The select code is the backplane slot number (numbered from 0) on the
Series 500 HP-UX systems.

Valid select codes range from 0 to 7 for a Series 500 HP-UX system that
does not have a bus expander (HP part number ###) and an associated I/O
processor, or from 0 to 15 if one extra I/O processor and bus expander
are present, and from 0 to 23 if two extran I/O processors and bus
expanders are present.
