What is the Difference between RISC and CISC Architecture

The architecture of the Central Processing Unit (CPU) operates the capacity to function from “Instruction Set Architecture” to where it was designed. The architectural design of the CPU is Reduced instruction set computing (RISC) and Complex instruction set computing (CISC). A processor like CISC has the capacity to perform multi-step operations or addressing modes within one instruction set. It is the CPU design where one instruction works several low-level acts. For instance, memory storage, loading from memory, and an arithmetic operation. Reduced instruction set computing is a Central Processing Unit design strategy based on the vision that a basic instruction set gives great performance when combined with a microprocessor architecture that has the capacity to perform the instructions by using some microprocessor cycles per instruction. This article discusses the difference between RISC and CISC architecture. The hardware part of the Intel is named as Complex Instruction Set Computer (CISC), and Apple hardware is Reduced Instruction Set Computer (RISC).


Difference between RISC and CISC Architecture

Before we discuss the differences between the RISC and CISC architecture let us know about the concepts of RISC and CISC

RISC and CISC Processors
RISC and CISC Processors

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”.


The RISC is a Reduced Instruction Set Computer microprocessor and its architecture includes a set of instructions that are highly customized. The main function of this is to reduce the time of instruction execution by limiting as well as optimizing the number of commands. So each command cycle uses a single clock cycle where every clock cycle includes three parameters namely fetch, decode & execute.

The kind of processor is mainly used to execute several difficult commands by merging them into simpler ones. RISC processor needs a number of transistors to design and it reduces the instruction time for execution. The best examples of RISC processors include PowerPC, SUN’s SPARC, RISC-V, Microchip PIC processors, etc.

RISC Architecture

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


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.

Characteristics

The characteristics of RISC include the following.

  • Pipeline architecture
  • The number of instructions is restricted as well as decrease
  • The instructions like load as well as store have right of entry to memory
  • Addressing modes are less
  • Instruction is uniform and its format can be simplified

Advantages

The advantages of the RISC processor include the following.

  • The performance of this processor is good because of the easy & limited no. of the instruction set.
  • This processor uses several transistors in the design so that making is cheaper.
  • RISC processor allows the instruction to utilize open space on a microprocessor due to its simplicity.
  • It is very simple as compared with another processor due to this; it can finish its task within a single clock cycle.

Disadvantages

The disadvantages of a CISC processor include the following.

  • The performance of this processor may change based on the executed code because the next commands may depend on the earlier instruction for their implementation within a cycle.
  • The complex instruction is frequently used by the compilers and programmers
  • These processors need very quick memory to keep different instructions that use a huge collection of cache memory to react to the command within less time.

What is CISC?

It was developed by the Intel Corporation and it is Complex Instruction Set Computer. This processor includes a huge collection of simple to complex instructions. These instructions are specified in the level of assembly language level and the execution of these instructions takes more time.

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 ”.

So, this processor moves to decrease the number of instructions on every program & ignore the number of cycles for each instruction. It highlights to assemble complex instructions openly within the hardware as the hardware is always as compared with software. However, CISC chips are relatively slower as compared to RISC chips but utilize small instruction as compare with RISC. The best examples of the CISC processor include AMD, VAX, System/360 & Intel x86.

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 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.

Characteristics

The main characteristics of the RISC processor include the following.

  • CISC may take more time to execute the code as compared with an only clock cycle.
  • CISC supports high-level languages for simple compilation and complex data structure.
  • It is collected with more addressing nodes, fewer registers normally from 5 to 20.
  • For writing an application, less instruction is required
  • The code length is very short, so it needs extremely small RAM.
  • It highlights the instruction on hardware while designing as it is faster to design than the software.
  • Instructions are larger as compared with a single word.
  • It gives simple programming within assembly language.

Advantages

The advantages of CISC include the following.

  • This processor will create a procedure to handle the usage of power that regulates the speed of clock & voltage.
  • In the CISC processor, the compiler needs a small effort to change the program or statement from high-level to assembly otherwise machine language.
  • A single instruction can be executed by using different low-level tasks
  • It doesn’t use much memory due to a short length of code.
  • CISC utilizes less instruction set to execute the same instruction as the RISC.
  • The instruction can be stored within RAM on every CISC

Disadvantages

The disadvantages of CISC include the following.

  • The existing instructions used by the CISC are 20% within a program event.
  • As compared with the RISC processor, CISC processors are very slow while executing every instruction cycle on every program.
  • This processor use number of transistors as compared with RISC.
  • The pipeline execution within the CISC will make it difficult to use.
  • The machine performance reduces because of the low speed of the clock.

