RISC vs CISC: Difference and Comparison

Key Takeaways

1. Instruction Set Complexity: RISC (Reduced Instruction Set Computing) and CISC (Complex Instruction Set Computing) represent two different philosophies in designing computer architectures. RISC focuses on simple instructions that can be executed within one clock cycle, whereas CISC includes complex instructions that might take multiple clock cycles to execute but achieve more with each instruction.
2. Performance and Efficiency: RISC architectures tend to have a higher number of registers and spend more transistors on registers rather than on complex instructions, allowing for more parallelism and potentially higher performance in certain situations. CISC architectures, with their more complex instructions, can perform more complex tasks with fewer instructions, potentially leading to more efficient memory usage.
3. Applications and Examples: RISC architectures are commonly used in systems where heat and power efficiency are crucial, such as in mobile devices and embedded systems. Notable examples include ARM processors. CISC architectures, like those seen in Intel’s x86 series, are used in desktop and server environments where raw processing power and the ability to handle complex instructions can be critical.

What is RISC?

RISC stands for Reduced Instruction Set Computer. It is a type of computer architecture that focuses on simplicity and efficiency in the design of the processor’s instruction set. In a RISC architecture, the instruction set is deliberately kept small and simple, consisting of a limited number of instructions that can be executed in a single clock cycle. This design philosophy contrasts with Complex Instruction Set Computers (CISC), which have larger and more complex instruction sets.

RISC processors have a reduced number of instructions, ranging from around 30 to 200, which are optimized for specific tasks. Each instruction performs a simple operation, and complex operations are built by combining multiple simpler instructions. RISC processors follow a load/store architecture, where data is loaded from memory into registers, manipulated within registers, and then stored back to memory. Arithmetic and logical operations are primarily performed within registers.

What is CISC?

CISC stands for Complex Instruction Set Computer. It refers to a type of computer architecture that emphasizes a large set of instructions with varying complexity. In CISC architecture, instructions can perform complex operations and access memory directly. These instructions are variable in length and can take multiple clock cycles to execute.

CISC processors are designed to handle complex instructions and minimize the number of instructions required to perform a task. They provide a rich set of addressing modes and support for high-level languages, allowing programmers to write more compact code. Examples of CISC architectures include the x86 family of processors, such as Intel’s Pentium and AMD’s Ryzen.

Difference Between RISC and CISC

1. RISC architectures have a simplified and streamlined instruction set, consisting of a small number of simple instructions. In contrast, CISC architectures have a larger and more complex instruction set with instructions that can perform more intricate operations.
2. RISC processors execute instructions in a single clock cycle, making their execution time more predictable. CISC processors, on the other hand, may require multiple clock cycles to execute instructions, and the execution time can vary depending on the complexity of the instruction.
3. RISC architectures employ a load/store architecture, where all data processing instructions operate on data stored in registers, and memory access is limited to specific load and store instructions. CISC architectures allow instructions to directly access memory, reducing the need for explicit load/store instructions.
4. RISC architectures are designed with a simple and regular pipeline structure, where each stage of the pipeline performs a specific task. This allows for easier pipelining and higher instruction throughput. CISC architectures, with their more complex instructions, may have longer pipelines and face challenges in achieving efficient pipelining.
5. RISC architectures tend to generate code that is more compact and efficient. The simplified instruction set and focus on optimizing common operations contribute to smaller code sizes. CISC architectures, on the other hand, can perform complex operations in a single instruction, potentially reducing the number of instructions required but resulting in a larger code size.

Comparison Between RISC and CISC

Parameters of Comparison	RISC	CISC
Instruction Encoding	Fixed-length instructions	Variable-length instructions
Register Usage	Emphasizes extensive use of registers	Relies more on memory access
Control Unit Complexity	Simple control unit design	Complex control unit design
Performance per Watt	Typically more power-efficient	May consume more power for complex instructions
Development Time	Faster development due to simpler architecture	Longer development time due to complexity

References

1. https://dl.acm.org/doi/abs/10.1145/250015.250016
2. https://link.springer.com/chapter/10.1007/978-3-540-93799-9_4