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