Caching technology and techniques.

Direct Cache - specific 'lines' in memory mapped to one and only
one line in cache. 

High competition for lines, especially lower indexed ones.

More common in single task (RISC?) CPUs.


Associative cache - any 'line' in memory can be mapped to any line in cache.
Requires search of all entries to find target line. 

Does not use line index.

But requires longer tag field.

More useful when working with a small number of large lines or blocks, such as 
hard drive track caching.


Set Associative cache - specific 'lines' in memory mapped to one line in cache.
But cache has 2 or more instances of that line. 

Choice of fill or replacement determined by an algorithm.

2-way or 4-way common on CPUs with small number of tasks.
8-way for modern CISC CPUs.

The more ways, the more complex the logic to quickly ID correct 'way'.


Skewed Associative - similar to set associative. But line index from different  
  segments are hashed so they don't compete for the same line in cache.

Competition for lines in a standard cache are not evenly distributed, but often 
  favor lower line indexes. 

Having a way to predictably map a second line x to a different line in the 
  cache can improve hit performance. 2-way skew may perform as well as a 4-way 
  set associative.

However, calculating hash on line may had some time.

May not completely eliminate line competition.

And replacement policies may be more complex.

see : https://sites.google.com/site/paulclaytonplace/andy-glew-s-comparch-wiki/skewed-associativity

Acts more like an associative cache but with the ability to predict where any line 
  may land. 


Review

Fast index.
Direct Cache 

Direct cache access 

Set associative 

Associative cache features 

Selecting slots 

When to refresh main memory