CISC AND RISC PROCESSOR

RISC and CISC Processors

RISC stands for Reduced Instruction Set Computer and

CISC stands for Complex Instruction Set Computer.

There are two approaches of the design of the control unit of a microprocessor i.e.-

• Hardware approach and
• Software approach.

RISC Processors:- To execute an instruction, a number of steps are required. By the control unit of the processor, a number of control signals are generated for each step. To execute each instruction, if there is a separate electronic circuitry in the control unit, which produces all the necessary signals, this approach of the design of the control section of the processor is called RISC design. It is hardware approach. It is also called hard-wired approach.

Examples of RISC processors are:

• DEC's Alpha 21064, 21164 and 21264 processors;
• SUN's SPARC and ULTRA SPARC;
• PowerPC processors etc.

CISC Processors:- If the control unit contains a number of micro electronic circuitry to generate a set of control signals and each micro circuitry is activated by a microcode, this design approach is called CISC design. This is a software approach of designing a control unit of the processor.

Examples of CISC processors are:

• Intel 386, 486;
• Pentium Pro, Pentium, Pentium II, Pentium III, Pentium 4;
• Motorola's 68000, 68020, 68030, 68040, etc.

Difference between RISC and CISC

S.No.	RISC	CISC
1.	Simple instruction set	Complex instruction set
2.	Consists of Large number of registers.	Less number of registers
3.	Larger Program	Smaller program
4.	Simple processor circuitry (small number of transistors)	Complex processor circuitry (more number of transistors)
5.	More RAM usage	Little Ram usage
6.	Simple addressing modes	Variety of addressing modes
7.	Fixed length instructions	Variable length instructions
8.	Fixed number of clock cycles for executing one instruction	Variable number of clock cycles for each instructions