Method and apparatus for maintaining transaction ordering and arbitrating in a bus bridge

A computer system comprising:

• (a) a first bus;
• (b) a second bus;
• (c) a first agent coupled to the first bus;
• (d) a second agent coupled to the second bus;
• (e) a bus bridge coupled to the first and second buses, the bus bridge including,
• an inbound queue coupled to the first bus and the second bus,
• an outbound queue coupled to the first bus and the second bus,
• an outbound posting enable state machine coupled to the first bus for determining whether a first transaction is to be committed to the outbound queue and an indication given to the first agent that the bridge has committed to the first transaction,
• an inbound posting enable state machine coupled to the
• second bus for determining if a second transaction is to be committed to the inbound queue and an indication given to the second agent that the bridge has committed to the second transaction, and
• a priority state machine coupled to the first bus and the second bus for determining whether a transaction in the inbound queue is to be executed on the first bus or a transaction in the outbound queue is to be executed on the second bus;
• (f) the first bus for transferring data between the first agent and the bus bridge;
• and
• (g) the second bus for transferring data between the second agent and the bus bridge.

A bridge being coupled to a first bus and a second bus, wherein a first agent is coupled to the first bus and a second agent is coupled to the second bus, the bridge comprising:

• (a) an outbound queue coupled to the first bus and the second bus for transferring a first transaction from the first bus to the second bus;
• (b) an inbound queue coupled to the first bus and the second bus for transferring a second transaction from the second bus to the first bus;
• (c) a transaction arbitration unit coupled to the inbound queue and the outbound queue, the transaction arbitration unit including,
• an outbound posting enable state machine coupled to the first bus for determining whether the first transaction is to be committed to the outbound queue and an indication given to the first agent that the bridge has committed to the first transaction,
• an inbound posting enable state machine coupled to the second bus for determining whether the second transaction is to be committed to the inbound queue and an indication given to the second agent that the bridge has committed to the second transaction, and
• a priority state machine coupled to the first bus and the second bus for determining whether a transaction in the inbound queue is to be executed on the first bus or a transaction in the outbound queue is to be executed on the second bus.

A method of maintaining transaction ordering in a bus bridge coupled to a first bus and a second bus, wherein the bus bridge has a first queue and a second queue, and wherein a requesting agent is coupled to the first bus, the method comprising the steps of:

• (a) determining whether posting to the second queue is enabled, based on a posting enable signal received from a transaction arbitration unit;
• (b) placing a request originating from the requesting agent on the first bus into the first queue:
• (c) indicating to the requesting agent that the request has been committed to, provided posting to the first queue is enabled;
• (d) determining whether ownership of the second bus is available;
• and
• (e) placing the request on the second bus when the request propagates to the top of the first queue and ownership of the second bus is available.

A method of performing transaction arbitration in a bus bridge coupled to a first bus and a second bus, the bus bridge having a first queue and a second queue, the method comprising the steps of:

• (a) determining whether posting to the second queue is enabled;
• (b) determining whether the second queue contains posted transactions;
• (c) placing a transaction originating on the second bus in the first queue when the posting to the second queue is disabled and the second queue contains no posted transactions;
• and
• (d) executing the transaction on the first bus.