Cache

Direct Mapped Cache

Cache's are directed mapped if each memory location is mapped to exactly one location in the cache. An example would be:

     location = (Block Address) MOD (Number of cache blocks in the cache)

In this way we can map a memory location to a cache location.

Example: Suppose we have a cache with 8 slots 2^3. Then a word at 45 would be found at slot 5 in the cache.

https://www.scotnpatti.com/images/directmappedcache.jpg

Tags contain the address information required to identify if the information in a cache location coresponds to the data needed. These tags contain the upper bits of the memory address. NOTE: The byte offset is the number of bits required to specify bytes in a block. Since this example uses 4 byte (word) sized blocks the byte offset is 2 bits

https://www.scotnpatti.com/images/directmappedcache2.jpg

To determine the size of the cache above:

     cache size = (number of locations=1024 = 2^10) X (Block size = 32 + Validity bit = 1 + (32 - TagSize=10 - Block bits = 2) 
                = 2^10 X (32 + 1 + 32 - 10 - 2) 
                = 1024 X (53)
                = 54,272 bits or 6,784 bytes

64/4 = 16 KWords = 2^16
Cache Size = 2^16 X (32 + (32 - 16 - 2) + 1) = 2^16 X 49 = 802816 bits = 100,352 bytes = 98 KB.