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1
CLK
BREQ0#
BREQ1#
BREQ2#
BREQ3#
BPRI#
BNR#
ADS#
AAAAAA
A
AAAAA
A
A
{REQUEST}
{rotating id}
3
3
I
I
{
o
w
n
e
Figure 4-6. Symmetric Arbitration with no Transaction Generation
This figure is the same as Figure 4-5 up until T9.
In T9, the clock that bus agent 2 wins bus ownership, bus agent 2 deasserts BREQ2# because
the need to drive the transaction was removed (for example, on the Pentium Pro processor, if a
transaction is pending to writeback a replaced cache line and it gets snooped, HITM# will be
asserted and the line will be written out as an implicit writeback. The pending transaction to
writeback the line gets cancelled).
In T10, all agents observe an inactive BREQ2# and an active BREQ0#. During T10 they recog-
nize that agent 0 is the only symmetric agent arbitrating for the bus. In T11, all agents update
the Rotating ID to 0. The ownership remains busy and agent 0 initiates request 0b. Because no
other agent has requested the bus, agent 0 parks on the bus by keeping its BREQ0# signal active.
4.1.4.6.
BUS EXCHANGE AMONG SYMMETRIC AND PRIORITY AGENTS
WITH NO LOCK#
Figure 4-7 illustrates bus exchange between a priority agent and two symmetric agents. A sym-
metric agent relinquishes physical bus ownership to a priority agent as soon as possible. A max-
imum of one unlocked ADS# can be generated by the current symmetric bus owner in the clock
after BPRI# is asserted because BPRI# has not yet been observed. Note that the symmetric bus
owner (Rotating ID) does not change due to the assertion of BPRI#. BPRI# does not affect sym-
metric agent arbitration, or the symmetric bus owner. Finally, note that in this example BREQ0#
must remain asserted until T12 because transaction 0b has not yet been driven. An agent can not
drive a transaction unless it owns the bus in the clock in which ADS# is to be driven for that
transaction.
2
3
4
5
6
7
0a
1a
A
A
AAAAAA
A
A A AA
A
A A
AAA
A A AA
A
3
0
0
1
1
I
B
B
B
B
r
s
h
i
p
8
9
10
11
12
0b
AAA
A A
AAAAAA
A
A
AAA
AA A
AAAA
1
2
2
0
0
B
B
B
B
B
}
BUS PROTOCOL
13
14
15
16
AAAAAAAAAAAAAAAA
A
A
0
0
0
0
B
B
B
B
4-11
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