INT 67H 4fxxH: Get/Set Partial Page Map

                                                     Compatibility: EMS 4.0+ 
 Expects: AH    4fH
          AL    subfn: 00H=Save some mapping registers
                           DS:SI=list of registers to query
                           ES:DI=buffer to receive data
                       01H=Restore some mapping registers
                           DS:SI=data obtained via subfn 00H
                       02H=Query buffer size for subfns 00H and 01H
                           BX=count of pages to be saved/restored
 Returns: AH    EMM status (0=successful)
          AL    (subfn 02H) size, in bytes of buffer for data buffers
          ES:DI (subfn 00H) buffer contains returned information
    Info: This fn is similar to INT 67H 4exxH, in that it saves/restores
          the page mapping registers.  This version lets you specify a
          subset (e.g., save/restore just one or two pages).

          Before using subfn 4f00H, you should use subfn 4f02H to see how
          large of a buffer you will need.

    DS:SI (subfn 4f00H) is a list that identifies which mapping registers
          you wish to save.  The list is formatted as:

           Offset Size Contents
           ▀▀▀▀▀▀ ▀▀▀▀ ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
            +0      2  wPgCnt    number of 16K pages to save
            +2      2  wSeg1     segment of first physical pg to save
            +4      2  wSeg2     segment of second physical pg to save

    ES:SI (subfn 4f00H) is a buffer to receive mapping-register data.  You
          should use subfn 4f02H to see how large this should be.

See Also: INT 67H: EMM Services
          Extended Memory Specification (XMS)
          Interrupts and BIOS Services