APM Device ID

 Several APM API functions require a device ID in BX.  This is a 16-bit
 value in the form:

  1 1 1 1 1 1
 ║ device classdevice unit  ║
 ╙─┴─┴─┴─┴─┴─┴─┴─╨─┴─┴─┴─┴─┴─┴─┴─╜  bits
  ╚══════╦══════╝ ╚══════╦══════╝
         ║               ╚═══════►  0-7: device unit; ffH=ALL (see below)
         ╚═══════════════════════► 8-15: device class
                                           00H = system
                                           01H = display
                                           02H = secondary storage
                                           03H = parallel ports
                                           04H = serial ports
                                           05H-FFH (reserved)

   bits 8-15  (high byte) identify the device type.  The value 00xxH has
              the special meaning "the system as a whole".

    bits 0-7  (low byte) identify the unit number of the (possibly several)
              units attached of the device type in the high byte.

              This value is 0-based.  For instance, an ID of 0301H is the
              second parallel printer.

              A value of ffH indicates all devices of that type.

   Notes: ID 0000H identifies the system BIOS.
          ID 0001H identifies all devices power-managed by the system BIOS.

See Also: APM API
          About APM