RISC vs CISC

RISC (Reduced Instruction Set Computer) and CISC (Complex Instruction Set Computer) are two different ways of designing computer processors. RISC uses a small set of simple, fixed-size instructions designed to execute in a single clock cycle. On the other hand, CISC includes a larger set of instructions, many of which are complex and can perform multiple operations (e.g., memory access and computation) in a single instruction, often requiring multiple clock cycles.

Reduced Instruction Set Architecture (RISC)

RISC simplifies processor design by using a small, uniform set of instructions. Each instruction performs a basic operation (e.g., load, compute, store) and is designed to execute in a single clock cycle, enabling efficient pipelining and simpler hardware.

Characteristics of RISC

• Simpler instruction, hence simple instruction decoding.
• Instruction comes in the form of one word.
• An instruction takes a single clock cycle to get executed.
• More general-purpose registers for register-to-register operations.
• Simple Addressing Modes.
• Optimized for pipelining due to uniform instruction size and simplicity.

Advantages of RISC

• Simpler instructions: RISC processors use a smaller set of simple instructions, which makes them easier to decode and execute quickly. This results in faster processing times.
• Faster execution: Because RISC processors have a simpler instruction set, they can execute instructions faster than CISC processors.
• Lower power consumption: RISC processors consume less power than CISC processors, making them ideal for portable devices.

Disadvantages of RISC

• More instructions required: RISC processors require more instructions to perform complex tasks than CISC processors.
• Increased memory usage: More instructions may need more memory to store programs.
• Compiler Dependency: RISC relies on optimized compilers to generate efficient code.

Complex Instruction Set Architecture (CISC)

CISC reduces the number of instructions a program needs by using a large set of complex, variable-length instructions. A single instruction can perform multiple operations (e.g., load, compute, and store), which may take multiple clock cycles.

Characteristics of CISC

• Complex instruction, hence complex instruction decoding.
• Instructions 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 operations get performed in memory itself.
• Complex Addressing Modes.

Advantages of CISC

• Reduced code size: CISC processors use complex instructions that can perform multiple operations, reducing the amount of code needed to perform a task.
• More memory efficient: Because CISC instructions are more complex, they require fewer instructions to perform complex tasks, which can result in more memory-efficient code.
• Widely used: CISC processors have been in use for a longer time than RISC processors, so they have a larger user base and more available software.

Disadvantages of CISC

• Slower execution: CISC processors take longer to execute instructions because they have more complex instructions and need more time to decode them.
• More complex design: CISC processors have more complex instruction sets, which makes them more difficult to design and manufacture.
• Higher power consumption: CISC processors consume more power than RISC processors because of their more complex instruction sets.

CPU Performance

Both approaches try to increase the CPU performance

• RISC: Reduce the cycles per instruction at the cost of the number of instructions per program.

• CISC: The CISC approach attempts to minimize the number of instructions per program but at the cost of an increase in the number of cycles per instruction.

Earlier when programming was done using assembly language, a need was felt to make instruction do more tasks because programming in assembly was tedious and error-prone due to which CISC architecture evolved but with the uprise of high-level language dependency on assembly reduced RISC architecture prevailed.

Example:

Suppose we have to add two 8-bit numbers:

• CISC approach: There will be a single command or instruction for this like ADD which will perform the task.
• RISC approach: Here programmer will write the first load command to load data in registers then it will use a suitable operator and then it will store the result in the desired location.

So, add operation is divided into parts i.e. load, operate, store due to which RISC programs are longer and require more memory to get stored but require fewer transistors due to less complex command.

RISC vs CISC

RISC and CISC are two processor designs. Here is a comparision table between them.