What is the difference between CISC and RISC architectures?

 The main difference between CISC (Complex Instruction Set Computing) and RISC (Reduced Instruction Set Computing) architectures lies in their instruction set design philosophy and execution approach:

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🔹 CISC (Complex Instruction Set Computing)

Key Characteristics:

• Large and complex instruction set: A single instruction can execute several low-level operations (e.g., memory access + ALU operation).

• Variable-length instructions: Instructions can vary in size and complexity.

• Microprogrammed control unit: Complex instructions are decoded and executed using internal microcode.

• Fewer instructions per program, but each instruction is more complex and may take multiple clock cycles.

Advantages:

• Easier to write assembly code due to powerful instructions.

• Can lead to more compact code.

Examples: x86 (Intel, AMD)

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🔹 RISC (Reduced Instruction Set Computing)

Key Characteristics:

• Small and simple instruction set: Each instruction performs a very basic operation.

• Fixed-length instructions: Simpler to decode and pipeline.

• Hardwired control unit: Faster execution due to simpler instruction decoding.

• More instructions per program, but each executes in one clock cycle (ideal case).

Advantages:

• Easier to pipeline and optimize for performance.

• Lower power consumption and faster execution per instruction.

Examples: ARM, MIPS, RISC-V

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🔸 Summary Table:

Feature	CISC	RISC
Instruction Set	Complex, many instructions	Simple, fewer instructions
Instruction Length	Variable	Fixed
Execution Time	Multiple cycles per instruction	One cycle per instruction (ideal)
Memory Usage	Lower (compact code)	Higher (more instructions)
Hardware Complexity	More complex	Simpler
Pipeline Efficiency	Less efficient	Highly efficient
Common Architectures	x86	ARM, MIPS, RISC-V

In short, CISC aims to do more per instruction, while RISC aims to do one thing fast per instruction.