A cache line can be in one of three states: invalid, clean or dirty. Invalid means that the cache line has no useful data, and no memory operation will hit it. Clean means that the cache line contains an up-to-date copy of a piece of main memory. Dirty means that the cache line has been written to, and main memory is out of date.

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In the Intel world, the CPU performs something called "cache snooping". This means that a dedicated piece of hardware looks at all memory operations to main memory, and checks to see if the CPU's cache has a more up-to-date version of the memory. It also looks at memory writes, and makes sure that the CPU's cache has the most up to date version of the data.
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