Using Dual Displays During Startup or in DOS
This application note discusses using two displays running under DOS or when booting into an OS such as Windows or Linux. This document explains the issues with pre-OS dual displays and provides workarounds to address the situation using step-by-step examples. Dual display is allowed in DOS on Twin and Clone modes which are described in “Example: Enabling Clone Mode for Dual Display” and “Example: Enabling Twin Mode with LVDS and CRT”.
To be completely technically accurate, most likely we are talking about Clone mode, not Twin mode. To understand the difference between these two modes it is helpful to understand the graphics “three P's” – Planes, Pipes and Ports.
A “plane” refers to an area of graphics memory allocated to hold the data that needs to be displayed. The display's frame buffer memory is a “plane,” as are the overlay plane, sprite plane, hardware cursor plane, etc.
A “pipe” is the data pipeline that gets the display data from the plane to the display technology (the “port”). The pipe establishes structure to the raw data from the plane and establishes the timing of the data appropriate to display. The pipe also handles getting data from the various planes that may need to be displayed in the proper order such as the main framebuffer until it needs to get data from the overlay plane to display video data, then back to the framebuffer, then the cursor plane, etc. The pipe handles all this switching around seamlessly and without CPU or GPU intervention. It is a “set and forget” type of capability and the right data is displayed in the correct place once it is set correctly.
Read the full Using Dual Displays During Startup or in DOS Application Note.