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        Open disk file (INT 21h, 0Fh)

        Entry:
          AH = 0Fh
          DS:DX -> FCB (see below)
        Return:

          AL = 00h file found
               FFh file not found

        Note: (DOS 3+) file opened in compatibility mode

        Format of File Control Block:
                Offset  Size    Description
                 -7     BYTE    extended FCB if FFh
                 -6   5 BYTEs   reserved
                 -1     BYTE    file attribute if extended FCB
                 00h    BYTE    drive number (0 = default, 1 = A, etc)
                 01h  8 BYTEs   blank-padded file name
                 09h  3 BYTEs   blank-padded file extension
                 0Ch    WORD    current block number
                 0Eh    WORD    logical record size
                 10h    DWORD   file size
                 14h    WORD    date of last write (see AX=5700h)
                 16h    WORD    time of last write (see AX=5700h)
                 18h  8 BYTEs   reserved (see below)
                 20h    BYTE    record within current block
                 21h    DWORD   random access record number
                                (if record size is > 64 bytes, high byte
                                is omitted)

        Format of reserved field for DOS 1.x:
                Offset  Size    Description
                 18h    BYTE    bit 7: set if logical device
                                bit 6: set if open???
                                bits 5-0: disk number or logical device ID
                 19h    WORD    absolute current cluster number
                 1Bh    WORD    starting cluster number
                 1Dh    WORD    relative current cluster number
                 1Fh    BYTE    unused???

        Format of reserved field for DOS 2.x:
                Offset  Size    Description
                 18h    BYTE    bit 7: set if logical device
                                bit 6: set if open???
                                bits 5-0: ???
                 19h    WORD    starting cluster number
                 1Bh    WORD    ???
                 1Dh    BYTE    ???
                 1Eh    BYTE    ???
                 1Fh    BYTE    ???

        Format of reserved field for DOS 3.x:
                Offset  Size    Description
                 18h    BYTE    number of system file table entry for file
                 19h    BYTE    attributes
                                bits 7,6: 00 = SHARE.EXE not loaded,
                                               disk file
                                          01 = SHARE.EXE not loaded,
                                               character device
                                          10 = SHARE.EXE loaded, remote file
                                          11 = SHARE.EXE loaded, local file
                                bits 5-0: low six bits, device attribute word

                ---SHARE.EXE loaded, local file---
                 1Ah    WORD    starting cluster of file
                 1Ch    WORD    offset within SHARE of sharing record
                                (see AH=52h)
                 1Eh    BYTE    file attribute
                 1Fh    BYTE    ???
                ---SHARE.EXE loaded, remote file---
                 1Ah    WORD    number of sector containing directory entry
                 1Ch    WORD    relative cluster within file of last
                                cluster read
                 1Eh    BYTE    absolute cluster number of last cluster read
                 1Fh    BYTE    ???
                ---SHARE.EXE not loaded---
                 1Ah    BYTE    (low byte of device attribute word AND 0Ch)
                                OR open mode
                 1Bh    WORD    starting cluster of file
                 1Dh    WORD    number of sector containing directory entry
                 1Fh    BYTE    number of directory entry within sector

                Note: if FCB opened on character device, DWORD at 1Ah is set to the address
                      of the device driver header, then the BYTE at 1Ah is overwritten.