INT 16 - Microsoft Word internal - MICROSOFT WORD COOPERATION WITH TSR
AX = 5500h
Return: AX = 4D53h ('MS') if keyboard TSR present
Notes:  during startup, Microsoft Word tries to communicate with any TSRs
that are present through this call.
if the return is not 4D53h, Word installs its own INT 09 and INT 16
handlers; otherwise it assumes that the TSR will handle the keyboard

See also: 1A3601

165500 - Microsoft Word internal - MICROSOFT WORD COOPERATION WITH TSR