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.