Difference between RISC and CISC Architecture

Difference between RISC and CISC
Difference between RISC and CISC

RISC

CISC

1. RISC stands for Reduced Instruction Set Computer.1. CISC stands for Complex Instruction Set Computer.
2. RISC processors have simple instructions taking about one clock cycle. The average clock cycle per instruction (CPI) is 1.52. CSIC processor has complex instructions that take up multiple clocks for execution. The average clock cycle per instruction (CPI) is in the range of 2 and 15.
3. Performance is optimized with more focus on software3. Performance is optimized with more focus on hardware.
4. It has no memory unit and uses separate hardware to implement instructions..4. It has a memory unit to implement complex instructions.
5. It has a hard-wired unit of programming.5. It has a microprogramming unit.
6. The instruction set is reduced i.e. it has only a few instructions in the instruction set. Many of these instructions are very primitive.6. The instruction set has a variety of different instructions that can be used for complex operations.
7. The instruction set has a variety of different instructions that can be used for complex operations.7. CISC has many different addressing modes and can thus be used to represent higher-level programming language statements more efficiently.
8. Complex addressing modes are synthesized using the software.8. CISC already supports complex addressing modes
9. Multiple register sets are present9. Only has a single register set
10. RISC processors are highly pipelined10. They are normally not pipelined or less pipelined
11. The complexity of RISC lies with the compiler that executes the program11. The complexity lies in the microprogram
12. Execution time is very less12. Execution time is very high
13. Code expansion can be a problem13. Code expansion is not a problem
14. The decoding of instructions is simple.14. Decoding of instructions is complex
15. It does not require external memory for calculations15. It requires external memory for calculations
16. The most common RISC microprocessors are Alpha, ARC, ARM, AVR, MIPS, PA-RISC, PIC, Power Architecture, and SPARC.16. Examples of CISC processors are the System/360, VAX, PDP-11, Motorola 68000 family, AMD, and Intel x86 CPUs.
17. RISC architecture is used in high-end applications such as video processing, telecommunications, and image processing.17. CISC architecture is used in low-end applications such as security systems, home automation, etc.

Key Differences between RISC and CISC

The key differences between RISC and CISC include the following.

  • The size of an instruction set is small as compared with RISC.
  • In RISC, the CPU control can be done with hardwired without comprising a control memory whereas CISC is micro coded that uses ROM, however, the current CISC processor also utilizes hardwired control.
  • RISC processor works with 32-bits for each instruction and frequently based on the register while CISC utilizes an uneven format that ranges from 16 bits to 64 bits for each instruction.
  • RISC architecture includes the design of instruction cache and split data whereas CISC architecture includes a unified cache intended for data & instructions, even though most recent designs also utilize split caches.
  • In the RISC processor, the mechanism of memory used is register to register including the instructions like STORE & independent LOAD. In CISC, the mechanism of memory used is memory to memory to execute different operations including the instructions like LOAD & STORE.
  • The general purpose registers used in the RISC processor are 32 to 192 whereas RISC uses 8 to 24 GPR’s.
  • In the RISC processor, the single clock is used, and addressing modes are limited whereas, in CISC, it uses the multi clock, and addressing modes ranges from 12 to 24.
  • The difference between RISC and CISC instruction set is, RISC ISA highlights software as compared with hardware. The instruction set of the RISC processor uses more efficient software like code or compilers through fewer instructions. CISC ISAs employ a number of transistors within the hardware to execute several instructions as well as additional complex instructions also.

The advantages of RISC over CISC include the following.

In the current developments of computer processors, the RISC (reduced instruction set computer) microprocessor is the most frequently used and significant one. Beneath certain conditions, the devices based on this processor will offer important benefits over CISC (complex instruction set computer). In the above, a brief comparison between both the processors is discussed.

The RISC processor performance is two to four times higher as compared with CISC processors due to the basic instruction set. The architecture of this processor uses very little space because of the decreased instruction set and this will make additional functions such as memory management or floating-point arithmetic units on a similar chip.

This article discusses the concepts of RISC, CISC, and differences. When the first microprocessors, as well as microcontrollers, were introduced, there is no better and suitable architecture. Once these processors were implemented, the CISC architecture is used mostly due to the lack of software support in the RISC processor. This is mainly doing to build all their hardware as well as software back well-suited through their first 8086 processors. We hope that you have got a better understanding of this concept. Furthermore, for any doubts regarding this concept, or implementation of any electrical and electronic projects, please give your feedback by commenting on the comment section below.

10 Comments

  1. Charlie Richard says:

    Which is better, RISC or CISC?

    1. Tarun Agarwal says:

      Hi Charlie Richard
      RISC has many more advantages over CISC.So RISC is better

  2. Manish Maurya says:

    Aap class nahi lete hai sir YouTube pe

    1. Tarun Agarwal says:

      Hi Manish Maurya
      No, we don’t take any classes.

  3. Chitrakala says:

    Very useful. clear & crisp explanation

  4. Anuj Gandhat says:

    thanks for risc and cisc very helpful

  5. Ashok amirtharaj says:

    Nice…..this help me prepare my exams….

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