31:20

0x0

RW/O

Next Capability Offset (NCO)

This field contains the offset to the next PCI Express Capability structure or 000h if no other items exist in the linked list of Capabilities.
For Extended Capabilities implemented in Configuration Space, this offset is relative to the beginning of PCI compatible Configuration Space and thus must always be either 000h (for terminating list of Capabilities) or greater than 0FFh.
The bottom 2 bits of this offset are Reserved and must be implemented as 00b and software must mask them to allow for future uses of these bits.

19:16

0x0

RW/O

Capability Version (CV)

This field is a PCI-SIG defined version number that indicates the version of the Capability structure present.
For systems that support Secondary PCI Express Extended Capability, BIOS should write a 1h to this register else it should write 0.

15:0

0x0

RW/O

PCI Express Extended Capability ID (PCIEECID)

This field is a PCI-SIG defined ID number that indicates the nature and format of the Extended Capability.
PCI Express Extended Capability ID for the Secondary PCI Express Extended Capability is 0019h. For systems that support Secondary PCI Express Extended Capability, BIOS should write a 0019h to this register else it should write 0.