Intel Itanium 2 Processor manual BREQ30# I/O, Table A-6. BR30# Signals and Agent IDs, Pin Sampled

Signals Reference

Table A-4. BR0# (I/O), BR1#, BR2#, BR3# Signals for 4P Rotating Interconnect

Bus Signal	Agent 0 Pins	Agent 1 Pins	Agent 2 Pins	Agent 3 Pins
BREQ[0]#	BR[0]#	BR[3]#	BR[2]#	BR[1]#
BREQ[1]#	BR[1]#	BR[0]#	BR[3]#	BR[2]#
BREQ[2]#	BR[2]#	BR[1]#	BR[0]#	BR[3]#
BREQ[3]#	BR[3]#	BR[2]#	BR[1]#	BR[0]#

Table A-5. BR0# (I/O), BR1#, BR2#, BR3# Signals for 2P Rotating Interconnect

Bus Signal	Agent 0 Pins	Agent 3 Pins
BREQ[0]#	BR[0]#	BR[1]#
BREQ[1]#	BR[1]#	BR[0]#
BREQ[2]#	Not Used	Not Used
BREQ[3]#	Not Used	Not Used

During power-on configuration, the priority agent must assert the BR[0]# bus signal. All symmetric agents sample their BR[3:0]# pins on asserted-to-deasserted transition of RESET#. The pin on which the agent samples an asserted level determines its agent ID. All agents then configure their pins to match the appropriate bus signal protocol as shown in Table A-6.

Table A-6. BR[3:0]# Signals and Agent IDs

Pin Sampled
Asserted on	Arbitration ID	Agent ID Reported
RESET#
BR[0]#	0	0
BR[3]#	1	2
BR[2]#	2	4
BR[1]#	3	6

A.1.15 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 deasserted), or the current symmetric owner deasserts BREQn# to release the bus ownership to a new bus owner n. On a new arbitration event, all symmetric agents simultaneously determine the new symmetric owner using BREQ[3:0]# and the rotating ID. 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 asserted. On

Datasheet 95