. The instruction comes back to the CPU on the data bus.

Match the Pairs

address bus → A one-way bus that carries the address of a memory location from the CPU to RAM

MAR → address bus → RAM → The path an address takes when the CPU wants to access a memory location

The address bus is one-way: addresses only travel from the CPU to memory, never back → CPU → address bus → RAM

The address in the MAR is the value placed on the address bus → MAR = 0042, address bus carries 0042

The address bus carries data from RAM to the CPU → The address bus carries addresses, not data. Data travels along the data bus.

RAM → address bus → CPU → The address bus is one-way and goes from the CPU to RAM, not from RAM to the CPU.

Key Term

address bus

A one-way bus that carries the address of a memory location from the CPU (sent from the MAR) to RAM. It carries addresses, not data.

Fill in the Blanks

1. Address on the bus comes from the MAR

Hint: Type the register that holds the address sent to RAM

2. Address bus: CPU → RAM

Hint: Type the component that receives the address

3. The address bus carries addresses, not data

Hint: Type what the address bus carries

4. Address bus direction: one-way

Hint: Type the direction of travel, as a hyphenated word

5. MAR ← PC address bus ← MAR

Hint: Type the name of the bus that carries the MAR contents to RAM

Spot the Error

Question 1

A MAR holds address 0042

B CPU puts it on address bus

C Address bus carries it one way

D RAM sends the address to CPU

Line D — The address bus is one-way, from the CPU to RAM. RAM receives the address, so line 4 should be 'RAM receives the address'.

Question 2

A Address bus: one-way

B Direction: CPU to RAM

C Carries: memory addresses

D Sender: MDR

Line D — The address sent on the address bus comes from the MAR. The MDR holds data being transferred, so it should be 'Sender: MAR'.

Question 3

A CPU needs data at address 7

B MAR <- 7

C Address bus sends data to RAM

D MDR <- data from RAM

Line C — The address bus carries the address (7), not data. The line should be 'Address bus carries 7 to RAM'.

Question 4

A Address bus: RAM to CPU

B It is a one-way bus

C The MAR sends the address

Line A — The address bus carries addresses from the CPU to RAM, never the other way. It should be 'Address bus: CPU to RAM'.

Question 5

A PC holds next address

B MDR <- PC

C Address bus carries MAR

Line B — The address is copied from the PC into the MAR, which then sends it on the address bus. It should be 'MAR <- PC'.

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