Int 16 Fn FFDD  U - Pc Tools V6.0+ Pcrun Api - ???                         [U]

   AX = FFDDh
   BX = 000Bh
   ???

Return: CX = 5555h if PCRUN active
   DX = 5555h

Note:  also clears flag set by AX=FFDDh/BX=000Ah