Most users would say that there is no major difference between the RISC and CISC microprocessors. They may be correct partially because the purpose of these two types of microprocessors is basically the same.
However, there are more to it that are unknown to them and this article is specially created to make them more knowledgeable. When you read this article, you will know which kind of microprocessor will be more suitable for your kind of computing jobs.
- The RISC microprocessors reduce the number of cycles needed by an instruction but the CISC microprocessors reduce the number of instructions in a program.
- Every instruction in a RISC microprocessor has a fixed length but the length of the instructions in a CISC microprocessor is variable.
- The set of instructions in a RISC microprocessor is less complicated in comparison to the instruction sets of a CISC microprocessor which makes it easy to design a RISC.
- The lower number and less complicated set of instructions in the RISC microprocessors reduce the time taken by it to execute them in comparison to a CISC microprocessor.
- RISC supports simple addressing formats, it does not use any condition codes and it needs multiple registers to store instructions which the CISC does not.
In This Article
The 24 Differences Between RISC and CISC Microprocessors
RISC is the short for Reduced Instruction Set Computer and CISC is the acronym for Complex Instruction Set Computer.
The main idea of the RISC processors is to simplify the process by using an instruction set made up of the fundamental steps of loading, evaluating and storing operations.
It is more about how these instructions are used and not about having less instruction sets.
On the other hand, the main idea behind the CISC microprocessors is to use a single instruction to do all the jobs such as loading, evaluating and storing the applications.
The effect of the RISC architecture is that it reduces the number of cycles required for every instruction but at the same time it comes at the cost of an increased number of instructions for every program.
On the other hand, the goal of the CISC microprocessors is to reduce the number of instructions in each program, which however, increases the number of cycles for each instruction.
4. Instruction Length
Each and every instruction in the RISC processor comes with a fixed size. This makes it very easy to decode them and execute.
On the other hand, the instructions in the CISC processors come with varying lengths.
This significantly increases the time taken for processing in comparison to the RISC processors.
The RISC processors come with a set of instructions that are much less complicated.
On the other hand, the set of instructions of the CISC processors are much more complicated.
6. Design Comfort
Due to the less complicated nature, it makes designing a RISC CPU much easier, quicker, and cheaper.
However, in comparison, the more complicated set of architecture of the CISC processors makes designing them far more difficult and costlier.
Apart from that, it also takes a lot of time.
7. Number of Instructions
There are very few instructions present in the RISC architecture which is often less than 100.
On the other hand, in CISC, there is a large number of instructions present which may range anywhere between 100 and 250.
8. Execution Time
In RISC, there are no instructions that need a long execution time due to the very simple instruction set.
It may not even have an integer multiply instruction that needs the compilers to use multiplication as a series of additions.
On the other hand, in the CISC, there may be quite a few instructions that need a long time to execute such as instructions to copy a complete block from one part to another of the memory or copy several registers to and from the memory.
9. Format Support
RISC supports simple addressing formats for specifying operands such as base and displacement register addressing.
On the other hand, CISC supports multiple formats to specify operands in different combinations that include base, displacement, and index register.
10. Array Support
RISC processors do not support arrays but the CISC processors, in comparison, support arrays.
11. Arithmetic and Logical Operations
As for the arithmetic and logical operations, a RISC processor uses only register operands.
In RISC, memory referencing is allowed only by loading and storing the instructions which means reading from the memory into the register and writing from the register to the memory respectively.
On the other hand, the arithmetic and logical operations in the case of CISC processors are applicable to both the register and memory operands.
12. Implementations Programs
Implementation programs in RISC are typically exposed to machine-level programs where few processors may not allow a definite instruction sequence.
On the other hand, the implementation programs in CISC are usually hidden from machine-level programs where the Instruction Set Architecture offers a clear abstraction between the programs and the ways in which they are executed.
13. Condition Codes
In RISC usually there are no condition codes used but, in comparison, in CISC such condition codes are used.
14. Procedure Arguments and Return Addresses
In the RISC processors, usually the registers are used for procedure arguments and return addresses.
Also, in a few specific procedures even the memory references may be averted.
On the other hand, in the CISC processors, it is the stack that is used for procedure arguments and return addresses.
15. Optimizing Instruction Set
RISC emphasizes on the software for optimizing the instruction sets while, in comparison, the CISC emphasizes on the hardware.
In RISC, it is actually a hard wired unit of programming. On the other hand, in CISC, it is a microprogramming unit.
17. Register Set
RISC typically needs multiple register sets for storing instructions. In comparison, CISC needs only a single register set for the same purpose.
RISC involves a simple decoding of instructions but, in comparison, CISC involves a complex decoding of instructions.
19. Use of Pipeline
In the RISC processors the use of pipeline is pretty simple but, in the CISC processors, the use of pipeline is quite complex.
20. LOAD and STORE Use
RISC processors use independent LOAD and STORE instructions for register to register interaction of programs.
