Basic Jobs Of A HyperTransport Bridge

As in the case of PCI bridges, a HyperTransport bridge has a number of responsibilities:

1. It extends the topology through the addition of one or more secondary buses. Each HyperTransport chain (bus) can support up to 32 UnitIDs. Because a device is permitted to consume multiple UnitID's, implementing a bridge is a reasonable way to add a new chain that can support 32 additional UnitIDs (the bridge secondary interface consumes at least one of the new UnitIDs).

2. It acts as host for each of its secondary chains. There are many aspects to this, including ordering responsibilities, error handling, maintaining a queue for outstanding transactions routed to other buses, reflecting peer-to-peer transactions originating below it, decoding memory addresses so it may claim and forward transactions moving between the primary and secondary bus, forwarding/converting configuration cycles based on target bus number, etc.

3. In cases where it bridges between HyperTransport and PCI/PCI-X, the bridge also must translate protocols for transactions going in either directiion. It may also have to remap address ranges between the 40-bit HyperTransport address range and the 32/64-bit PCI or PCI-X range.