Assert BNR# When One Entry Remains

Refer to Figure 48-1 on page 1191. When the maximum number of transactions (minus one) that a device can track are currently outstanding on the FSB at various stages of completion, the agent cannot permit any other agent to initiate a new transaction. If a new transaction were initiated, the agent would be incapable of tracking it and consequently would lose track of all activity on the FSB.

Figure 48-1. Don't Wait Until It's Too Late!

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For this reason, agents must have the ability to throttle the ability of other agents to initiate new transactions. That is the purpose of the BNR# (Block Next Request) signal. An agent must assert BNR# when its In Order Queue (IOQ) has one entry remaining empty. This is necessary because a new transaction request could be issued by another agent at the same time that an agent begins to assert BNR#. The one entry that is remaining can then be used to latch and track the newly-issued transaction. There is no danger that another transaction will be issued to the FSB because all agents have detected BNR# by this time.