AT&T 6300 Programmer's Manual page 125

Graphics
The SCREEN statement must precede any I/O
statements to the screen, as it selects the
"screen attributes" to be used by subsequent
statements. The system assumes SCREEN
O,OjO,O by default if no screen attributes are
specified. This selects 80 columns Text Mode,
B/W, and only one display page.
You can also use more than one SCREEN
statement to define different screen attributes
for different sections of your program.
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