Inside Macintosh: Imaging with QuickDraw
The user can use the Monitors control panel to set the desired pixel depth of each screen; whether it displays color, grayscale, or black and white; and the position of each screen relative to the main screen. The Monitors control panel stores all configuration information for a multiscreen system in the System file in a resource of type 'scrn' that has a resource ID of 0. Your application should never create this resource, and should never alter or examine it.
When the InitGraf procedure (described in the chapter "Basic QuickDraw" in this book) initializes Color QuickDraw, it checks the System file for the 'scrn' resource. If the 'scrn' resource is found and it matches the hardware, InitGraf organizes the video devices according to the contents of this resource; if not, then Color QuickDraw uses only the video device for the startup screen.