GATE CSE IT » Difference Between RISC and CISC Processor

Difference Between RISC and CISC Processor

RISC and CISC are two different types of instruction set architectures (ISA) used in most computer processors. Here is a comparison between CISC and RISC Processor.

Table of Contents

  1. What is RISC?
  2. What are CISC?
  3. Key differences between RISC and CISC
  4. Below are a few differences between RISC and CISC

You may have heard the terms Reduced Instruction Set Computer (RISC) & Complex Instruction Set Computer (CISC) if you are a computer enthusiast (CISC). And if you know anything about computers, you might realise that these names relate to various approaches to constructing a CPU. The RISC architecture, for example, is used by the Embedded system in your phone. On the other hand, the x86 CPU in your computer is a CISC architecture.

What is RISC?

It is an abbreviation of Low Instruction Set Computer and is pronounced the same as RISK. It is a sort of microprocessor that is meant to perform a few instructions at once. Until the 1980s, hardware manufacturers attempted to create CPUs that could execute a high number of commands at the same time. However, the tendency was reversed, and manufacturers chose to design computers that could perform relatively few instructions. Because the instructions were basic and few, CPUs could process them rapidly. Another benefit of RISC is that it uses fewer transistors, making it less expensive to manufacture.

Features of RISC

  • – Requires less decoding
  • – A standard set of instructions
  • – Identical general-purpose registers that may be utilised in any situation
  • – Basic addressing modes
  • – Fewer hardware data types

Advantages of RISC

  •  The RISC processor performs better due to its basic and constrained instruction set.
  • It necessitates using multiple transistors, making the design less expensive.
  • Because of its simplicity, RISC allows this instruction to utilise spare space on a CPU.

What is CISC?

CISC is an abbreviation for Complex Instruction Computer. It is a CPU that can do several tasks with a single instruction. These fundamental processes might include loading from memory, performing a mathematical calculation, etc.

CISC Features

  • – Difficult directions
  • – A greater variety of addressing modalities
  • – Extensive pipeline
  • – Extra data types within the hardware

Advantages of CISC

  • The data size is rather small, which reduces the memory demand.
  • It takes extremely little RAM to store instructions on each CISC.
  • CISC develops a power management technique that modulates clock frequency and voltage.

Key differences between RISC and CISC

The words RISC and CISC have practically become useless over time as both have evolved, and the distinction between the two has increasingly got muddled with both being utilised in computer systems. Many modern RISC chips provide the same instructions as yesterday’s CISC devices. There are CISC processors that employ the very same techniques that have been previously thought to be reserved for RISC chips exclusively. However, the fundamental distinctions between the two are simple to grasp and are described as follows. Regarding differences, RISC costs software developers by requiring them to write additional lines for similar activities. Because fewer transistors are required, RISC is less expensive than CISC. 

Below are a few differences between RISC and CISC:

RISCCISC
The architecture has many instructions. There are very few instructions. In most cases, the number of directions is fewer than 100.
Some commands take a long time to execute. These instructions include those that duplicate an entire city block from one section of memory to another and those that duplicate multiple registers from and to memory.Because of the limited instruction set, there are no instructions with extended execution times. Some early RISC computers lacked a numeric multiply instruction, forcing compilers to construct multiplication as a series of adds.

Instruction encodings with variable lengths.

For example, the size of an IA32 instruction can vary from one to 15 bytes.

The instructions are encoded using fixed-length encodings.

In IA32, all instructions are normally encoded as 4 bytes.

For specifying operands, many forms are supported. A storage operand identifier can have many distinct displacements, base, & index register combinations. Simple addressing formats can be used. Base & displacement addressing are the only options.
Both memory and register operands can be subjected to arithmetic and logical operations. Only register operands are used in arithmetic and logical operations. Storage referencing is only possible with loading & storing instructions, which are read from memory into the register and written from a registered to memory.
Machine-level programmes cannot see implementation programmes. The ISA establishes a clear separation between programmes and how they are executed.Machine-level programs are exposed to implementation programs. Only a few RISC devices enable specified instruction sequences.

Procedure inputs and return addresses are stored on the stack.

 

Procedure parameters and return addresses are stored in registers. Some processes can avoid using memory references.