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Exam Board:


1.1a: The CPU & The FDE Cycle


A-Level 2015 

The Central Processing Unit (CPU) is the most important component in any computer system.

The purpose of the CPU is to process data and instructions by constantly repeating the fetch - decode - execute cycle.

CPU Components

The control unit directs the flow of data and information into the CPU.


It also controls the other parts of the CPU.

ALU stands for ‘Arithmetic and Logic Unit’.


It performs simple calculations and logical operations.

CPU Diagram (without Cache).png

The registers are temporary storage spaces for data and instructions inside the CPU.

The registers are used during the FDE cycle.

Five essential registers are explained below.

Important Registers

A register is a small storage space for temporary data in the CPU.


Each register has a specific role. There are five essential registers used in the FDE cycle:

Program Counter (PC) 


A register that tracks the RAM address of the next instruction to be fetched.

RAM (1).png

Memory Address Register (MAR) 


A register that tracks the RAM address of data that is currently being accessed.

RAM (1).png

Memory Data Register (MDR)

The MDR stores the data that  is transferred from RAM to the CPU.