Lecture 6: The Walls to Sequential Computing. Moore’s Law.

Lecture Summary

  • Wrap up Caches

  • Virtual Memory


  • Handling a write-hit

    • Write-through

    • Write-back

  • Handling a write-miss

    • Write-allocate

    • No-write-allocate

  • Typical combos in practice

    • Write-back + Write-allocate (more common)

    • Write-through + No-write-allocate

Miss rate is more important than the hit rate: 97% hit rate is ~2 times worse than 99% hit rate

Case Study: Rearranging Loops to Improve Spatial Locality

Virtual Memory

Why memory virtualization?

  • Ease of use (running programs that require more memory than physically available)

  • Isolation (running multiple programs simultaneously)

  • Protection

  • A page of virtual memory corresponds to a frame of physical memory

  • Page table enables the translation of virtual address into physical addresses

  • The page table is stored in main memory

    • If the page table is accessed for each address translation, this would be very costly

  • Translation Lookaside Buffer (TLB): "Cache" for the addr translation process

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