Lecture Notes: RISC vs. CISC

Introduction

• Presenter: Gary Sims
• Topic: Discussion on RISC (Reduced Instruction Set Computing) vs. CISC (Complex Instruction Set Computing) and its relevance today.

Definitions and History

• CISC: Complex Instruction Set Computing
• RISC: Reduced Instruction Set Computing
  • Historically, processors became more complex in the 1980s, aiming to mirror software complexity for speed.
  • Early programming languages: C, Fortran.
  • RISC emerged as a notion to simplify instructions and improve performance.

Key Characteristics

RISC

• Fewer and simpler instructions.
• Aim: Execute one instruction per clock cycle.
• Fixed instruction size for easy decoding.
• Does not perform operations directly on memory:
  • Example: To add 1 to a number in memory, three instructions are needed:
    1. Fetch memory content.
    2. Add to register.
    3. Write back to memory.
• Compiler plays a crucial role in performance optimization.
• Concept of pipelining to enhance instruction execution.
• Delayed jump techniques to handle branching without disrupting instruction flow.

CISC

• More complex instructions that can perform multiple operations in one command.
• May require several clock cycles to execute a single instruction.
• Variable instruction lengths complicate decoding.

Comparisons and Misconceptions

• The 1990s saw a rise in CISC dominance, particularly in desktop PCs (e.g., x86 architecture).
• Common misconception: RISC lost in the competition, but:
  • RISC architecture is prevalent in smartphones, tablets, and modern computing devices.
  • Examples: ARM architecture in mobile devices, IBM’s Power architecture in AI applications.

The Current Landscape

• Evolution: Today's processors have significantly more transistors (e.g., Kirin 980 has 6.9 billion transistors).
• Branch predictors are now standard in CPUs, allowing for better instruction management and fewer pipeline stalls.
• Intel's approach since the Pentium Pro involves breaking down complex instructions into simpler RISC-like operations (micro-operations).
  • This hybrid approach allows CISC to benefit from RISC strategies while maintaining backward compatibility.

Conclusion

• The RISC vs. CISC debate is not as clear-cut today, with both architectures evolving.
• RISC techniques have been integrated into CISC processors to improve performance and efficiency, particularly in power-sensitive environments like smartphones.
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