HyperTransport System Limits

HyperTransport shares PCI terminology in describing a system in terms of the number of buses, devices, functions, and configuration space.

256 Buses In A System

PCI permits 256 buses in a system and each PCI host bridge or PCI-to-PCI bridge secondary interface is host to a new bus with a unique bus number. Unlike PCI, a HyperTransport bus may not end with a single electrical connection. Tunnel devices enable the construction of device chains which are still viewed as a single logical bus. The 256 bus limit in HyperTransport, then, is actually 256 chains.

32 UnitIDs Per Bus

PCI permits a maximum of 32 physical devices per bus. In HyperTransport, each functional device can request multiple device numbers , called UnitIDs. The reason for this is because HyperTransport ordering rules consider the transactions from each UnitID to be a unique transaction stream ; owning multiple UnitIDs enables a device to source more than one transaction stream (e.g. a standard transaction stream and an isochronous transaction stream for its high priority traffic). The 32 device per bus limit in PCI is a 32 UnitID per bus limit in HyperTransport.

One To Eight Functions Per Device

As in PCI, HyperTransport allows 1-8 logical functions in a physical device package. Each function has its own 256 byte configuration space, and will be assigned unique UnitID(s).

256 Bytes Of Configuration Space

Just as in other PCI devices, each function of a HyperTransport device must implement a 256 byte configuration space memory. The first one-fourth of the configuration space is the header. In addition to the header, devices also must implement at least one set of HyperTransport advanced capability registers.