When the cache is full and a memory word is referenced that is not in the cache, a decision must be made as to which block should be removed from the cache to create space to bring the new block to the cache that contains the referenced word. Replacement algorithms are used to make the proper selection of block that must be replaced by the new one.

When a write request is received from the CPU, there are two ways that the system can proceed. In the first case, the cache location and the main memory location are updated simultaneously. This is called the store through method or write through method.

The alternative is to update the cache location only. During replacement time, the cache block will be written back to the main memory. This method is called write back method. If there is no new write operation in the cache block, it is not required to write back the cache block in the main memory. This information can be kept with the help of an associated bit. This bit it set while there is a write operation in the cache block. During replacement, it checks this bit, if it is set, then write back the cache block in main memory otherwise not. This bit is known as dirty bit. If the bit gets dirty (set to one), writting to main memory is required.

The write through method is simpler, but it results in unnecessary write operations in the main memory when a given cache word is updated a number of times during its cache residency period.