课 12, 主题 2

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risc-vs-cisc

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RISC and CISC

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

RISC (Reduced Instruction Set Computer)

Reduced Instruction Set Computer (RISC) 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

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

What’s the difference between RISC & CISC?

	RISC	CISC
Features	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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