2.1.1 – Outline the architecture of the central processing unit (CPU) and the functions of the arithmetic logic unit (ALU) and the control unit (CU) and the registers within the CPU

The CPU comprises three main components: the control unit, the Arithmetic and Logic Unit (ALU) and the registers.

Control Unit

  • The control unit sends out signals to other parts of the computer system and is responsible for organising the fetching, decoding and execution of instructions.

Arithmetic Logic Unit (ALU)

  • The ALU carries out arithmetic operations (+, -, * , /, etc.) and logic operations (AND, NOT, etc.) It can include the ability to compare a number with zero or to test if two numbers are equal.


The registers are individual storage locations which hold an instruction, data or address of a memory location.

Each type of processor has a number of different registers each designed for a specific function to hold a particular bit of information.

Most processors have the following registers:

Instruction Register or Current Instruction Register

  • The ‘instruction register’ holds the instruction that is currently being executed by the processor.


  • The ‘accumulator’ is a register that holds the accumulated total of results performed in the ALU.

Program Counter

  • The ‘program counter’ is a register that holds the memory address of the next instruction to be executed.

2.1.2 – Describe primary memory. Distinguish between random access memory (RAM) and read-only memory (ROM), and their use in primary memory

What is primary memory?

Primary storage, also known as main storage or memory, is the area in a computer in which data is stored for quick access by the computer’s processor. 

Random Access Memory (RAM) – Primary Memory

Contains the data and instructions the computer has loaded since starting up and everything the user has opened/loaded.

Is volatile = loses its contents if power is lost

Read Only Memory – ROM

  • Originally its contents were static (hence ‘read only’) and could not be changed – not true any more (“flash upgrades”).
  • Non-volatile = does not lose its contents if power is lost
  • Stores the BIOS (Basic Input Output System) – a small program that allows the computer to know what to do to find the operating system to ‘boot’ the computer after power is restored.


2.1.3 – Explain the use of cache memory

A type of small, high-speed memory inside the CPU used to hold frequently used data, so that the CPU needs to access the much slower RAM less frequently

2.1.4 – Explain the machine instruction cycle

2.1.5 – Identify the need for persistent storage

  • Once the CPU has done the processing (which has no permanent storage)
  • It is stored in RAM. As this is volatile it will clear all the data when the power is lost
  • It is therefore imperative that computers have persistent storage

What is Secondary Memory?

Secondary memory is non-volatile, which means it does not require an electrical current to retain its data. Therefore, secondary memory is sometimes called “persistent storage” or “permanent memory.”

Secondary memory is where programs and data are kept on a long-term basis. Common secondary storage devices are the hard disk and optical disks.

The hard disk has enormous storage capacity compared to main memory. The hard disk is usually contained inside the case of a computer.