Mac OS: Display Resolution Switching at System Startup Time

When you turn on your Macintosh computer, the display may appear to momentarily flash or flicker, or it may display a very large image that suddenly turns into a smaller image as the computer turns on. This is normal, as long as the Macintosh uses the resolution you chose in the Monitors control panel when the Finder finishes starting up. This behavior varies from Macintosh to Macintosh depending on the model, video card, software versions, and monitor. Essentially, the computer is determining which resolution, or screen size, to use as it starts up. As it determines and sets the correct resolution, it may flash, flicker, or display images at smaller resolutions until the final resolution is displayed.

If the preferred resolution is not available on the display after the computer has started up, you may not have set up the display correctly. If your computer and display have ADB or USB connections, make sure the display is connected properly, and that the latest version of Apple Displays software is installed.

There are a number of variables that determine a display's resolution setting at system startup time:

• computer model (video driver in ROM varies)
• version of Apple Displays software (resolution stored in PRAM or preference file)
• type of video card

On earlier computers, the resolution is stored in PRAM and is read at system startup time by the video driver in ROM. Later versions of Apple Displays software rely on a preference stored on the hard drive to set this resolution instead of the PRAM setting. This preference is read during the startup sequence, and is applied either during system startup time, or when the Finder starts up, depending on the video controller.

On some computers, you can use the Monitors control panel to set the PRAM setting to the resolution of your choice. This is not the case with all Macintosh computers.

Different video cards and built-in video handle resolution preference loading at different times, depending on when the driver for the video card and built-in video is loaded. Some computers and video cards (usually older models) do not recognize multiple scan displays when the computer is turned on, because the required driver is not built into ROM, and is only loaded when the Apple Displays Software loads.

AppleVision/ColorSync models have a unique sense pin setting that, when combined with the ADB connection, tells the computer that it is an AppleVision display. Until the AppleVision extension loads, some computers and video cards will read this sense pin as a 640 by 480 display since the sense pin algorithm is not yet complete. Therefore, the computer will start up in 640 by 480. After the AppleVision extension loads, the computer will recognize the display properly, and apply the user's preference for resolution setting. Again, depending which video controller is used, the preference may load the computer is turning on, or after it has started up.