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.

Negative Binary Numbers

Negative numbers are trickier to deal with in binary. There are two methods you need to be able to use:

• sign and magnitude
• Two's Complement

Sign and magnitude is relatively straight forward, but less useful when doing binary maths. Two's complement is much more useful - but also quite hard to get your head around!

Negative numbers in binary - both methods in one

Two's Complement - just Two's complement using 8-bit numbers

It's important to be able to deal with both positive and negative numbers using sign and magnitude and Two's Complement. Make sure you know how to deal with them in 8-bit binary as this tends to be the way they ask questions on exams.

There's a very handy tool for converting Two's Complement numbers at Exploring Binary. Handy for checking you have answers right and confirming that you know what you're doing.

Revision exercises

This stuff is tricky, so it's well worth going through. There will only be a few marks to get here, but they're well worth getting.

Negative number revision exercises - both sign and magnitude and Two's Complement

The BBC Bitesize website has a section on negative binary numbers which is worth looking through. There's a particularly useful video on Two's Complement.