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School of Computer Science & Engineering
University of New South Wales
 Advanced Operating Systems 
 COMP9242 2002/S2 

Cache access

Virtually indexed:

looked up by virtual address,
operates concurrently with address translation.

Physically indexed:

looked up by physical address

Cache access mechanics

• Address is hashed to produce index of line set.
• Associative lookup of line within set
  lines per set: -way set associative cache.
  • Typically .
  • is called direct mapped.
  • is called fully associative, unusual for CPU caches.
• Hashing must be simple (complex hardware is slow)
  ==> use least-significant bits of address.

Cache indexing

• The tag is used to distinguish lines of set...
• ... consists of the address bits not used for indexing.