On the other hand, the CISC processor uses the LOAD and STORE instructions for memory to memory interaction of a program.
RISC typically can be used for high-end applications such as image and video processing, telecommunication, and others.
On the other hand, CISC architecture is typically used for low-end applications which include and are not limited to security systems, home automation, and others.
22. Memory Space Requirement
It takes more memory space for the programs written in RISC while, in comparison, in CISC less memory space is required to write a program.
23. Power Consumption
The RISC processors typically can operate by consuming low power, but, in comparison, the consumption of power in the CISC processors is pretty high.
24. Clock Cycles per Instruction
In the RISC processors the average Clock Cycles per Instruction or CPI is 1.5 while, in comparison, the average CPI of the CISC processors ranges between 2 and 15.
Which is Better – RISC or CISC Microprocessors?
The short and simple answer to this question is that both are good and none of these microprocessors are better than the other.
Each of these processors or architectures come with its significant pros and cons which makes them suitable for different users and in different scenarios.
Both these refer to the specific category of the processor, or to be more precise, the Instruction Set Architecture or ISA.
This is the specific way in which a processor communicates with the human user.
Ideally, both RISC and CISC are different designs for microprocessors developed with different architectures so that the performance is increased.
However, even though the goal is almost the same for both, it is achieved in different ways by each of them.
Generally speaking, most people think that the RISC processors are an improved version of the CISC processors.
If you want to choose the most appropriate Instruction Set Architecture for you, what you need to do is match those advantages and disadvantages along with their respective features with your computing needs and intended application.
Typically, the early computers, especially those that existed before the 1980s, came with the CISC architecture.
However, this was increasingly complex and therefore a relatively simple RISC Instruction Set Architecture was developed in the 1980s to overcome those complexities.
Ideally, when a RISC processor will perform a job in one instruction per clock cycle due to the architectural difference, the CISC processors will perform a series of different actions for the same.
Therefore, it will take several clock cycles and longer time to do the same job.
If you consider the architecture of RISC, it is highly customized, comprising a set of instructions that offer higher system reliability and therefore can be used in portable devices. These devices include:
- Apple iPod and
- Nintendo DS.
Some of the important features and characteristics of the RISC processors are:
- One cycle execution time
- Every CPI includes the fetch, decode and execute process
- Pipelining technique to carry out multiple stages or parts
- Large number of registers to store instructions and respond quickly in order to minimize interaction with the computer memory and
- Load and Store instruction to access memory location.
And, as for the architecture of the CISC processors, it embeds several operations on every program to reduce program code and memory cost. This however, makes these processors much more complex.
Few notable features and characteristics of the CISC processors are:
- Shorter length of codes which needs very little memory space
- Fewer instructions required for writing an application
- Easier encoding in assembly language
- Complex data structure
- Simple set of high-level languages
- Fewer registers
- Large number of addressing nodes usually ranging between 5 and 20
- Larger instructions than a single word and
- Emphasis on building instructions on hardware since it is faster than building on the software.
Knowing the advantages and disadvantages of each of these two types of processors will also help you a lot in making the right choice.
As for the advantages of the RISC processors, the list includes:
- The performance is better and faster because it comes with limited number and simple set of instructions
- It is cheaper to design since it uses several transistors
- It helps in the memory management due to its broad addressing capabilities and
- Using the free space by the instruction is allowed due to its simplicity.
On the other hand, the list of advantages of CISC processors includes:
- Little effort needed by the compiler to translate statement languages or high-level programs machine or assembly language
- Process created to manage power usage and adjust voltage and clock speed
- Easy to create new commands and add them into the chip since it does not need changing the structure of the set of instructions and
- Use of fewer instructions set.
As for the downsides of the RISC processor, the list includes:
- Performance variance depending on the code executed due to the dependence of the subsequent instructions on the previous instruction for executing in a cycle
- Using complex instructions frequently by the programmers and compilers and
- Need for a very fast and a large group of cache memory for saving different instructions to respond in quick time.
As for the downsides of the CISC processors, the list includes:
- Slower speed
- Reduced performance
- Difficult to execute the pipeline
- Need for more transistors which makes them quite large and
- Only a small portion of the existing instructions is used in an encoding event.
A few examples of the RISC architecture are:
- Power Architecture
- ARC and
In comparison, the examples of CISC are:
Most importantly, with every generation of computer chip hardware and instruction set, the CISC architecture gets more complex because the earlier generations of its family are mostly contained as a subset in the new versions.
And, as for the RISC architecture it needs the on-chip hardware to be reprogrammed continuously.
Also, the level of performance offered by it largely depends on the compiler or the programmer, especially when it is required to convert the CISC code to a RISC code.
So, after reading this article, you now surely will not mix up the two types of microprocessors, RISC and CISC, knowing about the differences in their architecture, instruction set numbers, and lots of other aspects including their respective pros and cons.