AT&T 6300 Programmer's Manual page 380

OPEN
Statement
7-224
The GWBASIC file I/O system allows you to
utilize user installed devices.
Character devices are opened and used in the
same manner as disk files except that
characters are not buffered by GWBASIC as
they are for disk files. The record length is set
to one.
GWBASIC only sends a carriage return as
end of line. If the device requires a LF (line
feed), the driver must provide it.
*
When writing
device drivers, keep in mind that GWBASIC
users want to read and write control
information. Writing and reading of device
control data is handled by the GWBASIC
IOCTL statement and IOCTL$ function.
Rules
• If you enter a value outside of the
corresponding range, an "Illegal function call"
error is returned, and the file will not be
opened.
• If the file is opened for INPUT, attempts to
write to the file result in a "Bad File Mode"
error. If a file that is opened for input does not
exist, a "File not found" error message is
displayed.
*The exception to this is output sent to a printer (LPTl,
LPT2, or LPT3), where each line ends with a linefeed unless
the printer is opened as a random file and WIDTH is set to
255.