Int 21H, AH=0FH

1 and higher

OPEN FILE USING FCB

DosOpen

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

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

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)

SeeAlso: #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) SeeAlso: #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.

SeeAlso: #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

AH=10h,AH=16h,AH=3Dh,AX=4303h

Text based on Ralf Brown Interrupt List Release 61