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Format of DOS 3.0 system file tables and FCB tables

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Offset Size Description

00h

DWORD

pointer to next file table (offset FFFFh if last)

04h

WORD

number of files in this table

06h

38h BYTEs

per file
Offset Size Description

00h

WORD

number of file handles referring to this file

02h

WORD

file open mode (see AX=6C00h, #01402 at AH=3Dh)
bit 15 set if this file opened via FCB

04h

BYTE

file attribute (see #01420 at AX=4301h)

05h

WORD

device info word (see #01423 at AX=4400h)
bit 15 : set if remote file
bit 14 : set means do not set file date/time on closing
bit 12 : set means don't inherit on EXEC
bits 5-0 : drive number for disk files

07h

DWORD

pointer to device driver header if character device
else pointer to DOS Drive Parameter Block (see #01395 at AH=32h)

0Bh

WORD

starting cluster of file

0Dh

WORD

file time in packed format (see #01665 at AX=5700h)
not used for character devices in DR DOS

0Fh

WORD

file date in packed format (see #01666 at AX=5700h)
not used for character devices in DR DOS

11h

DWORD

file size

---system file table---

15h

DWORD

current offset in file (may be larger than size of file; INT 21/AH=42h does not check new position)

---FCB table---

15h

WORD

counter for last I/O to FCB

17h

WORD

counter for last open of FCB (these are separate to determine the times of the latest I/O and open)

---

19h

WORD

relative cluster within file of last cluster accessed

1Bh

WORD

absolute cluster number of last cluster accessed
0000h if file never read or written???

1Dh

WORD

number of sector containing directory entry (see #01352)

1Fh

WORD

byte offset of directory entry within sector

21h

11 BYTEs

filename in FCB format (no path/period, blank-padded)

2Ch

DWORD

(SHARE.EXE) pointer to previous SFT sharing same file

30h

WORD

(SHARE.EXE) network machine number which opened file
(Windows Enhanced mode DOSMGR uses the virtual machine ID as the machine number; see INT 2F/AX=1683h)

32h

WORD

PSP segment of file's owner (first three entries for AUX/CON/PRN contain segment of IO.SYS startup code)

34h

WORD

(SHARE.EXE) offset in SHARE code seg of share record

36h

WORD

??? apparently always 0000h

See Also:

#01639,#01641,#01642