Specifications

E PENTIUM® PRO PROCESSOR AT 150, 166, 180, and 200 MHz
103
Table 48. BR[3:0]# Signals Rotating Interconnect
Bus Signal Agent 0 Pins Agent 1 Pins Agent 2Pins Agent 3 Pins
BREQ0# BR0# BR3# BR2# BR1#
BREQ1# BR1# BR0# BR3# BR2#
BREQ2# BR2# BR1# BR0# BR3#
BREQ3# BR3# BR2# BR1# BR0#
During power-up configuration, the central agent
must assert the BR0# bus signal. All symmetric
agents sample their BR[3:0]# pins on active-to-
inactive transition of RESET#. The pin on which the
agent samples an active level determines its agent
ID. All agents then configure their pins to match the
appropriate bus signal protocol, as shown in
Table 49.
Table 49. BR[3:0]# Signal Agent IDs
Pin Sampled Active on RESET# Agent ID
BR0# 0
BR3# 1
BR2# 2
BR1# 3
A.17 BREQ[3:0]# (I/O)
The BREQ[3:0]# signals are the Symmetric-agent
Arbitration Bus signals (called bus request). A
symmetric agent n arbitrates for the bus by asserting
its BREQn# signal. Agent n drives BREQn# as an
output and receives the remaining BREQ[3:0]#
signals as inputs.
The symmetric agents support distributed arbitration
based on a round-robin mechanism. The rotating ID
is an internal state used by all symmetric agents to
track the agent with the lowest priority at the next
arbitration event. At power-on, the rotating ID is
initialized to three, allowing agent 0 to be the highest
priority symmetric agent. After a new arbitration
event, the rotating ID of all symmetric agents is
updated to the agent ID of the symmetric owner. This
update gives the new symmetric owner lowest
priority in the next arbitration event.
A new arbitration event occurs either when a
symmetric agent asserts its BREQn# on an Idle bus
(all BREQ[3:0]# previously inactive), or the current
symmetric owner de-asserts BREQm# to release the
bus ownership to a new bus owner n. On a new
arbitration event, based on BREQ[3:0]#, and the
rotating ID, all symmetric agents simultaneously
determine the new symmetric owner. The symmetric
owner can park on the bus (hold the bus) provided
that no other symmetric agent is requesting its use.
The symmetric owner parks by keeping its BREQn#
signal active. On sampling active BREQm# asserted
by another symmetric agent, the symmetric owner
de-asserts BREQn# as soon as possible to release
the bus. A symmetric owner stops issuing new
requests that are not part of an existing locked
operation upon observing BPRI# active.
A symmetric agent can not deassert BREQn# until it
becomes a symmetric owner. A symmetric agent can
reassert BREQn# after keeping it inactive for one
clock.
On observation of active AERR#, RESET#, or
BINIT#, the BREQ[3:0]# signals must be deasserted
in the next clock. BREQ[3:0]# can be reasserted in
the clock after sampling the RESET# active-to-
inactive transition or three clocks after sampling
BINIT# active and RESET# inactive. On AERR#
assertion, if bus agent n is in the middle of a bus-
locked operation, BREQn# must be re-asserted after
two clocks, otherwise BREQ[3:0]# must stay inactive
for at least 4 clocks.
A.18 D[63:0]# (I/O)
The D[63:0]# signals are the data signals. They are
driven during the Data Phase by the agent
responsible for driving the data. These signals
provide a 64-bit data path between various Pentium
Pro processor bus agents. 32-byte line transfers