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System File Table (Undocumented)
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): |
File Descriptor
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
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 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”.
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