Advanced Computer Architecture Chapter 7: shared memory and cache consistency

Advanced Computer Architecture, Imperial College 2001

The “Berkeley" Protocol

•Four cache line states:

Broadcast invalidations on bus unless cache line is exclusively “owned” (DIRTY)

•Write hit:

•No action if line is DIRTY

•If VALID or SHARED-DIRTY,

•an invalidation is sent, and

•the local state set to DIRTY

•Write miss:

•Line comes from owner (as with read miss).

•All other copies set to INVALID, and line in requesting cache is set to DIRTY

•Read miss:

–If another cache has the line in SHARED-DIRTY or DIRTY,

•it is supplied

•changing state to SHARED-DIRTY

–Otherwise

•the line comes from memory. The state of the

•line is set to VALID

–INVALID

–VALID : clean, potentially shared, unowned

–SHARED-DIRTY : modified, possibly shared, owned

–DIRTY : modified, only copy, owned