Caches
The processor sends an access request for a particular address to the cache. If we get a then the data for the address is in the cache and we can immediately return it.
If we get a then the data for the address is not in the cache, so the request is passed to the main memory (causing high latency), the memory returns data to the processor via the cache. We can choose to retain this data using a number of strategies including spacial locality, temporal locality, and sequential locality.
Effective Access Time
Hit ratio
Hit ratio is the fraction of hits to total attempts, $HR = \frac{\text{hits}}{\text{hits} + \text{misses}}$.
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Miss ratio
Miss ratio is the fraction of misses to total attempts, $HR = \frac{\text{misses}}{\text{hits} + \text{misses}}$.
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Effective Access Time
The Effective Access Time is the average time it takes to access something in memory: ?
Our aim is to reduce the effective time as much as possible.
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Overlapped access
Overlapped access
With cache, access can also be overlapped: the cache is checked in parallel with checking main memory. If the address is not found in the cache, there is slightly less of a penalty. However, this requires more advanced hardware and more power. With overlapped access, the effective access time becomes:
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