CISC & RISC
Computer Science: OCR A Level, topic 1.2.1
CISC & RISC Processors
Processors usually fall under one of two categories: CISC or RISC.
CISC
- CISC stands for Complex Instruction Set Computer.
- CISC processors have a large instruction set built into it.
CISC instruction set
- The large instruction set allows programmers of assembly language to use fewer instructions as the instruction set built-in would deal with the additional needed steps.
- Most of the built-in instructions are made up of multiple simple instructions.
- Only about 20% of the built-in instructions were used in the average program.
RISC
- RISC stands for Reduced Instruction Set Computer.
- A RISC processor has a small number of essential instructions built-in.
- Each of these instructions can be completed in a single clock cycle.
- This reduced instruction set means assembly programmers have to code more lines of code to achieve more complex results.
CISC vs RISC
- RISC processors have a library of simple efficient instructions whereas a CISC processor has a large library of complex instructions made up of simpler instructions.
- A RISC processor has few addressing modes where a CISC processor has many addressing modes.
- CISC processors are more expensive than RISC processors.
Test yourself on CISC & RISC
What does CISC stand for?
Which type of processor has a large instruction set built into it?