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Copyright 1991 David Jurgens |
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
DOS 3.x System File Table and FCB 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 (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
DOS 4.0+ System File Table and FCB Table
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 21,67 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
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