PATH
Every PCI expansion card should contain code in its expansion ROM conforming to IEEE Standard 1275. Among other tasks, this code helps build a configuration structure called a device tree. The requirements for this code (and the benefits of its inclusion in expansion ROMs) are discussed in The Open Firmware Startup Process.
Expansion cards should follow the mechanical specifications given in
PCI Local Bus Specification, Revision 2.0, exactly. In particular, short PCI cards for Macintosh computers should not be longer than the 6.875-inch (174.63 mm) dimension specified. In some Macintosh models, 6.875 inches represents the maximum length for a PCI card, while in other models cards may be any length up to 12.283 inches.