Function 51h            Get PSP Address

    Returns the segment address of the program segment prefix (PSP)
    for the current program.

        Entry   AH = 51h

        Return  BX = Segment address of PSP


    Note
    Functions 51h and 62h are identical. Programs can use either
    to get the segment address of the current PSP.

    Function 51h (undocumented until DOS 5.0 was released) is
    supported by DOS 2.0+, function 62h by DOS 3.0+.