CISC vs RISC: Difference Between Architectures, Instruction Set

What is CISC?

CISC was developed to make compiler development easier and simpler. The full form of CISC is Complex Instruction Set Computer. They are chips that are easy to program that makes efficient use of memory.

CISC eliminates the need for generating machine instructions to the processor. For example, instead of having to make a compiler, write lengthy machine instructions to calculate a square-root distance, a CISC processor offers a built-in ability to do this.

Many of the early computing machines were programmed in assembly language. Computer memory was slow and expensive. CISC was commonly implemented in such large computers, such as the PDP-11 and the DEC system.

In this tutorial, you will learn:

• What is CISC?
• What is RISC?
• Characteristics of CISC
• Characteristics of RISC
• Difference between CISC and RISC
• CISC Advantages
• RISC Advantages
• CISC Disadvantages
• RISC Disadvantages

What is RISC?

RISC is designed to perform a smaller number of types of computer instruction. Hence, it can operate at a higher speed. The full form of RISC is Reduced Instruction Set Computers. It is a microprocessor that is designed to perform smaller number of computer instruction so that it can operate at a higher speed.

RISC instruction sets hold less than 100 instructions and use a fixed instruction format. This method uses a few simple addressing modes that use a register-based instruction. In this compiler development mechanism, LOAD/STORE is the only individual instructions for accessing memory.

Characteristics of CISC

Here, are important characteristics Of CISC

One instruction is needed to support multiple addressing modes.

• A large number of instructions.
• Instruction-decoding logic will be complex.
• Instructions for special tasks used infrequently.
• A large variety of addressing modes
• It offers variable-length instruction formats.
• Instruction are larger than one-word size.
• Instruction may take more than a single clock cycle to get executed.
• Less number of general-purpose registers as operation get performed in memory itself.
• Various CISC designs are set up with two special registers for the stack pointer for managing interrupts

Characteristics of RISC

Here, are an important characteristic of RICS:

• Simpler instruction decoding
• A number of general-purpose registers.
• Simple Addressing Modes
• Fewer Data types.
• A pipeline can be achieved
• One instruction per cycle
• Register-to-register operations
• Simple instruction format
• Instruction execution would be faster
• Smaller Programs

Difference between CISC and RISC

Here, are important differences between CISC vs. RISC

CISC	RISC
It has a microprogramming unit.	It has a hard-wired unit of programming.
The instruction set has various different instructions that can be used for complex operations.	The instruction set is reduced, and most of these instructions are very primitive.
Performance is optimized with emphasis on hardware.	Performance is optimized which emphasis on software
Only single register set	Multiple register sets are present
They are mostly less or not pipelined	This type of processors are highly pipelined
Execution time is very high	Execution time is very less
Code expansion is not a problem.	Code expansion may create a problem.
Decoding of instructions is complex.	The decoding of instructions is simple.
It requires external memory for calculations	It doesn’t require external memory for calculations
Examples of CISC processors are the System/360, VAX, AMD, and Intel x86 CPUs.	Common RISC microprocessors are ARC, Alpha, ARC, ARM, AVR, PA-RISC, and SPARC.
Instructions can take several clock cycles	Single-cycle for each instruction
More efficient use of RAM than RISC	Heavy use of RAM (can cause bottlenecks if RAM is limited)
Simple, standardized instructions	Complex and variable-length instructions
A small number of fixed-length instructions	A large number of instructions
Limited addressing modes	Compound addressing modes
Important applications are Security systems, Home automation.	Important applications are : Smartphones, PDAs.
Varying formats (16-64 bits for each instruction).	fixed (32-bit) format
Unified cache for instructions and data.	Separate data and instruction cache.

CISC Advantages

Here, are pros/benefits of CISC

• In CISC it is easy to add new commands into the chip without need to change the structure of the instruction set
• This architecture allows you to make efficient use of main memory
• The compiler should not be very complicated, as with the case of CISC. The instruction sets can be written to match the structures of high-level languages.

RISC Advantages

Here, are pros/benefits of RISC

• Complex and efficient machine instructions.
• It offers extensive addressing capabilities for memory management.
• Relatively few registers when compared with RISC processors
• It helps you to reduce the instruction set.
• Offers limited addressing schemes for memory operands

CISC Disadvantages

Here, are Cons/ Drawbacks of CISC

• Earlier generations of a processor family mostly contained as a subset in every new version. Hence, instruction set & chip hardware becomes complex with each generation of computers.
• The performance of the machine slows down because of clock time taken by different instructions will never be similar.
• They are larger as they require more transistors

RISC Disadvantages

Here, are Cons/Drawbacks of RISC

• The performance of the RISC processors depends on the programmer or compiler. Compiler plays an important role while converting the CISC code to a RISC code
• RISC processors have large memory caches on the chip itself.
• RISC architecture necessitates on-chip hardware to be continuously reprogrammed.