Function 42h (66)        Move File Pointer (LSEEK)

Move the file-pointer the specified number of bytes from the
beginning, end, or current location, as determined by the mode code in
AL.

On entry:      AH         42h
BX         File handle
CX:DX      Offset, in bytes (signed 32-bit integer)
AL         Mode code (see below)

Mode Code:     AL         Action
0          Move pointer CX:DX bytes from beginning of
file
1          Move pointer CX:DX bytes from current
location
2          Move pointer CX:DX bytes from end of file

Returns:       DX:AX      New pointer location (signed 32-bit integer),
or AX      Error code, if CF is set

Error codes:   1          Invalid mode code
6          Invalid handle

Call Function 59h for extended error code information
(DOS 3.0 and above).

                                                                          

This function changes the logical read/write position in the specified
file, by incrementing or decrementing the value assigned to the file
pointer. The offset to add to the file pointer is given as a signed
32-bit integer in the CX:DX register pair--the more significant
portion of the offset in CX. A mode code, specified in AL, indicates
whether the offset specified in CX:DX is referenced to the beginning
of the file (mode 0), the pointer's current location (mode 1), or the
end of the file (mode 2).

Notes:         No matter what mode is specified in AL, a positive
offset value in CX:DX always increases the value of
the file pointer. Note that using mode 2 with a
positive value in CX:DX will therefore move the file
pointer beyond the end of the file.

Use a mode code of 2 with an offset of 0 to move the
file pointer to the end of the file, or to find the
length of the file.

It is possible to move the pointer to a location
before the beginning of the file or after the end of
the file. Moving the pointer to a position before
the start of the file will not generate an error,
but the next read or write operation will fail.

A file can be extended by moving the file pointer
past the end of the file and writing one or more
bytes of data.

A file can be truncated my moving the file pointer
to the new end of file and writing zero bytes using
function 40h.

See also: 40h
See also: 59h

42h (66) Move File Pointer (LSEEK)