Lecture Notes: RISC vs. CISC

Introduction

• Speaker: Gary Sims
• Topic: Discussion on CISC (Complex Instruction Set Computing) and RISC (Reduced Instruction Set Computing)
• Context: Comments from viewers on recent videos regarding RISC vs. CISC.

Definitions

• CISC: Complex Instruction Set Computing
• RISC: Reduced Instruction Set Computing
• Historical Context:
  • Until the 1980s, CPUs were becoming increasingly complicated.
  • The idea was to make hardware mirror software for efficiency.
  • Programming languages used: C, Fortran.

Evolution from CISC to RISC

• Motivation for RISC:
  • Simplifying CPU design leads to cost reduction.
  • Easier and quicker CPU design due to reduced complexity.
• Key Features of RISC:
  • Each instruction executes once per clock cycle.
  • Fixed instruction size for ease of decoding.
• CISC Limitations:
  • Variable instruction length complicates decoding.
  • More complex instructions require multiple cycles to execute.

Instruction Execution

• CISC Example:
  • One instruction can fetch, add, and write back to memory in one command.
• RISC Example:
  • Requires three separate instructions:
    1. Fetch memory content.
    2. Add to a register.
    3. Write back to memory.
• RISC emphasizes compiler optimization for better performance.

Pipeline Concept

• RISC architecture allows for instruction pipelining.
• Early RISC CPUs had shorter pipelines (e.g., two stages).
• Delayed Jump:
  • Next instruction already in the pipeline during a jump, minimizing impact on execution.

Historical Perspective

• The dominance of x86 in PCs does not reflect the entire processor landscape.
• RISC processes prevalent in smartphones and tablets (ARM architecture).
• Example of Power Architecture in IBM used for AI tasks.

Modern Context

• As of today, processors have billions of transistors (e.g., Kirin 980 has 6.9 billion).
• Branch predictors are now standard in modern CPUs, improving efficiency.
• Micro Operations:
  • Intel's Pentium Pro started breaking complicated instructions into RISC-like micro operations.
  • This method allows for efficient pipelining, similar to RISC's approach.
• CISC processors face challenges in power efficiency compared to ARM processors due to complex instruction decoding.

Conclusion

• The RISC vs. CISC debate continues to evolve with technology advancements.
• Current trends indicate a blending of RISC principles into CISC architectures.
• Importance of understanding both architectures in the modern computing landscape.

Closing

• Speaker: Gary Sims
• Encouragement to like the video, subscribe, and stay tuned for future content.