Int 16 Fn FFDD  U - Pc Tools V5.1-5.5 Pcshell Api - Get ???                [U]

   AX = FFDDh
   BX = 0002h

Return: AL =
       00h ???
       01h ???

Note:  PCShell v6.0+ displays the error message "Incorrect PCRUN version",
     awaits a keystroke, and aborts the current process