Difference between RISC and CISC

RISC and CISC architectures are fundamentally the basis of all current computing architectures and affect the performance, power consumption, and complexity of programs in different ways, so it is necessary to understand both types of architectures. In this article, we will discuss what RISC and CISC are, their characteristics, advantages, disadvantages, and the major differences between RISC and CISC. 

Table of Contents:

• What is a Microprocessor?
• What is RISC?
• What is CISC?
• Difference between RISC and CISC
  • Instruction Set
  • Instruction Length
  • Execution Time
  • Registers
  • Size of Programs
  • Hardware Complexity
  • Pipelining
  • Power Consumption
• Conclusion

What is a Microprocessor?

A microprocessor is an integrated circuit that works as the central processing unit (CPU) of a computer. A microprocessor’s function is to perform arithmetic and logic operations, control and perform the transmission of data through the CPU, as well as control the memory of the computer. Microprocessors are also called the brains of a computer. A microprocessor is basically of three types: RISC, CISC, and EPIC.

What is RISC?

RISC stands for Reduced Instruction Set Computer. It is a type of microprocessor architecture that has fewer simple and general instructions. RISC minimizes the number of clock cycles, which makes execution faster by executing each instruction in one clock cycle.

Characteristics of RISC Processors

• RISC uses a small and fixed set of instructions.
• This type of processor has a fixed number of instructions that are all the same length.
• Instructions in RISC are executed in one clock cycle.
• It has many general-purpose registers.
• It has a load/store architecture in which memory is accessed only with load and store.

RISC Architecture

RISC architecture is a simpler instruction set architecture, regardless of its programming complexity. In this architecture, programs require many simple instructions more than a few complex ones. Using instruction sets that are simple in design and execution makes it easier for a processor to work on multiple instructions at one time, as a result making RISC architecture faster and more efficient.

Advantages of Reduced Instruction Set Computer (RISC)

• Faster execution due to single-cycle instructions.
• Easier to design, test, and maintain.
• More efficient pipelining.
• Lower power consumption.
• Better performance per watt.

Disadvantages of Reduced Instruction Set Computer (RISC)

• Larger program size due to more instructions required to perform complex tasks.
• May need more memory bandwidth.
• Software development can be more challenging for complex operations.

What is CISC?

CISC is short for Complex Instruction Set Computer. It is a type of microprocessor that is designed with a large set of instructions. In CISC, some of the instructions perform complex functions with a single instruction. CISC is designed in such a way that it decreases the instruction count per program.

Characteristics of CISC Processors

• It has a large and complex instruction set.
• Instructions can have variable lengths.
• Single instructions can carry out several low-level operations.
• Has fewer general-purpose registers.
• More complexity in hardware.

CISC Architecture

CISC architecture is designed in such a manner that complex instructions can be used in a computing device. It basically decreases instruction counts per program, so that some instructions may be able to perform more complex tasks and have shorter, more memory-efficient programs. 

Advantages of Complex Instruction Set Computer (CISC)

• Uses complex instructions easily to produce smaller programs.
• Requires less memory for the program to run.
• Assembly codes can be written easily for complex tasks.
• Works better with applications that have limited amounts of memory.

Disadvantages of Complex Instruction Set Computer (CISC)

• Execution is typically slower than a RISC processor because fewer instructions take more cycles to complete.
• More complex hardware design.
• It can be difficult to implement an efficient pipelining.
• Overall, the power consumption is greater than that of the RISC processor.
• Has higher power consumption.

Difference between RISC and CISC

Here are the main differences between RISC and CISC:

1. Instruction Sets

RISC uses a small number of simple instructions that perform basic operations and result in fast and predictable execution. While the CISC uses a large number of complex instructions, many of which can perform more than one task in a single instruction, which also decreases the number of instructions that a program would need to execute.

2. Instruction Length

RISC instructions are of a fixed instruction length, which allows for easier decoding and faster execution. While the CISC can have variable instruction lengths, which provides more flexibility, but also needs more complex hardware to decode.

3. Execution Time

Most RISC instructions are complete in a single clock cycle, which promotes faster and more efficient execution. On the other hand, the CISC instructions frequently take multiple clock cycles to execute, as they often perform more than one operation per instruction.

4. Registers

RISC architectures have many general-purpose registers that can hold temporary data, and thus reduce the memory accesses that are traditionally slower for maintaining data temporarily. In contrast, CISC only has a few registers and also has more frequent data access to memory to store temporary values.

5. Size of Programs

RISC programs are typically larger than CISC programs because the instructions that RISC handles fewer and simpler tasks than CISC’s complex instructions. Although individual RISC instructions consume fewer resources, the overall RISC program may end up being larger.

6. Hardware Complexity

Due to the small set of consistent instructions that RISC processors rely on for decoding and executing instructions, hardware complexity and optimization improved categorically. Alternatively, CISC instruction sets are complex and different (in terms of size).

7. Pipelining

Because of RISC’s simple, fixed-length instructions, it can use pipelining by enabling the execution of several instructions simultaneously at different stages. On the other hand, the CISC’s complex and variable-length instructions make pipelining complex and more difficult to implement.

8. Power Consumption

RISC processors also tend to consume less overall power due to simpler hardware, and they execute the instructions faster and in a more predictable way. While CISC tends to consume more power simply due to the complexity of the instruction decoding and execution units.

Here is a comparison table that shows the difference between RISC and CISC in a precise manner:

Conclusion

RISC and CISC are two different types of processors, and each has its own advantages and disadvantages. RISC uses simple and fast instructions, which makes this processor more efficient while consuming less power, while CISC uses complex instructions and is also designed in such a way as to minimize the size of a program in memory. When choosing between a RISC and a CISC, the choice would depend on an application’s needs based on power, performance, or compactness. So knowing the difference between RISC and CISC is very useful when determining which is better suited to your computing device, given your needs.

Difference between RISC and CISC – FAQs