Difference Between RISC and CISC

Key Difference between RISC and CISC Processors

  • In RISC, the instruction set is reduced, and most of these instructions are very primitive, while in CISC, the instruction set is very large and can be used for complex operations.
  • The RISC computer’s execution time is very low, whereas the CISC computer’s execution time is very high.
  • RISC code expansion may create a problem, while CISC code expansion is not a problem.
  • In RISC, the decoding of instructions is simple, whereas in CISC, the decoding of instructions is complex.
  • RISC doesn’t require external memory for calculations, but CISC requires external memory for calculations.
  • RISC has multiple register sets present, while CISC has only a single register set.
Difference Between RISC and CISC
Difference Between RISC and CISC

Here, I have analyzed the difference between RISC and CISC and will comprehensively evaluate their pros and cons.

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 a smaller number of computer instructions 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 instruction for accessing memory.

Characteristics of RISC

In our analysis, an important characteristic of RICS is:

  • Simpler instruction decoding
  • A number of general-purpose registers.
  • Simple Addressing Modes
  • There are 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

RISC Processor Examples

  • Alpha
  • ARC
  • ARM
  • AVR
  • MIPS
  • PIC
  • Power Architecture

RISC Advantages

From what I’ve observed, here are the key pros of RISC:

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

RISC Disadvantages

Below, I share the cons of RISC as we understand them.

  • The performance of the RISC processors depends on the programmer or compiler. The compiler plays an important role in 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.

What is CISC?

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

CISC eliminates the need for generating machine instructions for the processor. For example, instead of having to make a compiler and write lengthy machine instructions to calculate a square-root distance, a CISC processor offers the 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 as the PDP-11 and the DEC system.

Characteristics of CISC

In my experience, here are some important characteristics of CISC.

One instruction is needed to support multiple addressing modes.

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

CISC Processor Examples

  • Intel x86 CPUs
  • System/360
  • VAX
  • PDP-11
  • Motorola 68000 family
  • AMD

CISC Advantages

In my experience, here are the benefits of CISC:

  • In CISC, it is easy to add new commands to the chip without needing 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 in the case of CISC. The instruction sets can be written to match the structures of high-level languages.

CISC Disadvantages

Here, let me share the drawbacks of CISC that we’ve encountered.

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

RISC vs. CISC: Key Differences

From what I’ve observed, here are the important differences between RISC and CISC.

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

How to Choose Between a RISC and a CISC

I have observed that while RISC architectures excel in environments where simplicity and speed are paramount, CISC systems are indispensable for tasks that require comprehensive command sets.