| As in the case of PCI bridges, a HyperTransport bridge has a number of responsibilities: -
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). -
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. -
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. |