DOS Fn 51H: Query Current PSP

                                            Compatibility: 2.0+ (doc'd: 5.0)
 Expects: AH    51H
 Returns: BX    Process ID (i.e, PSP segment) of current process
    Info: This function obtains the segment address of the PSP (a.k.a, the
          Process ID) of the calling program.

          A TSR should save its PSP during initialization.  Before popping
          up, the TSR should use fn 51H to get the ID of the interrupted
          program, then use fn 50H to set the PID to the saved value.  Be
          sure to change the PID back before popping down.

   Notes: ■ On program startup, DS and ES both contain the PSP segment.

          ■ Undocumented fn 62H performs this same function.  You may use
            either one (62H is more commonly used, perhaps because it was
            uncovered earlier).

See Also: PSP (Program Segment Prefix)
          Process Control Functions
          Program Startup & Exit
          DOS Environment
          DOS Functions