RISC and CISC | Computer Architecture and Organization

Exam Notes For COA

What is RISC and CISC  With Characteristic (Important)

                                   OR

Instruction set based Classification of  Processor

Anwser.  The important Aspect of the computer Architecture is the design of Instruction Set  for the Processor.

 Instruction set of processor includes the two types i.e

(a) RISC  ( Reduced Instruction Set of Computer)

(b) CISC (Complex Instruction  set of Computer)

What is RISC?

A reduced instruction set computer is a computer that only uses simple commands that can be divided into several instructions that achieve low-level operation within a single CLK cycle, as its name proposes “Reduced Instruction Set”.

RISC Architecture

The term RISC stands for ‘’Reduced Instruction Set Computer’’. It is a CPU design plan based on simple orders and acts fast.

RISC Architecture

This is a small or reduced set of instructions. Here, every instruction is expected to attain very small jobs. In this machine, the instruction sets are modest and simple, which help in comprising more complex commands. Each instruction is of a similar length; these are wound together to get compound tasks done in a single operation. Most commands are completed in one machine cycle. This pipelining is a crucial technique used to speed up RISC machines.

Characteristic of RISC –

1. Simpler instruction, hence simple instruction decoding.
2. Instruction come under size of one word.
3. Instruction take single clock cycle to get executed.
4. More number of general purpose register.
5. Simple Addressing Modes.
6. Less Data types.
7. Pipeling can be achieved.

What is CISC?

A complex instruction set computer is a computer where single instructions can perform numerous low-level operations like a load from memory, an arithmetic operation, and a memory store or are accomplished by multi-step processes or addressing modes in single instructions, as its name proposes “Complex Instruction Set ”.

CISC Architecture

The term CISC stands for ‘’Complex Instruction Set Computer’’. It is a CPU design plan based on single commands, which are skilled in executing multi-step operations.

CISC Architecture

CISC computers have small programs. It has a huge number of compound instructions, which takes a long time to perform. Here, a single set of instructions is protected in several steps; each instruction set has additional than 300 separate instructions. Maximum instructions are finished in two to ten machine cycles. In CISC, instruction pipelining is not easily implemented.

Characteristic of CISC –

1. Complex instruction, hence complex instruction decoding.
2. Instruction are larger than one word size.
3. Instruction may take more than single clock cycle to get executed.
4. Less number of general purpose register as operation get performed in memory itself.
5. Complex Addressing Modes.
6. More Data types.

Difference between RISC and CISC Architecture

Difference between RISC and CISC

 RISC                                                          CISC

Focus on software	Focus on hardware
Uses only Hardwired control unit	Uses both hardwired and micro programmed control unit
Transistors are used for more registers	Transistors are used for storing complex Instructions
Fixed sized instructions	Variable sized instructions
Can perform only Register to Register Arthmetic operations	Can perform REG to REG or REG to MEM or MEM to MEM
Requires more number of registers	Requires less number of registers
Code size is large	Code size is small
A instruction execute in single clock cycle	Instruction take more than one clock cycle
A instruction fit in one word	Instruction are larger than size of one word