Table of contents

Introduction

What is Reduced Instruction Set Computer (RISC)?

2.1.

Architecture of RISC

2.2.

RISC Processor of Features

2.3.

Characteristics of RISC

2.4.

Advantages of RISC

2.5.

Disadvantages of RISC

Complex Instruction Set Computer (CISC)

3.1.

Architecture of CISC

3.2.

Features of CISC Processor

3.3.

Characteristics of CISC

3.4.

Advantages of CISC

3.5.

Disadvantages of CISC

RISC vs CISC in Computer Organization

Frequently Asked Questions

5.1.

How much Clock Cycle does RISC take to execute code?

5.2.

How much Clock Cycle does CISC take to execute code?

5.3.

Does RISC architectures simple or complex?

5.4.

How memory is accessed in CISC?

Conclusion

Last Updated: Oct 3, 2024

Easy

RISC vs CISC

Author Rajat Agrawal

Introduction

RISC stands for Reduced Instruction Set Computer Processor, and CISC stands for Complex Instruction Set Computer Processor, both are used to increase CPU performance.

RISC reduces the cycles per instruction at the cost of the number of instructions per program.

CISC tries to minimize the number of instructions per program but at the cost of increasing the number of cycles per instruction.

A few decades earlier, when the programming was done using assembly language, a need was felt to make instruction do more tasks because programming in assembly was tedious and error-prone; because of this, CISC architecture evolved but with the uprise of high-level language dependency on assembly reduced the RISC architecture prevailed.

Let’s understand both RISC and CISC concepts in-depth.

Recommended Topic, Microinstruction in Computer Architecture

What is Reduced Instruction Set Computer (RISC)?

RISC is a microprocessor architecture that uses a simple set of instructions that can be substantially modified. It is designed to reduce the time it takes for instructions to execute by optimizing and reducing the number of instructions. It means that each instruction cycle has only one clock per cycle, and each cycle consists of three parameters: fetch, decode, and execute. The RISC processor can also combine multiple complex instructions into a simple one. RISC chips require several transistors, making them less expensive to develop and reducing instruction execution time.

Examples of RISC processors are PowerPC, Microchip PIC, SUN's SPARC, RISC-V.

Architecture of RISC

RISC architecture is a computing paradigm that emphasizes simplicity and efficiency by using a smaller set of instructions with uniform execution times. It prioritizes performance optimization by streamlining instruction execution, minimizing complexity, and maximizing hardware utilization. RISC processors typically feature a simpler instruction set, fewer addressing modes, and a focus on pipelining and parallelism for faster execution. This architecture is prevalent in modern microprocessors, embedded systems, and high-performance computing environments due to its ability to deliver efficient, scalable, and cost-effective solutions.

RISC Processor of Features

Some of the crucial features of the RISC processor are:-

1.) RISC processors use one clock per cycle (CPI) to execute each instruction in a computer. Each CPI also comprises the methods for fetching, decoding, and executing computer instructions.

2.) Multiple registers in RISC processors allow them to hold instructions, reply fast to the computer, and interact with computer memory as little as possible.

3.) The RISC processors use the pipelining technique to execute multiple parts or stages of instructions to perform more efficiently.

4.) RISC has a simple addressing mode and fixed instruction length for the pipeline execution.

5.) It uses LOAD and STORE instruction to access the memory location.

Also See, Shift Registers in Digital Electronics and what is middleware

Characteristics of RISC

Simplified Instruction Set: RISC architectures feature a reduced and simplified instruction set compared to Complex Instruction Set Computing (CISC) architectures.
Uniform Instruction Format: Instructions in RISC architectures typically have a uniform length and format, simplifying decoding and execution.
Single Clock Cycle Execution: Most instructions execute in a single clock cycle, leading to predictable and consistent performance.
Register-based Operations: RISC architectures heavily rely on registers for operand storage and manipulation, reducing memory access overhead.
Hardwired Control: Control logic in RISC processors is often hardwired, resulting in faster instruction decoding and execution.
Pipelining: RISC architectures commonly employ pipelining techniques to overlap instruction execution stages, enhancing throughput.
Highly Optimized for Compiler: RISC designs are optimized for compiler efficiency, enabling better code generation and optimization strategies.

Advantages of RISC

Simplified instruction set facilitates faster decoding and execution.
Uniform instruction format simplifies hardware design and instruction decoding.
Single clock cycle execution ensures predictable and consistent performance.
Register-based operations reduce memory access overhead and enhance performance.
Hardwired control logic speeds up instruction decoding and execution.
Pipelining techniques increase throughput and overall system performance.
Optimized for compiler efficiency, enabling better code generation and optimization.

Disadvantages of RISC

Limited flexibility due to a reduced instruction set.
May require more instructions to perform complex operations compared to CISC architectures.
Increased reliance on compiler optimization for performance.
Reduced support for complex memory addressing modes.
Less suitable for applications requiring extensive hardware support for specific tasks.
Initial development and optimization may require more effort and resources.
Compatibility issues may arise when porting legacy software from CISC architectures.

