A form of cache in which each memory block from the main store may only ever be placed in one cache block. Each location in main store is directly mapped to to single location in the cache.

This might be done by indexing the cache with the most significant bits of the main store block's address. This means that the remaining address bits must also be stored in the cache, to indicate which of the possible main store blocks is there. These bits are known as the "Tag". Finally a bit is stored for each cached address to show if the address is valid (the "Valid" bit). This is required so that cache locations that are as-yet unused (ie, unfilled by main store locations) are never read.