Notes on State Changes

It is possible to have any number of simultaneous active output devices. It is possible to have 0, 1, 2 … and so on active output devices. For example, it is possible for both the LCD device and the CRT device to be active simultaneously. It is also possible for all display outputs devices to be inactive (this could happen in a system where multiple graphics cards are present).

The state of the output device is separate from the power state of the device. The “active” state represents whether the image being generated by the graphics adapter would be sent to this particular output device. A device can be powered off or in a low-power mode but still be the active output device. A device can also be in an off state but still be powered on.

Example of the display-switching mechanism:

The laptop has three output devices on the VGA adapter. At this moment in time, the panel and the TV are both active, while the CRT is inactive. The automatic display-switching capability has been disabled by OSPM by calling _DOS(0), represented by global variable display_switching = 0.

The platform runtime firmware, in order to track the state of these devices, will have three global variable to track the state of these devices. There are currently initialized to:

crt_active - 0 panel_active - 1 tv_active - 1

The user now presses the display toggle switch, which would switch the TV output to the CRT.

The platform runtime firmware first updates three temporary variables representing the desired state of output devices:

want_crt_active - 1 want_panel_active - 1 want_tv_active - 0

Then the platform runtime firmware checks the display_switching variable. Because this variable is set to zero, the platform runtime firmware does not do any device reprogramming, but instead generates a Notify (VGA, 0x80/0x81) event for the display. This event will be sent to OSPM.

OSPM will call the _DGS method for each enumerated output device to determine which devices should now be active. OSPM will determine whether this is possible, and will reconfigure the internal data structure of the OS to represent this state change. The graphics modes will be recomputed and reset.

Finally, OSPM will call the _DSS method for each output device it has reconfigured.


OSPM may not have called the _DSS routines with the same values and the _DGS routines returned, because the user may be overriding the default behavior of the hardware-switching driver or operating system-provided UI. The data returned by the _DGS method (the want_XXX values) are only a hint to the OS as to what should happen with the output devices.

If the display-switching variable is set to 1, then the platform runtime firmware would not send the event, but instead would automatically reprogram the devices to switch outputs. Any legacy display notification mechanism could also be performed at this time.