Int 21 Fn 0F  - DOS 1+ - Open File Using Fcb                               [D]

   AH = 0Fh
   DS:DX -> unopened File Control Block (see below)

Return: AL = status
       00h successful
       FFh file not found or access denied

Notes: (DOS 3.1+) file opened for read/write in compatibility mode
   an unopened FCB has the drive, filename, and extension fields filled
     in and all other bytes cleared
   not supported by MS Windows 3.0 DOSX.EXE DOS extender
   DR-DOS checks password attached with AX=4303h

See Also: AH=10h,AH=16h,AH=3Dh,4303h

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) (DOS 1.1+)
 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)

Note:  to use an extended FCB, you must specify the address of the FFh flag at
     offset -7, rather than the address of the drive number field

Format of reserved field for DOS 1.0:
Offset Size    Description
 16h   WORD    location in directory (if high byte = FFh, low byte is device
       ID)
 18h   WORD    number of first cluster in file
 1Ah   WORD    current absolute cluster number on disk
 1Ch   WORD    current relative cluster number within file
       (0 = first cluster of file, 1 = second cluster, etc.)
 1Eh   BYTE    dirty flag (00h = not dirty)
 1Fh   BYTE    unused

Format of reserved field for DOS 1.10-1.25:
Offset Size    Description
 18h   BYTE    bit 7: set if logical device
       bit 6: not dirty
       bits 5-0: disk number or logical device ID
 19h   WORD    starting cluster number on disk
 1Bh   WORD    current absolute cluster number on disk
 1Dh   WORD    current relative cluster number within file
 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 on disk
 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 or device
       bits 5-0: low six bits of device attribute word
---SHARE.EXE loaded, local file---
 1Ah   WORD    starting cluster of file on disk
 1Ch   WORD    (DOS 3.x) 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 accessed
 1Eh   BYTE    absolute cluster number of last cluster accessed
 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.

Format of reserved field for DOS 5.0:
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 or device
       bits 5-0: low six bits of device attribute word
---SHARE.EXE loaded, local file---
 1Ah   WORD    starting cluster of file on disk
 1Ch   WORD    unique sequence number of sharing record
 1Eh   BYTE    file attributes
 1Fh   BYTE    unused???
---SHARE.EXE loaded, remote file---
 1Ah   WORD    network handle
 1Ch   DWORD   network ID
---SHARE not loaded, local device---
 1Ah   DWORD   pointer to device driver header
 1Eh  2 BYTEs  unused???
---SHARE not loaded, local file---
 1Ah   BYTE    extra info
       bit 7: read-only attribute from SFT
       bit 6: archive attribute from SFT
       bits 5-0: high bits of sector number
 1Bh   WORD    starting cluster of file
 1Dh   WORD    low word of sector number containing directory entry
 1Fh   BYTE    number of directory entry within sector