What this chapter is about

A computer is an electronic machine that follows a simple cycle: it takes in input, does some processing, and gives useful output, and it can store data for later. In this chapter we look at the units that carry out this cycle, how memory keeps data, and how software controls the whole system.

1. The parts of a computer

Four functional units share the work. The input unit takes data in, the CPU processes it and controls everything, the memory stores data and results, and the output unit shows the result.

2. Inside the CPU

The CPU is the brain of the computer. It has three parts. The Arithmetic Logic Unit (ALU) does calculations and comparisons. The Control Unit (CU) directs the whole system, fetching and decoding instructions. Registers are very fast, very small stores that hold data while it is being processed.

3. Memory: primary and secondary

Primary memory is used directly by the CPU. RAM is the working memory and is volatile, so it loses its contents when the power is off. ROM is non volatile and keeps its start-up instructions. Secondary memory, such as the hard disk, SSD and pen drive, is slower but permanent.

4. Software and the operating system

Software is the set of programs that tell the hardware what to do. System software runs the computer (the operating system, utilities and device drivers). Application software does a task for the user, such as a word processor. A compiler translates a whole program at once, while an interpreter works line by line, and Python uses an interpreter. The operating system manages processes, memory, files and devices, and gives the user interface.

Practise with the interactive

Tap each unit in the block diagram to read its job and watch the data flow, then sort the memory types into volatile and non volatile. The interactive opens right here in the lesson.

Practice set A, multiple choice

Practice set B, short answer

Quick summary