Many snooping coherence protocols have additional states, state transitions, or bus transactions to reduce the overhead of maintaining cache coherency. In Implementation 1 of Exercise 4.2, misses are incurring fewer stall cycles when they are supplied by cache than when they are supplied by memory. Some coherence protocols try to improve performance by increasing the frequency of this case.
A common protocol optimization is to introduce an Owned state (usually denoted O). 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' read and write misses to Owned blocks. A read miss to a block in either the Modified or Owned states supplies data to the requesting node and transitions to the Owned state. A write miss to a block in either state Modified or Owned supplies data to the requesting node and transitions to state Invalid. This optimized MOSI protocol only updates memory when a node replaces a block in state Modified or Owned.
Draw new protocol diagrams with the additional state and transitions.