If those fields are zero, it then uses the Vendor ID and Device ID also in the Configuration Space. If not, the Open Firmware probing process constructs one for you. It does so by writing all 1's to the Base Address Registers. It then reads back the values. These registers are hardwired in powers of two to define your memory needs.
Bits returning a zero effectively define the memory requirements.
It is not, however, contained in the expansion ROM but is placed in the device tree. A Technote on these memory related properties and the device tree is scheduled for release mid 1996. A word of caution is required. All device types except type PCI to PCI bridges use a PCI Configuration Register Map. Its absence defines no interrupt needs.