Get or set page map

        Entry:
          AH = 4Eh
          AL = 00h if getting mapping registers
               01h if setting mapping registers
               02h if getting and setting mapping registers at once
               03h if getting size of page-mapping array
          DS:SI = pointer to array holding information (AL=01/02)
          ES:DI = pointer to array to receive information (AL=00/02)
        Return:
          AH = status
                  AL = bytes in page-mapping array (AL=03h only)
                  array pointed to by ES:DI receives mapping info (AL=00h/02h)
               80h internal error
               81h hardware malfunction
               84h undefined function requested
               8Fh undefined subfunction parameter
               A3h contents of source array corrupted (EMS 4.0?)

        Note: This function was designed to be used by multitasking
              operating systems and should not ordinarily be used by
              appplication software.