Transferring Snoop Responsibility; Figure 5-2. Response Responsibility Pickup Effect On An Outstanding Invalidation - Intel Pentium Pro Family Developer's Manual

5.3.2.

Transferring Snoop Responsibility

A requesting agent picks up snoop responsibility for the cache line after observing a transac-
tion's Snoop Phase. When a requesting agent accepts snoop responsibility for a cache line and
immediately drops that responsibility in response to a subsequent transaction, it is allowed to use
the cache line exactly once for internal use, before performing an implicit writeback.
Figure 5-2 illustrates the effect of response agent responsibility pickup on an outstanding Inval-
idation Transaction (Read Invalidate Line, or an Invalidate Line Transaction). It also illustrates
that a cache line can be returned in response to an Invalidate Line Transaction if two competing
agents request ownership of a Shared cache line simultaneously.
1
CLK
1
ADS#
AA AAAA A
AA AAAA A
{REQUEST}
HIT#
HITM#
DEFER#
TRDY#
RS[2:0]#
DBSY#
AA AAAAAA A A AAAAA
D[63:0]#
DRDY#
?
Owner

Figure 5-2. Response Responsibility Pickup Effect on an Outstanding Invalidation

In T1, the requesting agent P1 asserts ADS# and drives the {REQUEST} group to issue Invali-
date Request 1. In T4, a different requesting agent, P2, asserts ADS# and intends to drive the
{REQUEST} group to issue Invalidation request 2 to the same cache line. However, the snoop
of Invalidate Request 1 will invalidate the shared line in P2, forcing P2 to instead issue Read In-
validate Request 2, to the same cache line.
2 3 4 5 6 7
2
AAAA AA AA AAA A A AA AAA A AAAA A AA A A AAAA
AA AAAA AA A AAA A AA AAA AAAA A A AAAA AA A A
1
1
AA A AAAAAA AA A AAAAA A AAAAAA A AAAAAA AA A AA AA AA AAAAAA A AAAAAA A A AAAAA AAAAA A AA AA AAAAAA A
? ? ? ? ? 1
Transaction
BUS TRANSACTIONS AND OPERATIONS
9 10 11 12 13
8
AA AA A AAAA AAAA A A A AAAA AA AA AA AA A A AAAA AAAA A A A AA AA
AA AA AAAA A A A A AAAA AA AA AA AAAA AA AAAA A A A A A AAAA AA AA
2
2
AA AAAAA A A AAAAAA AA A AAAAAA AA A A AA A A
1 1 2 2
2
2
14 15 16
AAAA A A AAAA A A AA AAAA AA AA AA A
AAAA A AAAA A A A AAAA AA AA AA AA A
AAAA AA A
2
2 2
5-15