Int 16 Fn 5500  C - Microsoft Word Internal - Microsoft Word Cooperation   [A]

   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: AX=55FFh,INT 1A/AX=3601h