Hardware Reference
In-Depth Information
FIGURE 5.23 The state transition diagram for the directory has the same states and
structure as the transition diagram for an individual cache . All actions are in gray be-
cause they are all externally caused. Bold indicates the action taken by the directory in re-
sponse to the request.
To understand these directory operations, let's examine the requests received and actions
taken state by state. When a block is in the uncached state, the copy in memory is the current
value, so the only possible requests for that block are
Read miss —The requesting node is sent the requested data from memory, and the requestor
is made the only sharing node. The state of the block is made shared.
Write miss —The requesting node is sent the value and becomes the sharing node. The block
is made exclusive to indicate that the only valid copy is cached. Sharers indicates the iden-
tity of the owner.
When the block is in the shared state, the memory value is up to date, so the same two requests
can occur:
Read miss —The requesting node is sent the requested data from memory, and the request-
ing node is added to the sharing set.
Write miss —The requesting node is sent the value. All nodes in the set Sharers are sent in-
validate messages, and the Sharers set is to contain the identity of the requesting node. The
state of the block is made exclusive.
 
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