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4.1.5.4.
OWNERSHIP FROM BUSY STATE
When the ownership state is busy, the next arbitration event begins with the deassertion of
BREQn# by the current symmetric owner.
4.1.5.4.1.
Bus Parking and Release with a Single Bus Request
When the ownership state is busy, bus parking is an accepted mode of operation. The symmetric
owner can retain ownership even if it has no pending requests, provided no other symmetric
agent has an active arbitration request.
The symmetric owner "n" may eventually deassert BREQn# to release symmetric ownership
even when other requests are not active. When the owner deasserts BREQn#, all agents update
the ownership state to idle, but maintain the same Rotating ID.
4.1.5.4.2.
Bus Exchange with Multiple Bus Requests
When the ownership state is busy, on observing at least one other BREQm# active, the current
symmetric owner n can hold the bus for back-to-back transactions by simply keeping BREQn#
active. This mechanism must be used for bus-lock operations and can be used for unlocked op-
erations, with care to prevent other symmetric agents from gaining ownership. (The Pentium Pro
processor limits the number of additional unlocked requests to one.)
A new arbitration event begins with deactivation of BREQn#. On observing release of owner-
ship by the current symmetric owner, all agents assign the ownership to the highest priority sym-
metric agent arbitrating for the bus. In the following clock, all agents update the Rotating ID to
the new symmetric owner Agent ID and maintain bus ownership state as busy.
A symmetric agent n shall deassert BREQn# for a minimum of one clock.
4.1.6.
Priority Agent Arbitration Protocol Rules
4.1.6.1.
RESET CONDITIONS
On observation of active RESET# or BINIT#, BPRI# must be deasserted in one or two clocks.
On observation of active AERR# (with AERR# observation enabled), BPRI# must be deasserted
in the next clock.
When the reset condition is generated by the activation of BINIT#, BPRI# must remain deas-
serted until 4 clocks after BINIT# is driven inactive. The first BPRI# sample point is 4 clocks
after BINIT# is sampled inactive.
When reset condition is generated by AERR#, the priority agent must keep BPRI# inactive for
a minimum of four clocks unless it has issued the second or subsequent transaction of a locked
operation. The priority owner that has issued the second or subsequent transaction of a locked
operation must activate its BPRI# two clocks from inactive BPRI#. This ensures that the locked
operation remains indivisible.
BUS PROTOCOL
4-17
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