Int 21/AH=0Fh

DOS 1+ - OPEN FILE USING FCB

AH = 0Fh
DS:DX -> unopened File Control Block (see #01345,#01346)

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. (FAT32 drive) this function will only succeed for creating a volume label; FAT32 does not support FCBs for file I/O

BUG: APPEND for DOS 3.3+ corrupts DX if the file is not found

See Also: AH=10h - AH=16h - AH=3Dh - AX=4303h


Format of File Control Block:

Offset  Size    Description     (Table 01345)
00h    BYTE    drive number (0 = default, 1 = A, etc)
FFh is not allowed (signals extended FCB, see #01346)
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 #01666 at AX=5700h)
16h    WORD    time of last write (see #01665 at AX=5700h) (DOS 1.1+)
18h  8 BYTEs   reserved (see #01347,#01348,#01349,#01350,#01351)
20h    BYTE    record within current block
21h    DWORD   random access record number (if record size is > 64 bytes, high
byte is omitted)

See Also: #01346

Format of Extended File Control Block (XFCB): Offset Size Description (Table 01346) 00h BYTE FFh signature for extended FCB 01h 5 BYTEs reserved 06h BYTE file attribute if extended FCB 07h 36 BYTEs standard FCB (all offsets are shifted by seven bytes)

See Also: #01246

Format of FCB reserved field for DOS 1.0: Offset Size Description (Table 01347) 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 FCB reserved field for DOS 1.10-1.25: Offset Size Description (Table 01348)

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 FCB reserved field for DOS 2.x: Offset Size Description (Table 01349)

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 FCB reserved field for DOS 3.x: Offset Size Description (Table 01350) 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 #01637 at AH=52h) 1Eh BYTE file attribute 1Fh BYTE ??? ---SHARE.EXE loaded, remote file--- 1Ah WORD number of sector containing directory entry (see #01352) 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 (see #01352) 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.

See Also: #01646

Format of FCB reserved field for DOS 5.0: Offset Size Description (Table 01351) 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 (see #01646) 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 (see #01352) 1Fh BYTE number of directory entry within sector

Category: DOS Kernel - Int 21h - D

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