Symmetric And Priority Bus Exchange During Lock; Figure 4-8. Symmetric And Priority Bus Exchange During Lock - Intel Pentium Pro Family Developer's Manual

4.1.4.7.

SYMMETRIC AND PRIORITY BUS EXCHANGE DURING LOCK#

Figure 4-8 illustrates an ownership request made by both a symmetric and a priority agent during
an ongoing indivisible sequence by a symmetric owner. When this is the case, LOCK# takes pri-
ority over BPRI#. That is, the symmetric bus owner does not give up the bus to the priority agent
while it is driving an indivisible locked operation. Note that bus agent 1 can hold bus ownership
even though BPRI# is asserted. Like the BREQ[3:0]# signals, if the priority agent is going to
issue a transaction, BPRI# must not be driven inactive until the clock in which ADS# is driven
asserted.
1
CLK
BREQ0#
BREQ1#
BREQ2#
BREQ3#
BPRI#
LOCK#
0a
ADS#
A A AA A
{REQUEST}
{rotating id}
0 0

Figure 4-8. Symmetric and Priority Bus Exchange During LOCK#

Before T1, agent 0 owns the bus. In T1, agent 0 initiates the first transaction in a bus locked op-
eration by asserting LOCK# along with request 0a. Also in T1, the priority agent and agent 1
assert BPRI# and BREQ1# respectively to arbitrate for the bus. Agent 0 does not deassert
BREQ0# or LOCK# since it is in the middle of a bus locked operation.
In T7, agent 0 initiates the last transaction in the bus locked operation. At the request's success-
ful completion the indivisible sequence is complete and agent 0 deasserts LOCK# in T11. Since
BREQ1# is observed active in T10, agent 0 also deasserts BREQ0# in T11 to release symmetric
ownership.
The deassertion of LOCK# is observed by the priority agent in T12 and it begins new-request
generation from T13. The deassertion of BREQ0# is observed by all symmetric agents and they
assign the symmetric ownership to agent 1, the agent with active bus request. In T13, all sym-
metric agents update the Rotating ID to one, the Agent ID of the new symmetric owner.
2 3 4 5 6 7
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0 0 0 0 0
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BUS PROTOCOL
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1a
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1 1 1
4-13