Write sector buffer (XT,PORT,PS)
        (harddisk only)

        Entry:
          AH = 0Fh
          AL = number of sectors
          CH = cylinder (bits 8,9 in top two bits of CL)
          CL = sector number
          DH = head number
          DL = drive number
          ES:BX = pointer to buffer

        Return:
          AL = number of sectors actually transferred
          AH = status code (see AH=1 above)

        Notes:  Should be called before formatting to initialize the
                controller's sector buffer.
                Used for diagnostics only on PS/2 systems