RISC vs CISC

Understanding the differences between RISC and CISC processor architectures

Introduction

RISC (Reduced Instruction Set Computer) and CISC (Complex Instruction Set Computer) are two different approaches to processor architecture design, each with its own advantages and trade-offs.

RISC Architecture

Key characteristics of RISC:

  • Simple Instructions
    • Fixed instruction length
    • Few addressing modes
    • Load/store architecture
  • Hardware Features
    • Large register set
    • Pipelined execution
    • Simple control unit

CISC Architecture

Key characteristics of CISC:

  • Complex Instructions
    • Variable instruction length
    • Multiple addressing modes
    • Memory-to-memory operations
  • Hardware Features
    • Microprogrammed control
    • Specialized instructions
    • Complex instruction decoding

Comparison

Feature RISC CISC
Instruction Set Small, simple Large, complex
Instruction Length Fixed Variable
Clock Cycles per Instruction 1 Multiple
Memory Access Load/Store only Memory-to-memory
Hardware Complexity Simple Complex

Important Concepts

Key aspects of RISC vs CISC:

  • Performance considerations
  • Power efficiency
  • Code density
  • Compiler optimization
  • Modern implementations