
8.6 Cache-Based Directory Protocols: The Sequent NUMA-Q 627
Requestor Old head
Requestor
Old head
INVALID
\
\
\
\
ONLY__
FRESH
\t
FRESH
Home memory
PENDING
\
ONLY_
FRESH
FRESH
Home memory
(a) Initial state, plus first round-trip
(b) After first round-trip
Requestor Old head
Requestor
Old head
PENDING
*+-
-
-
N
ONLY_
FRESH
FRESH
HEAD_
FRESH
\
TAIL_
VALID
FRESH
Home memory
Home memory
(c) Second round-trip
(d) Final state
FIGURE 8.26 An example of a read miss in the SCI protocol. The figure shows the messages and
state transitions for a read miss to a block that is initially in the FRESH state at home, with one node on
the sharing list. Solid