Function 42h Move a File Read/Write Pointer (LSEEK) entry AH 42h AL method code 00h offset from beginning of file 01h offset from present location 02h offset from end of file BX file handle CX most significant half of offset DX least significant half of offset return AX low offset of new file pointer DX high offset of new file pointer CF 0 successful move 1 error AX error code (1, 6) note 1) If pointer is at end of file, reflects file size in bytes. 2) The value in DX:AX is the absolute 32 bit byte offset from the beginning of the file 3) for origins 01h and 02h, the pointer may be positioned before the start of the file; no error is returned in that case (except under Windows NT), but subsequent attempts at I/O will produce errors 4) if the new position is beyond the current end of file, the file will be extended by the next write (see AH=40h); for FAT32 drives, the file must have been opened with AX=6C00h with the "extended size" flag in order to expand the file beyond 2GB bugs using this method to grow a file from zero bytes to a very large size can corrupt the FAT in some versions of DOS; the file should first be grown from zero to one byte and then to the desired large size
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