Some Directory Protocols Add An Owned O State To The

Some directory protocols add an Owned (O) state to the protocol, similar to the optimization discussed for snooping protocols. The Owned state behaves like the Shared state, in that nodes may only read Owned blocks. But it behaves like the Modified state, in that nodes must supply data on other nodes’ Get requests to Owned blocks. The Owned state eliminates the case where a Get Shared request to a block in state Modified requires the node to send the data both to the requesting processor and to the memory. In a MOSI directory protocol, a Get- Shared request to a block in either the Modified or Owned states supplies data to the requesting node and transitions to the Owned state. A Get Modified request in state Owned is handled like a request in state Modified. This optimized MOSI protocol only updates memory when a node replaces a block in state Modified or Owned.
a. Explain why the MSA state in the protocol is essentially a “transient” Owned state.
b. Modify the cache and directory protocol tables to support a stable Owned state.

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