Write long (XT,AT,XT286,PS)
        (harddisk only)

        Entry:
          AH = 0Bh
          DL = drive ID
          DH = head
          CH = cylinder (bits 8,9 in high bits of CL)
          CL = sector
          ES:BX = pointer to buffer containing data

        Return:
          CF set on error
          AH = status code (see AH=1 above)

        Note: used for diagnostics only on PS/2 systems