Complex Instruction Set Computer (CISC)

Intel developed the CISC processor. It has an extensive collection of complex instructions that range from simple to very complex and specializes in the assembly language level, which takes a long time to execute the instructions. So, CISC approaches reducing the number of instructions on each program and ignoring the number of cycles per instruction. It emphasizes building complex instructions directly in the hardware because the hardware is always faster than software. However, CISC chips are relatively slower than RISC chips but use little instructions.

Examples of CISC processors are AMD, Intel x86, and the System/360.

Architecture of CISC

CISC architecture is a computing paradigm characterized by a rich and diverse instruction set, supporting complex operations that can be executed with a single instruction. Unlike RISC architectures, CISC processors can perform multi-step operations in a single instruction, which may include memory accesses, arithmetic operations, and control flow instructions. CISC architectures often feature variable-length instructions and a wide range of addressing modes to provide flexibility and convenience to programmers. While historically prevalent, modern CISC processors often incorporate RISC-like features to balance complexity and performance.

Features of CISC Processor

Some of the crucial features of the RISC processor are:-

1.) CISC may take longer than a single clock cycle to execute the code.

2.) The length of the code is short, so it requires minimal RAM.

3.) It provides more accessible programming in assembly language.

4.) It focuses on creating instructions on hardware rather than software because they are faster to develop.

5.) It comprises fewer registers and more addressing nodes, typically 5 to 20.

Characteristics of CISC

Rich Instruction Set: CISC architectures feature a wide range of complex instructions, allowing for versatile operations in a single instruction.
Variable-Length Instructions: Instructions in CISC architectures may vary in length, accommodating diverse operations and addressing modes.
Memory Access: CISC processors often support direct memory access and complex addressing modes, reducing the need for explicit memory management instructions.
Microcoding: Complex instructions in CISC architectures are typically implemented using microcode, enabling flexibility and abstraction.
Hardware Support for High-Level Constructs: CISC architectures may include hardware support for high-level language constructs, such as string manipulation and control structures.
Efficient for Complex Algorithms: CISC architectures excel at executing complex algorithms with fewer instructions, reducing program size and memory overhead.
Backward Compatibility: CISC architectures often maintain backward compatibility with legacy software and instruction sets, ensuring seamless integration and support for existing applications.

Advantages of CISC

Rich instruction set enables versatile operations in a single instruction.
Variable-length instructions accommodate diverse operations and addressing modes.
Memory access and complex addressing modes reduce the need for explicit memory management instructions.
Microcoding allows for flexibility and abstraction in implementing complex instructions.
Hardware support for high-level language constructs enhances programmer productivity.
Efficient execution of complex algorithms with fewer instructions.
Backward compatibility ensures seamless integration with legacy software and instruction sets.

Disadvantages of CISC

Complexity of instruction set architecture may lead to longer instruction decoding times.
Variable-length instructions can complicate instruction fetching and decoding.
Microcoding introduces overhead and may impact performance.
Limited optimization opportunities for compiler-generated code due to complex instruction set.
Increased hardware complexity may result in higher power consumption and heat dissipation.
Compatibility issues may arise when transitioning to newer architectures or software environments.
Reduced emphasis on pipelining and parallelism may limit overall performance scalability.

RISC vs CISC in Computer Organization

The major difference between RISC and CISC in Computer Organization is listed below:-

Feature	RISC	CISC
Instruction Set	Reduced and simple instruction set	Rich and complex instruction set
Instruction Length	Fixed-length instructions	Variable-length instructions
Addressing Modes	Limited addressing modes	Wide range of addressing modes
Instruction Encoding	Simple encoding schemes	Complex encoding schemes
Pipeline Efficiency	Emphasizes pipelining and parallelism	Less emphasis on pipelining and parallelism
Execution Speed	Typically faster due to simpler design	May be slower due to more complex instructions
Memory Access	Registers heavily used for operations	Supports direct memory access and complex modes
Hardware Complexity	Reduced hardware complexity	Higher hardware complexity due to microcoding
Power Consumption	Generally lower power consumption	May consume more power due to complexity
Backward Compatibility	Less emphasis on backward compatibility	Often maintains backward compatibility with older architectures

Read about Repeater in Computer Network

Frequently Asked Questions

How much Clock Cycle does RISC take to execute code?

RISC architectures typically execute code in a single clock cycle per instruction, aiming for simplicity and efficiency, resulting in faster execution times.

How much Clock Cycle does CISC take to execute code?

CISC architectures may require multiple clock cycles to execute code, especially for complex instructions, potentially leading to longer execution times compared to RISC.

Does RISC architectures simple or complex?

RISC architectures prioritize simplicity, featuring a reduced and straightforward instruction set, with most instructions executing in a single clock cycle.

How memory is accessed in CISC?

In CISC architectures, memory access is facilitated through a wide range of addressing modes, allowing for direct memory access and complex memory operations, which contribute to the architecture's versatility and flexibility.

Conclusion

In this blog, we have learned about RISC vs CISC. While RISC architectures prioritize simplicity, efficiency, and faster execution times with single-cycle instructions, CISC architectures offer versatility, flexibility, and support for complex operations with multi-cycle instructions.