Set PSP segment (INT 21h, 50h)

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

        Entry:
          AH = 50h
          BX = Segment address of new PSP

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