CISC vs RISC (OCR A Level Computer Science)
Revision Note
Author
Jamie WoodExpertise
Maths
CISC & RISC
A computer processor will have an instruction set that it can use to execute programs
This will vary from one processor to the next
There are 2 types of processors:
Complex Instruction Set Computer
Reduced Instruction Set Computer
CISC (Complex Instruction Set Computer)
Complex Instruction Set Computer (CISC) consists of a larger instruction set which includes more complex instructions
As the instructions are more complex, they can take more than one clock cycle to execute
Has more general purpose registers
Instructions take up less space in memory
Is usually used in laptops and desktop computers
RISC (Reduced Instruction Set Computer)
Reduced Instruction Set Computer consists of a smaller instruction set with more simple instructions
Each instruction takes one clock cycle to execute which makes it more suitable for pipelining
Compilers are more complicated so will generate more instructions
Has fewer addressing modes
Is usually used in smartphones and tablets
What’s the Difference Between RISC & CISC?
|
|
|
---|---|---|
Feature | Has fewer transistors | Has more transistors |
Takes one clock cycle per instruction | Takes multiple clock cycles per instruction | |
Suited to pipelining | Not suited to pipelining | |
Compilers are more complicated | Compilers are less complicated | |
Has fewer general purpose registers | Has more general purpose registers | |
Used in smartphones and tablets | Used in laptops and desktops | |
Has fewer addressing modes | Has more addressing modes | |
Benefits / Drawbacks | Requires less power | Requires more power |
Costs less to manufacture | Costs more to manufacture | |
Takes up more space in memory | Takes up less space in memory |
A program that has been written for a RISC processor won’t work on a CISC processor and vice versa
A program that has been written for a RISC processor won’t necessarily work on another RISC processor as they may have different instruction sets
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