The system assigns an instance number to each device. The driver may not reliably predict the value of the instance number assigned to a particular device. The driver should retrieve the particular instance number that has been assigned by calling ddi_get_instance(9F), as shown in Example 5-4.
The reg property is used for SBus, PCI, VMEbus, ISA, EISA, and MCA devices. Non-self-identifying device drivers provide this property in the hardware configuration file. See sbus(4), pci(4), isa(4), and vme(4).
Once an instance number has been assigned to a particular physical device by the system, it stays the same even across reconfiguration and reboot. Because of this, instance numbers seen by a driver may not appear to be in consecutive order.