Follow the address, data and control buses
Distinguish the roles of the three buses that connect the CPU, memory and other components.
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 3.1–3.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which question does an address answer?
What you will learn
- Explain the purpose of the address bus
- Explain the purpose of the data bus
- Explain the purpose of the control bus
Buses carry different information
The address bus carries the address of the memory location or device being accessed. The data bus carries the data or instruction being transferred. The control bus carries signals such as read or write that coordinate the operation.
Apply and check the rule
A useful memory-access trace keeps these roles separate: address says where, data says what, and control says what action is required. Bus widths and directions depend on the design; do not assume the data bus carries an address.
Pause and explain
Which bus carries data read from memory to the CPU?
Worked example
The CPU reads a value from memory address 500. What travels on each bus?
Show the worked solution
- Address bus: 500.
- Control bus: a read signal.
- Data bus: the contents returned from location 500.
Answer Address bus carries 500; control bus carries the read signal; data bus carries the returned value.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For a memory write to address 64 with value 9, state what travels on each bus.
Give me a hint
Identify where, what and the action.
Compare my reasoning
- Address bus: 64.
- Data bus: value 9.
- Control bus: write signal.
Address 64; data value 9; write control signal.
Look for these in your work
- Address identified
- Data identified
- Write signal identified
A CPU sends a read request but the address bus carries no address. What information is missing?
Give me a hint
The control action cannot identify the target by itself.
Compare my reasoning
- The read signal says what action to perform.
- The address bus must specify the memory location.
- Without it, the intended location is unknown.
The target address is missing; the read signal alone does not identify which location to access.
Look for these in your work
- Read action distinguished
- Address requirement stated
A student describes the data bus as 'the wire that says where data is going.' Correct the explanation using one read example.
Give me a hint
Separate the destination address from the transferred value.
Compare my reasoning
- The address bus specifies the target location.
- The control bus carries the read instruction.
- The data bus carries the value returned from that location.
The address bus says where, the control bus signals read, and the data bus transfers the value. For example, address 500 goes on the address bus and the contents of 500 return on the data bus.
Look for these in your work
- Address role corrected
- Control role included
- Data role illustrated
Common mistakes
- Putting the memory address on the data bus
- Treating read/write signals as data values
What can you explain now?
Which bus carries a read signal?
Compare with the explanation
Control bus
The control bus carries signals that coordinate a read or write operation.
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Make it stick
Use the words where, what and action to name the three bus roles.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.