Memory: cache

Fig 7.4

Consider simple cache

Processor requests are each 1 word

Cache blocks are 1 word

Processor makes reference to word Xn, and Xn is not currently in the cache: cache miss

If there is space in the cache, then word Xn is copied into the cache

2 questions:

How do we know if a data item is in the cache now?

If so, where is it located?

Simplest method:

Base cache address directly on memory address: direct mapped cache

(Notice they are not equal, since the cache is smaller)

Typical method:

(block address) mod (number of cache blocks in  whole cache)

Easy to compute if number of cache blocks is power of 2: use low order n bits of address

where n is log 2 (number of cache blocks)

Example:

Fig 7.5

Cache of 8 words

Each data word is mapped to location whose address ends in same 3 bits

For example, all the gray words have addresses ending in 001,

mapped to cache block 001

Each cache location can contain several possible data words

If a word is in the cache, how do we know which one it is?

Add tags to the cache entries

Tag needs to contain only the upper bits of the address

In example, only need upper 2 bits for tag

Also need to recognize whether block is empty: valid bit