# That Blue Square Thing

## Computer Science GCSE

Note: this page deals with the old Edexcel GCSE (grades A-G) which no longer exists. I'm keeping it as an archive and because a number of the resources will still apply to newer (grades 1-9) courses. The AQA GCSE CompSci pages deal with a current course.

### Binary Numbers

Binary numbers use base 2. This means that only 1 and 0 are used. Computers use binary numbers for everything.

In particular:

• to store data
• to store instructions

You need to be able convert binary numbers to denary (base 10 - or "normal" numbers) and hexadecimal (and vice-versa) and to be able to do some basic maths in binary. There are also more complex elements you need to know, such as the ways in which negative and real numbers are dealt with.

Note that in general exam questions seem to deal only with 8-bit binary numbers.

#### Key Skills in Binary

The key conversion knowledge you need.

Binary basics - what it is and how to convert to and from denary

The remainder method - just the remainder method slide from the basics notes

Why Binary? - key knowledge elements

The key maths you need are addition, subtraction and multiplication.

Binary addition - adding up in binary

Binary subtraction - taking binary numbers away from each other

Complex binary subtraction - when you don't have anything to borrow from directly...

Binary multiplication - multiplying binary numbers

Make sure you understand the concept of overflow and the problems that it can cause. There's a helpful video on the BBC Bitesize website which might be helpful.

#### Revision exercises

Binary stuff comes up on exams all the time. There won't be a huge amount of it, but it's normally easy marks. Make sure you're happy dealing with all the key techniques and know all the technical terms.

Binary revision exercises - conversion and maths

Answers - no peeking before you have a go...

The BBC Bitesize website has a good section on binary and binary maths. Well worth a look, although some of it gets into more complex things such as negative numbers.