Refill lines are the unit of data transfer. Size of refill lines are all equal and generally contain n words, where n is a power of 2. Typical cache refill lines have between 4 and 64 bytes. The reason to have this refill line instead of accessing each individual word is to make use of spatial locality Cache memory uses different mapping techniques for mapping main memory addresses into its own local addresses. It is a fixed mapping.
Main memory is divided into regions.
Line of memory can go to any cache lines in that block. Any memory sector can be mapped into any cache sector. Inside a sector, tag memory uses validity bits to indicate which memory units are in use and which are not. When a read request is received, the cache is checked first to see if the entry is there. If it is there, it is a hit, and process is finished.