That Blue Square Thing

AQA Computer Science GCSE

May 2019: this area of the site is being built just now. I’m making progress but there are areas where there is no content yet. That will get added over the next 6 months or so.

Computer Systems - Fetch-Execute Cycle

The CPU processes data. To do that it needs to move data and instructions around the computer system - using buses to move things stored in RAM into and out of the CPU's registers.

The ways it does this can be simplified down to the Fetch-Decode-Execute cycle.

Diagram of CPU components

PDF iconFetch-Decode-Execute - slides from class

PDF iconRegisters - slides from class detailing how registers work

Decoding Instructions - Machine Code and Assembly Language

Note: most of this content is now Unit 2 content. It relates to Unit 4 as well as it helps develop an understanding of how the CPU works, but this will almost always be Paper 1 exam material.

Instructions for the CPU need to be converted into a low-level programming language called Machine Code. One Machine Code instruction is carried out on each clock pulse.

Assembly code is another low-level programming language that can be used to program computers and hardware devices directly. It has the advantage that it's easier to understand than Machine Code.

Each instruction in Assembly Code can be converted to a single Machine Code instruction.

PDF iconAssembly Code Instructions - for 6 x 7

PDF iconAssembly Code Trace Table