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

Valid bit (or validity bit): When the computer is first started the tags contain junk. Since we don't want to recognize this junk, we start with all the valid bits set to 0 for invalid. Even as time goes on we may still have invalid information and so we have to check this bit.

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

How many bits are required for a direct-mapped cache with 64 KB of data and one-word blocks, assuming a 32-bit address. 

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.

Cache Misses

A data cache miss requires us to "freeze" the processor on the instruction that caused the miss. We then retrieve the data from memory, place it in cache and restart the instruction - this time we are guaranteed a hit.

Aninstruction cache miss requires us to do the following

  1. Send the original PC value (Current PC - 4) to memory.
  2. Instruct main memory to performa read and wait for the memory to complete it's access.
  3. Write the cache entry, putting the data from maemory in the data portion of the entry, writing the upper bitsof the address (from the ALU) into the tag field, and turning the valid bit on.
  4. Restart the instruction execution at the first step, which will refetch teh instruction, this time finding it in the cache.