When a window is placed (set) on the video screen, the characters and attributes displayed within the window are copied from the memory screen to the video screen.
If the memory screen is larger than the window, the image you see represents the area of the memory screen that the window's work area covers. The memory screen has a coordinate system where (0,0) is the top-left position. The window structure contains information about the window's buffer origin. The buffer origin is the memory screen location of the character displayed in the top-left corner of the window's work area. For example, if a window has a buffer origin of (0,0), the top left corner of the memory screen (location 0, 0) is displayed in the top left corner of the window's work area. If a window has a buffer origin of (5, 3), the character in the top left corner of the window's work area corresponds to the character on the fifth row and third column of the memory screen.
Figure 27.1 shows what the window would look like on a video screen with two different origins in a memory screen.
Figure 27.1: How Windows Appear on the Video Screen
