BASIC manipulates graphics with respect to the problem coordinates，which do not depend on hardware pixels.
A SET WINDOW statement introduce a problem coordinates on the drawing pane.
SET WINDOW left , right , bottom , top
introduces the coordinate system on which the x-coordinate of the left-hand edge is left, the x-coordinate of right-hand edge is right, the y-coordinate of the bottom-end is bottom, and the y-coordinate of top-end is top, where left , right , bottom and top are numerical expressions.
SET WINDOW -4,4,0,8
The initial aspect of the drawing pane is square.
Thus, the ratio of the vertical length to the horizontal length is 1:1 when the coordinates are set so that right
- left =top
If no SET WINDOW statement has been executed, the left-hand edge coordinate and the bottom-end coordinate are 0, and the right-hand edge coordinate and the top-end coordinate are 1.
ASK WINDOW x1, x2, y1, y2
assign s the current coordinates to numerical variables x1, x2, y1, y2.