Atari 400 Technical Reference Manual page 65

OPEN Input (AUXl = $04)
The indicated file is opened for input. Any wild-card characters
are used to search for the first match. If the file is not found,
an error status is returned, and no file will be opened.
OPEN Output (AUXl
=
$08)
The indicated file is opened for output starting with the first
byte of the file, if the file is not locked. Any wild-card
characters are used to search for the first match. If the file
already exists, the existing file will be deleted before opening
the named file as a new file. If the file does not already exist,
it will be created.
A file opened for output will not appear in the directory until
it has been closed. If an output file is not properly closed,
some or all of the sectors that were acquired for it can be lost
until the disk is reformatted.
A file that is opened for output can not be opened concurrently
for any other access.
OPEN Append (AUXl
=
$09)
The indicated file is opened for output starting with the byte
after the last byte of the existing file (that must already
exist), if the file is not locked. Any wild-card characters are
used to search for the first match.
If a file opened for append is not properly closed, the appended
data will be lost. The existing file will remain unmodified and
some or all of the sectors that were acquired for the appended
portion can be lost until the diskette is reformatted.
OPEN Update (AUXl = $OC)
The indicated file (that must already exist) will be opened for
update provided it is not locked. Any wild-card characters are
used to search for the first match.
The GET, PUT, NOTE and POINT operations are all valid, and can be
intermixed as desired.
If a file opened for update is not properly closed, a sector's
worth of information can be lost to the file. A file opened for
update can not be extended.
OPERATING SYSTEM C016555 -- Section 5
80