What is write-back or copy-back protocol?

For a write operation using this protocol during write-hit: the method is to update only the cache and to mark it as updated with an associated flag bit, often called the modified or dirty bit. The main memory location of the word is updated later, when the block having this marked word is to be removed from the cache to make room for a latest block.

For a write-miss: the block having the addressed word is first brought into the cache, and then the deserved word in the cache is overwritten with the latest information.