Int 21H, AH=42H

2 and higher

“LSEEK” - SET CURRENT FILE POSITION

      AH = 42h
      AL = origin of move
          00h start of file
          01h current file position
          02h end of file
      BX = file handle
      CX:DX = (signed) offset from origin of new file position

CF clear if successful

          DX:AX = new file position in bytes from start of file
      CF set on error
          AX = error code (01h,06h) (see #01680 at AH=59h/BX=0000h)

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

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

BUG: 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

AH=24h,INT 2F/AX=1228h

Text based on Ralf Brown Interrupt List Release 61