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

ApmDevIdRec
1 1 1 1 1 1
 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 
  device class     device 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
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APM Device ID