Map memory

        Entry:
          AH = 44h
          AL = physical page number (0-3)
          BX = logical page number
          DX = handle
        Return:
          AH = status
               00h function successful
               80h internal error
               81h hardware malfunction
               83h invalid handle
               84h undefined function requested
               8Ah invalid logical page number
               8Bh illegal physical-page number