Q. Explain Cache Organisation?

Cache memories are found in nearly all latest computers.  They are very helpful for increasing speed of access of information from memory.  Let's look into their organisation in more detail in this section.  

The basic idea of cache organisation is that by keeping most frequently accessed instructions and data in fast cache memory therefore average memory access time will approach the access time of cache. 

The fundamental operation of cache is as follows. When CPU needs to access memory cache is examined. If word addressed by CPU isn't found in cache then main memory is accessed to read that word. A block of words is then transferred from main memory to cache memory.

Performance of cache memory is often measured in terms of a quantity known as hit ratio. When CPU refers to main memory and finds that word in cache it's said to generate a hit. If the word isn't found in cache it's in main memory and it counts as a miss. Ratio of number of hits divided by total CPU references to memory is Hit ratio.   

Average memory access time of a computer system can be improved significantly by use of a cache. E.g. if memory read cycle takes 100 ns and a cache read cycle takes 20 ns then for four continuous references first one brings main memory contents to cache and the subsequent three from cache.

The time taken with cache = (100+20)         +       (20×3)  

                               (For the first read operation)      (For the last three read operations)

                                            = 120                +               60

                                              = 180 ns

Time taken without cache   = 100×4 = 400 ns   

So the closer are reference then better is the performance of cache.