CISC vs RISC (OCR A Level Computer Science)

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?

• 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