CISC vs. RISC Processor Differences

May 13, 2025

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Differences between CISC and RISC Processors

Introduction

  • At the core of all computers is the instruction set.
    • Set of instructions recognized and executed by a CPU.
  • Two main categories:
    • Complex Instruction Set Computer (CISC)
    • Reduced Instruction Set Computer (RISC)

CISC (Complex Instruction Set Computer)

  • Aims to complete tasks in as few lines of assembly as possible.
  • Processor hardware and circuitry are complex.
  • Can execute a series of operations.
  • Example: MUL
    • Multiplies two numbers, stores result back.
    • Resembles high-level language commands.
    • Compiler does minimal work.
    • May take more than one clock cycle to execute.
  • Historically common in desktops/laptops.
  • Intel's x86 processors are based on CISC but incorporate RISC features.

RISC (Reduced Instruction Set Computer)

  • Uses simple instructions executed in a single machine or clock cycle.
  • Lacks complex commands like "MUL"; needs multiple simple commands.
    • E.g., requires four lines of assembly to perform a multiplication.
  • Compiler does more work, requires more RAM for assembly instructions.
  • Efficient as each instruction is completed in a single cycle.
  • Simpler hardware with fewer transistors.
  • Benefits from more registers and cache.
  • Allows pipelining for faster processing.
  • Lower energy requirements.
  • Popular in low-power, portable devices (smart TVs, thermostats, etc.).
  • ARM processors and similar architectures dominate the market.

Comparison

  • CISC:
    • More instructions, variable length.
    • More complex, less efficient for pipelining.
  • RISC:
    • Fewer instructions, fixed size.
    • Efficient for pipelining and performance.

Historical Context

  • RISC was slow to gain acceptance commercially.
  • Lack of software support initially (Windows designed for CISC).
  • Intel's dominance with CISC delayed RISC adoption.
  • Shift to RISC due to cheaper RAM and advanced compilers.
  • RISC processors now make up a significant majority of the market.

Key Question

  • "What are the differences between the RISC and CISC architectures?"
    • Understanding of efficiency, complexity, application, and historical evolution.

Additional Insight

  • Performance Equation:
    • CISC: Minimize instructions per program, more cycles per instruction.
    • RISC: Minimize cycles per instruction, more instructions in the program.
  • Fixed-size instructions in RISC help with pipelining.
  • RISC eventually outpaced CISC in commercial adoption and sales.

These notes summarize the key points about CISC and RISC processors, illustrating their differences, uses, and the evolution of processor architectures.

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