Symmetric Arbitration With No Lock; Figure 4-5. Symmetric Bus Arbitration With No Lock - Intel Pentium Pro Family Developer's Manual

4.1.4.4.

SYMMETRIC ARBITRATION WITH NO LOCK#

Figure 4-5 illustrates arbitration between two or more symmetric agents while LOCK# and
BPRI# stay inactive. Because LOCK# and BPRI# remain inactive, bus ownership is determined
based on a Rotating ID and bus ownership state. The symmetric agent that wins the bus releases
it to the other agent as soon as possible (the Pentium Pro processor limits it to one transaction,
unless the outstanding operation is locked). The symmetric agent may re-arbitrate one clock af-
ter releasing the bus. Also note that when a symmetric agent n issues a transaction to the bus,
BREQn# must stay asserted until the clock in which ADS# is asserted.
1
CLK
BREQ0#
BREQ1#
BREQ2#
BREQ3#
BPRI#
BNR#
LOCK#
ADS#
A AAAAAA AA A AAAAAA
{REQUEST}
{rotating id}
3 3
I
I
{ownership}

Figure 4-5. Symmetric Bus Arbitration with no LOCK#

In T1, all arbitration requests BREQ[3:0]# and BPRI# are inactive. The bus is not stalled by
BNR#. The Rotating ID is 3 and bus ownership state is idle(I). Hence, the round-robin arbitra-
tion priority is 0,1,2,3.
In T2, agent 0 and agent 1 activate BREQ0# and BREQ1# respectively to arbitrate for the bus.
In T3, all agents observe inactive BREQ[3:2]# and active BREQ[1:0]#. Since the Rotating ID is
3, during T3, all agents determine that agent 0 has the highest priority and is the next symmetric
owner. In T4, all agents update the Rotating ID to zero and the bus ownership state to busy(B).
2 3 4 5 6 7
0a 1a
AAAAAA AA AA A A AAA A AA AAA A A A A AAA AA
3 0 0 1 1
I B B B B
8 9 10 11 12 13
2a 0b
A A AAA A A AAA AA AAA AA A A A A AAA
1 2 2 2 0 0
B
B B B B B
BUS PROTOCOL
14 15 16
AAA A AA AAAAAA A A
0 0 0
B B
B
4-9