AT&T 6300 Programmer's Manual page 379

Remarks
OPEN
Statement
A disk file must be opened before any disk I/O
operation can be performed on that file. OPEN
allocates a buffer for I/O to the file or device
and determines the mode of access that will be
used with the buffer. The "filenum" parameter
specifies the number which will be associated
with the file as long as it is open and is used
by other I/O statements to refer to the file or
device.
For each device or file, the following OPEN
modes are allowed:
KYBD:
SCRN:
LPTl:
LPT2:
LPT3:
COMl:, COM2:,
COM3:, and COM4:
Disk files allow all modes
INPUT only
OUTPUT only
OUTPUT only
OUTPUT only
OUTPUT only
INPUT, OUTPUT, or random only
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