Page mapping

        Entry:
          AH = 6Ah
          AL = subfunction
              00h save partial page map
                  CH = first page frame
                  CL = number of frames
                  ES:DI -> buffer which is to be filled
              01h restore partial page map
                  CH = first page frame
                  CL = number of frames
                  DI:SI -> previously saved page map
              02h save and restore partial page map
                  CH = first page frame
                  CL = number of frames
                  ES:DI = buffer for current page map
                  DI:SI = new page map
              03h get size of save array
                  CH = first page frame
                  CL = number of frames
                  Return: AL = size of array in bytes
              04h switch to standard map register setting
              05h switch to alternate map register setting
              06h deallocate pages mapped to frames in conventional memory
                  CH = first page frame
                  CL = number of frames
        Return:
          AH = status

        Note:   Similar to EMS function 4Eh, except that a subrange of pages
                can be specified

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