RISC vs CISC: Difference and Comparison

The term processor refers to an integrated electronic circuit that is responsible for performing various calculations on a computer.

Input/ output, arithmetical, logical, and various other instructions are performed by the processor on the operating system’s command.

Many electronic devices such as tablets, PCs, etc utilize processors. Common processors include a central processing unit, microprocessors, etc.

Key Takeaways

1. RISC processors execute simpler instructions, enabling faster processing times than CISC processors.
2. CISC processors have more complex instructions, allowing them to perform multiple operations in a single instruction.
3. RISC architecture is more energy-efficient, making it suitable for low-power devices, while CISC architecture excels in high-performance computing.

RISC vs CISC

RISC (Reduced Instruction Set Computing) processors have a simpler and more streamlined instruction set, with a focus on executing instructions quickly and efficiently. CISC (Complex Instruction Set Computing) processors have the ability to execute multiple operations in a single Instruction.

RISC is a simple microprocessor offering instructions sets that are highly customizable.

The main purpose of building this processor is to reduce the time taken for executing each instruction through a limit on the maximum number of instructions.

Each of the cycles comprises only three parameters: decode, fetch, and execute. Complex instructions can also be performed by this processor by using them with simpler ones.

CISC processor is a more complex processor when compared to RISC. This is because several operations are combined into a single instruction, thereby reducing program code.

The main purpose of using this processor in a computer is to reduce the end costs as a lesser number of instructions mean less space for memory and lesser memory costs.

Comparison Table

Parameters of Comparison	RISC	CISC
Full-Form	Reduced Instruction Set Computer	Complex Instruction Set Computer
Instructions	Fewer and fixed instruction format	More and variable instruction format
Modes	Less	More
Registers	Multiple	Single
Examples	Alpha, Power Architecture, ARM, PA-RISC, etc	VAX, Intel x86 CPUs, PDP-11, System/360
Power Consumption	Low	High
Average Clock cycles Per Instruction (CPI)	1.5	2 and 15
Performance optimization	Focuses on software for performance optimization	Focuses on hardware for performance optimization
Code Size	Small	Large

What is RISC?

RISC stands for Reduced Instruction Set Computer. It is a microprocessor architecture that is considered the best one for use in a CPU. It is used as a replacement for CISC architecture.

A CPU involving this technology makes use of simplified instructions to improve execution speed. As a result, performance is also improved.

Apart from improving the executing speed of instructions, internal parallelism is also increased by allowing several threads to execute simultaneously.

Moreover, it optimizes energy by providing high performance per watt. Only one action is executed in every instruction which in turn is executed in the time frame of one cycle.

This architecture makes use of a fixed instruction length, making it easier to pipeline.

More registers are supported by it and less time is spent by a RISC processor on committing values to memory and also loading. This is because of the absence of complex instructions.

With the use of RISC processors, smaller components are required bringing down the cost of chips and the number of chips required.

It makes use of simple decoding logic making it possible to work with fewer transistors, leaving space for more general-purpose registers. It is used devices such as Nintendo DS, Apple iPod, smartphones, etc.

What is CISC?

CISC refers to Complex Instruction Set Computer. It is a microprocessor that makes use of complex instructions to complete various tasks by using the minimum number of codes.

In this, each instruction set takes more than one cycle to complete with each instruction being of different lengths and nature.

Its approach is based on completing the entire operation by not using more than a few parts of the assembly language code. It was first used in 1970.

Instead of using multiple codes, it executes instructions using complex instruction set architecture. It supports Higher Level- Languages supporting an array of modes.

It emphasizes the system’s hardware and supports its developer.

It reduces the work of the compiler, unlike other microprocessors by generating instructions in which a single one comprises all the tasks needing to be performed. Some of the features of a CISC Processor include:

• Decoding: As the instructions given are complex, they require complex decoding.
• General Purpose Registers: Lesser number of general-purpose registers are needed as operations are performed and stored in memory reducing the need for these registers.
•  Clock Cycle: More than one clock cycle can be used as instructions’ size is varied.

A major drawback in the case of CISC processors is the lack of pipelining of instruction.

Main Differences Between RISC and CISC

1. RISC processors process one instruction in the time frame of one clock cycle. On the other hand, CISC requires more than one clock cycle to execute the same. The average clock cycles per instruction of RISC is 1.5 and that of CISC is 2 and 15.
2. RISC processors need more RAM as compared to CISC because it executes an instruction, acts on it, and then moves on to the next one.
3. RISC processors make use of simplified instruction to do the work in a divided manner. On the other hand, CISC does the same in one go through a complex instruction.
4. RISC uses lesser power and smaller code size as compared to CISC which uses more power and larger code size.
5. RISC focuses on software for performance utilization while CISC focuses on hardware.

References

1. https://ieeexplore.ieee.org/abstract/document/464688/
2. https://www.computer.org/csdl/proceedings-article/ssst/1990/00138185/12OmNzlUKsE

Last Updated : 30 June, 2023