Difference Between RISC and CISC

The main difference between RISC and CISC is that the RISC (Reduced Instruction Set Computer) contains a small and a highly optimized set of instructions while the CISC (Complex Instruction Set Computer) contains a large, specialized and a complex set of instructions.

A processor is also known as the Central Processing Unit (CPU). It mainly performs the instruction fetching, decoding, execution. There are two types of instruction set architectures. They are RISC and CISC. There are differences between RISC and CISC in complexity, instruction and data formats, registers, etc. RISC has a simple set of instructions. Therefore, the program length is long. CISC has a complex set of instructions, so the program length is minimum. A single CISC instruction can perform an equivalent task related to multiple RISC instructions. Nowadays, there are hybrid processors that contain both RISC and CISC.

Key Areas Covered

1. What is RISC
     – Definition, Functionality, Characteristics 
2. What is CISC
     – Definition, Functionality, Characteristics 
3. Difference Between RISC and CISC
     – Comparison of Key Differences

Key Terms

CISC, RISC, OS

What is RISC

RISC stands for Reduced Instruction Set Computer. It is designed to reduce the execution time by simplifying the instruction set. It uses highly optimized instructions. RISC based processors are commonly used for portable devices such as mobile phones and tablets as they are more efficient.

Figure 1: RISC

In RISC architecture, the Hardwired Control Unit connects to the instruction cache. The data cache connects to the control unit via the data path. Both instruction and data from the instruction cache and data cache will be sent into the main memory.

RISC is a simple instruction set. It utilizes simple addressing modes. To access the memory locations, it uses the load and stores instructions. It also consists of a large number of registers. Moreover, each instruction requires one clock cycle to execute an instruction.  

What is CISC

CISC stands for Complex Instruction Set Computer. The main objective of designing CISC is to reduce the number of instructions in a program. Therefore, the same task can be obtained with a less number of instructions using CISC. It is mainly used for laptops and desktop computers.

In CISC architecture, Microprogrammed Control Unit connects to the instruction and data path. This instruction and data path is connected to the cache. Then the required instructions and data will be directed to the main memory from the cache.

CISC architecture has a variety of addressing modes. There is a large number of instructions. It also supports variable length instruction formats. As the instructions are complex, it requires a multiple number of clock cycles to execute a single instruction. Moreover, the instruction decoding is more complex.

Difference Between RISC and CISC

Definition

RISC is an instruction set architecture that is designed to perform a smaller number of computer instructions so that it can operate at a higher speed. CISC is a full set of computer instructions that intends to provide the necessary capabilities in an efficient way.

Long Form

RISC stands for Reduced Instruction Set Computer. CISC stands for Complex Instruction Set Computer.

Instructions

Further, RISC architecture utilizes a small, highly optimized set of instructions while CISC architecture utilizes a large, specialized and a complex set of instructions.

Orientation

Also, RISC is more machine oriented while CISC is more programmer oriented.

Number of Clock Cycles

Moreover, RISC is simple and requires one clock cycle to execute instructions. CISC is complex and requires multiple clock cycles to execute an instruction.

Number of Registers

Furthermore, RISC has more registers while CISC has fewer registers.

Addressing Modes

In RISC, the instructions have simple, fixed formats with few addressing modes.  In CISC, the instructions have variable formats with several complex addressing modes. 

Program Length

Since RISC has simple instructions, the program length is long. But since CISC has complex instructions, its program length is short.

Required RAM

Besides, CISC requires a minimum amount of RAM than RISC as the program has less number of instructions.

Usage

RISC is used in Hardwired Control Unit. It is used in applications such as mobile phones and tablets. CISC is used in Microprogrammed Control Unit.  It is used in applications such as desktop computer and laptops.

Conclusion

The difference between RISC and CISC is that the RISC contains a small and a highly optimized set of instructions while the CISC contains a large specialized and a complex set of instructions. In other words, RISC has a smaller and simple set of instructions while CISC has a large and a complex set of instructions.

Reference:

1. RISC vs CISC, David Keizer, 3 June 2016, Available here.
2. Risc Architecture | COA, Education 4u, 26 Feb. 2018, Available here.
3. Cisc Architecture | COA, Education 4u, 26 Feb. 2018, Available here.

Image Courtesy:

1. “DiagramaRISC” By Oriol.romani – Own work (CC BY-SA 3.0) via Commons Wikimedia