Cache coherency apparatus and method minimizing memory writeback operations

A processor comprising:

• a first caching agent to detect a first operation associated with a cache line at a first requestor cache, wherein the cache line being a block of data, and further detecting that a copy of the cache line exists in a first cache in a modified (M) state;
• a second caching agent to forward the copy of the cache line from the first cache to the first requestor cache, the first caching agent to store the copy of the cache line in the first requestor cache in a second modified (M′) state, and the second caching agent further to transition the copy of the cache line in the first cache to an invalid (I) state;
• a third caching agent to detect a second operation associated with the cache line at a second requestor cache;
• the first caching agent to responsively forward the copy of the cache line from the first requestor cache to the third caching agent, the third caching agent to store the copy of the cache line in the second requestor cache in an owned (O) state if the copy of the cache line has not been modified in the first requestor cache;
• and
• the first caching agent to transition the copy of the cache line to a shared (S) state in the first requestor cache.

A system comprising:

• a memory for storing instructions and data;
• a graphics processor for performing graphics operations in response to certain instructions;
• a network interface for receiving and transmitting data over a network;
• and
• a processor comprising:
• a first caching agent to detect a first operation associated with a cache line at a first requestor cache, wherein the cache line being a block of data, and further detecting that a copy of the cache line exists in a first cache in a modified (M) state;
• a second caching agent to forward the copy of the cache line from the first cache to the first requestor cache, the first caching agent to store the copy of the cache line in the first requestor cache in a second modified (M′) state, and the second caching agent further to transition the copy of the cache line in the first cache to an invalid (I) state;
• a third caching agent to detect a second operation associated with the cache line at a second requestor cache;
• the first caching agent to responsively forward the copy of the cache line from the first requestor cache to the third caching agent, the third caching agent to store the copy of the cache line in the second requestor cache in an owned (O) state if the copy of the cache line has not been modified in the first requestor cache;
• and
• the first caching agent to transition the copy of the cache line to a shared (S) state in the first requestor cache.