INT 21 - European MSDOS 4.0 - "GETPID" - GET PROCESS IDENTIFIER
        AH = 87h
Return: AX = PID
        BX = parent process's PID
        CX = Command Subgroup ID (CSID)
Desc:   determine an identifier by which to access the calling process
Notes:  called by MS C v5.1 getpid() function
        this function apparently must return AX=0001h for INT 21/AH=80h to
          succeed
        one possible check for European MSDOS 4.0 is to issue this call with
          AL=00h and check whether AL is nonzero on return
SeeAlso: AH=62h,AH=80h