AT&T 6300 Programmer's Manual page 452

SCREEN
Function
Remarks
Examples
7-296
In graphics mode the SCREEN function
returns zero if the specified location contains
graphics information.
100 X=SCREEN (10,10)
If the character at 10,10 is A, then x will
contain 65.
110 X=SCREEN (1,1,1)
Returns the color number of the character in
the upper left hand corner of the screen.