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

## Lecture Summary

* Wrap up Caches
* Virtual Memory

## Caches

![](/files/-MT4Gz0D9R0lC7yJDhbe)

* 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

![Cache Capacity Effects from Memory Mountain](/files/-MT4NLNO8skPv67NM0ir)

## 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

![](/files/-MT4TqtQ-6GaZW2HvDkn)

![](/files/-MT4TyJfcTfwIFBPdeA3)


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