How does the processor fetch and carry out a single program instruction?
The purpose of the CPU, the fetch-decode-execute cycle, the von Neumann architecture, and the function of the common CPU components and registers (ALU, control unit, PC, MAR, MDR, accumulator).
An Eduqas GCSE Computer Science answer on the purpose of the CPU, the von Neumann architecture, the fetch-decode-execute cycle, and the function of the ALU, control unit and the named registers (Program Counter, MAR, MDR and accumulator).
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What this dot point is asking
Eduqas wants you to state the purpose of the CPU, describe the von Neumann architecture, trace the fetch-decode-execute cycle in order, and give the function of the common CPU components and the named registers. This is recall plus the ability to follow one instruction through the cycle, which is a recurring Component 1 question.
The purpose and components of the CPU
The components work as a team. The control unit fetches and interprets each instruction and tells the ALU what to do; the ALU does the arithmetic and logic; the registers hold the small amounts of data each step needs, because they are far faster to reach than main memory. The processor is connected to memory by buses: the address bus carries the address of the location being accessed, the data bus carries the instruction or data itself, and the control bus carries the timing and control signals.
The von Neumann architecture
The registers Eduqas names
The fetch-decode-execute cycle
How the parts cooperate
It helps to see the components and the cycle together. During fetch, the control unit uses the Program Counter to find the next instruction and copies it from memory via the MAR and MDR. During decode, the control unit interprets the bit pattern to work out the operation and the data it needs. During execute, the control unit directs the work: if it is a calculation the values go to the ALU and the result lands in the accumulator; if it is a data move, values are copied between registers and memory. Registers are central throughout, because they are the only stores fast enough to keep up with the cycle.
Try this
Q1. Name the three stages of the fetch-decode-execute cycle in order. [3 marks]
- Cue. Fetch, decode, execute.
Q2. State which register holds the address of the next instruction to be fetched. [1 mark]
- Cue. The Program Counter (PC).
Q3. State one feature of the von Neumann architecture. [1 mark]
- Cue. Instructions and data are stored together in the same memory and fetched over the same shared bus.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas Component 1, 20224 marksDescribe the fetch-decode-execute cycle, naming the three stages in order and stating what happens at each stage.Show worked answer →
Fetch: the CPU copies the next instruction from main memory into the processor. The address of that instruction is held in the Program Counter, which is then incremented so the next cycle fetches the following instruction.
Decode: the control unit interprets the fetched instruction, working out which operation it is and what data or addresses it needs.
Execute: the CPU carries out the instruction, for example using the ALU to perform a calculation or comparison, or moving data between memory and registers. The cycle then repeats.
Markers reward the correct order (fetch, decode, execute), a sentence on each stage, and a reference to the Program Counter or to the cycle repeating. Reversing decode and execute loses marks.
Eduqas Component 1, 20233 marksState the function of each of the following: the arithmetic logic unit (ALU), the control unit, and a register.Show worked answer →
Award one mark for each correct function, up to three.
ALU: carries out all arithmetic (such as addition and subtraction) and logical operations (such as comparisons and AND, OR, NOT).
Control unit: decodes each instruction and sends out the control signals that direct and coordinate every other part of the CPU.
Register: a tiny, extremely fast store inside the CPU that holds a single value the processor is working on right now (for example the Program Counter holds the address of the next instruction).
Markers reward precise wording. Saying the ALU "controls the CPU" or that a register "stores all the programs" loses the mark.
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Sources & how we know this
- WJEC Eduqas GCSE Computer Science specification (from 2016) — Eduqas (2020)