Get PSP segment (INT 21h, 51h)

        Returns current PSP segment (used by resident programs, and
        pseodo-multitaskingsystems). This function may be called at
        any time (even when DOS is running).

        Entry:
          AH = 51h
        Return:
          BX = Current PSP Segment

        Note: Under DOS 2.xx, this function cannot be invoked inside an
              INT 28h handler without a prior call to function 5Dh.