Snooping To Solve Coherency
The cache controllers can “snoop” on the bus, meaning that they watch the events on the bus even if they do not issue them, noting any action applied to cache lines they hold
There are two possible actions when a memory location held by processor A is changed by processor B
- Invalidate -- mark local copy as invalid
- Update -- make the same change B made
The unit of cache coherency is a cache line or block