Inside the processor: ALU, control unit and registers

National 5 Computing Science: Computer systems

Questions on the parts of the processor are usually worth 1 mark each, and often tucked inside a programming question: "which part of the processor carries out this calculation?" In 2024, the examiners' report says, many candidates could name the part that does a calculation, but only a few could name the part that controls the transfer of instructions: the control unit.

One job each

Part	Its job
ALU (arithmetic and logic unit)	Does the calculations and comparisons: adding two prices, checking whether a score is bigger than 50.
Control unit	Controls the fetching and transferring of instructions and data, and makes sure everything happens in the right order.
Registers	Hold values inside the processor for a short time, while they are being used.
Data bus	Carries values between memory and the processor, in either direction.
Address bus	Carries the address of the memory location to be used. Every location has its own unique address.

How it is asked

A program contains this line:

SET total TO total + price

Which part adds the values? The ALU.
Where are total and price held while they are being added? In registers.
Which part controls fetching the instruction from memory? The control unit.
Which bus carries the value of price from memory to the processor? The data bus.

What gains the mark

The name of the part. These questions ask for a name, not a description.
Match the verb to the part: calculates or compares means ALU; temporarily holds means registers; controls means control unit; carries means a bus.

Try two exam-style questions

Choose an answer, say how sure you are, then press Check. Every wrong answer tells you why it is wrong.

The Inside the processor simulation — **In the course: Inside the processor.** Run instructions through a processor and watch the buses, the ALU, the registers and the control unit light up as each one does its job.

The full course covers every topic like this. Every topic in National 5 Computing Science has a short note, a simulation to try and a practice quiz with feedback on every answer, plus coding lessons, a full mock exam and a progress report for parents.

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This site is independent: it is not connected to the SQA or Qualifications Scotland. Points from the examiners' course reports are given in our own words.