ACPI r6.5, sec 6.5.6, currently says the low 16 bits of _SEG are the PCI Segment Group number. PCIe r6.0, sec 2.2.1.2, added Flit mode with TLP headers that may contain an 8-bit Segment number. ACPI currently says _SEG is purely a software thing and has no connection to any physical entities. But this may get a little blurry when Segment numbers appear in TLPs. For example, AER header logs will likely contain the Flit Segment, and we'll need to correlate that with the _SEG-derived identifiers Linux uses. One possibility is to reduce the width of _SEG to 8 bits to match the Flit mode Segment and require them to be identical. I'm trying to figure out whether that would break any existing systems. I've heard rumors that large systems like SGI UV may use more than 8 bits of _SEG. But I don't know any details. Bjorn
