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

Lecture Summary

• Wrap up Caches

• Virtual Memory

Caches

• 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