DOS Fn 42H: Set File Pointer

                                                         Compatibility: 2.0+ 
 Expects: AH    42H
          BX    file handle
          CX:DX distance to move file pointer: offset is (CX * 65536) + DX
          AL    subfn: 0 = move pointer to start of file + CX:DX
                       1 = move pointer to current position + CX:DX
                       2 = move pointer to end of file + CX:DX
 Returns: AX    error code if CF is set to CY
          DX:AX new position of file read/write pointer (if no error)
    Info: This moves the logical read/write pointer to the desired address.
          All file reads and writes will start from the new address.

   Notes: ■ One way to obtain the size of a file is to call with AL=2,
            CX=0, DX=0 (the length is returned in DX:AX --DX is the most
            significant word--actual length is (DX * 65536) + AX.

            A faster way is to use fn 4eH (find file).

          ■ To truncate a file, use LSEEK to move to the desired position,
            and use Fn 40H with CX=0.

          ■ To pre-allocate disk space to a file, use LSEEK to move to the
            desired file length, then use Fn 40H with CX=0.  Then close the
            file (Fn 3eH).

            (!) When you use this technique on a DoubleSpace compressed
            drive, DoubleSpace does not actually allocate disk storage from
            the physical medium.  It considers these bytes to be
            "infinitely compressible".  See MDFAT.

See Also: Handle-Oriented File I/O
          DOS Functions