System File Table (Undocumented)

DOS 2.x System File Table Format

Offset	Size	Description
00	dword	pointer to next system file table; offset of ffffh indicates last table in chain
04	word	number of file descriptors in table
06	nbytes	file descriptor table of the format (40 bytes each):

Offset	Size	Description
00	byte	count of handles referring to this file or zero if file is no longer open
01	byte	open mode
02	byte	file attribute
03	byte	drive (1=A, 2=B:, …, 0=char device)
04	11bytes	filename in FCB format
0F	word	unknown
11	word	unknown
13	dword	file size
17	word	file date
19	word	file time
1B	byte	device attribute
1C	word	starting cluster of file (block device)
	dword	pointer to device driver (if char device)
1E	word	if block device, relative cluster within file of the last cluster read; zero if file has never been read or written
20	word	absolute cluster number of current cluster
22	word	unknown
24	dword	current file position

Offset	Size	Description
00	dword	pointer to next system file table; offset of ffffh indicates last table in chain
04	word	number of file descriptors in table
06	nbytes	file descriptor table of the format (53 bytes each):

Offset	Size	Description
00	word	count of handles referring to this file or zero if file is no longer open
02	word	open mode, bit 15 set if file id opened via FCB
04	byte	file attribute
05	word	device info word
07	dword	if char device pointer to device driver header if block device pointer to DOS Device Control Block
0B	word	starting cluster of file
0D	word	file time
0F	word	file date
11	dword	file size
15	dword	current file position
19	word	if block device, relative cluster within file of the last cluster read
1B	word	absolute cluster number of last cluster read zero if file has never been read or written
1D	word	sector number containing the directory entry
1F	byte	number of dir entry within sector
20	11bytes	filename in FCB format
2B	dword	SHARE pointer to previous SFT sharing same file
2F	word	SHARE number of network machine opening file
31	word	PSP segment of file owner
33	word	offset within SHARE code segment of sharing record 0000h = none

Offset	Size	Description
00	dword	pointer to next system file table; offset of ffffh indicates last table in chain
04	word	number of file descriptors in table
06	nbytes	file descriptor table of the format (59 bytes each):

Offset	Size	Description
00	word	count of handles referring to this file or zero if file is no longer open
02	word	open mode, bit 15 set if file id opened via FCB
04	byte	file attribute
05	word	device info word
07	dword	if char device pointer to device driver header if block device pointer to DOS Device Control Block or REDIR data
0B	word	starting cluster of file
0D	word	file time
0F	word	file date
11	dword	file size
15	dword	current file position
19	dword	if network redirector, pointer to REDIRIFS record word if local block dev, relative cluster within file of the last cluster read
1B	dword	if local, sector number of directory entry
1F	byte	if local, number of directory entry within sector
20	11bytes	filename in FCB format
2B	dword	SHARE pointer to previous SFT sharing same file
2F	word	SHARE number of network machine opening file
31	word	PSP segment of file owner
33	word	offset within SHARE code seg of sharing record 0000h = none
35	word	absolute cluster number of last cluster read zero if file has never been read or written
37	dword	pointer to file IFS driver, NULL if native DOS

these structures are allocated by DOS during the scanning of the CONFIG.SYS file
INT 21h 67h can be used to allow an application to have more than 20 file handles; the total number of handles is limited still to the value of FILES=n in CONFIG.SYS
the structure of these fields is undocumented; I do not recommend relying on any of this information, instead use it as an aid in debugging
see Bibliography references to Bernd Schemmer and “Data Structures Used in IBM PC Compatibles and the PS/2”.
See INT 21,52