## AQA Computer Science GCSE

**July 2019:**the AQA CompSci area is almost complete. Most of the content for units 3-6 is now up and there's quite a lot of Unit 7 material. There's still some work to do, particularly on Paper 1 units, but it's getting there.

### Data Representation - Hexadecimal Numbers

Hexadecimal numbers use **base 16** - so you can use one digit to county up to 15. The letters A to F are added on after 9, so A = 10, B = 11 and so on until F = 15. That gives you 16 possible values - including the 0.

Hexadecimal isn't used by computers, but it is an easy thing for humans to use when we're trying to talk in binary. And it converts really easily.

You need to be able convert hexadecimal to and from 8-bit binary numbers and to and from decimal numbers. Just remember that A = 10!

Hexadecimal basics - notes on how it all works. The list on slide three is super important to know.

The 16 times table - and how not to panic when doing the maths. Slides used in class.

Hexadecimal questions - some hex to binary conversion questions

Answers - don't cheat, try and work things out first

One really important thing to remember is that the hexadecimal number FF = 255 - which is the same as the 8 bit binary number 11111111. So, 255 is the largest number which can be represented using 8 bit binary (a byte) and a 2 digit hexadecimal number.

Just remember that **A = 10** and **F = 15**. It's really easy to get into thinking that A is 11 and F is 16. It's not! Get this right and hexadecimal is a piece of cake.

#### Conversions using Hexadecimal

You need to be able to convert from:

- decimal to hexadecimal
- hexadecimal to decimal
- hexadecimal to binary (8 bit binary only)
- binary to hexadecimal (8 bit binary only)

I will write up the methods for this properly at some point (perhaps), but here are some photos taken in class which summarise the methods:

It's interesting to note that it's generally much easier to convert Hex to Binary. It's only when decimal gets involved that things become trickier.

#### Revision

There's likely to be one or two marks on hexadecimal on the exam.

The BBC Bitesize website has a section on hexadecimal that you might want to look at. Well worth a look.