The Registers — Computer Architecture | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 7% of your exam marks.

stable

Low

Stable7%

The fetch-decode-execute cycle and von Neumann architecture are tested almost every year.

A register is a tiny, extremely fast storage location built directly into the CPU. Each register holds a small amount of data needed during one step of the fetch-decode-execute cycle.

Registers are much faster than RAM but only hold a handful of bytes. Each register has a specific job. The five you must know:

Name	Acronym	Purpose
Program Counter	PC	Holds the memory address of the next instruction to be fetched. Increments by 1 each time an instruction is fetched, so the CPU naturally walks through the program in order
Memory Address Register	MAR	Holds the memory address that the CPU is about to read from or write to. The CPU puts the MAR's contents onto the address bus
Memory Data Register

How to memorise their roles

A useful one-line answer template for register questions:

"The [] stores [what value]. The then uses it to [purpose during the cycle]."

For example: "The MAR holds the memory address of the next instruction, which is sent to memory along the address bus so the instruction can be fetched."

Exam tip

Identifying or describing a register's role comes up most years (the PC and MAR catch people out), so you need to know that the PC holds the address of the next instruction to fetch and the MAR holds the address about to be read from or written to — not an instruction.