MultiMonitor Support and Windows Vista
Updated: November 8, 2006
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Desktop real estate has been found to be a key factor in enhancing productivity of an information worker. It is not uncommon these days to see the use of two or more than two displays connected to a single PC. Multimonitor - the common technical term for more than one display on a single PC - has been a critical feature supported through Microsoft Windows operating systems. This capability is sometimes referred to as 'multimon' or 'multimonitor' among users.
This article discusses the various multi-adapter scenarios that exist today, summarizes changes brought through WDDM, and discusses various solutions based on appropriate hardware choices.
Graphics Multimonitor Concepts
The basic concepts related to the various types of multiple-adapter configurations are not new or specific to Windows Vista, but are important to understand in relation to the design of the multi-adapter feature in Windows Vista.
Most modern graphics adapters have at least two display outputs. Each of these outputs on a single graphics adapter could be connected individually to a display device or monitor to provide an "extended desktop" or to have the same desktop presented on both displays - that is, a "clone view." This is the most prevalent case where multi-monitor capabilities are achieved through a single graphics adapter.
Multi-adapter refers to the use of more than one graphics adapter in a single PC. The term "homogeneous multi-adapter" is used to refer to cases when more than one graphics adapter is in use but all adapters use the same graphics driver.
Here are two examples:
Notice that the bus type - PCIe, AGP, or PCI - is irrelevant. You could have "n" cards in "n" PCIe slots of the same or different lane widths, or you could have "n-m" cards in PCIe slots and "m" cards in PCI slots. The key point to remember is that all "n" graphics adapters use a single graphics driver.
The term "heterogeneous multi-adapter" is used to refer to multiple graphics adapters using multiple graphics drivers in a single PC. A common example is the use of graphics adapters from two different manufacturers, each of which requires a different graphics driver from the respective manufacturer.
Multimonitor Support before WDDM
Although Windows NT 4.0 did not have native multi-monitor support in the operating system, graphics hardware vendors could incorporate some level of support through their customized graphics drivers, software utilities, or both.
Windows 2000 provided some native support for management of multiple displays to present the desktop. Windows XP took this a step further through the Windows XP Display Driver Model (XPDM) to support multiple graphics cards natively and provided the hardware vendors with the proper means to offer support in their drivers.
Multimonitor Support in WDDM
On Windows Vista, older XPDM drivers still work and the multi-monitor behavior with XPDM drivers hasn't changed, because the operating system uses the legacy graphics stack.
However, the Windows Vista Display Driver Model (WDDM) brings fundamental changes to the management of multiple graphics adapters and external displays. This includes a new restriction, because WDDM drivers do not support "heterogeneous multi-adapter" multi-monitor implementations. Specifically:
Background Notes: This restriction only affects a system that has WDDM drivers. WDDM was designed with stability as a key objective. Based on information gathered through Windows Error Reporting and the related Online Crash Analysis for Windows XP display drivers, Microsoft decided to simplify the graphics stack in Windows Vista.
The use of multiple graphics adapters occurred when graphics hardware vendors did not expose multiple connectors on graphics adapters. Today, almost all modern adapters support two or three connectors such as DVI, VGA, and S-Video. Also, most OEMs are now offering SLI/Crossfire configurations that support two or more graphics adapters that could also be used to connect more than two display devices when not in SLI/Crossfire mode.
WDDM and Multimonitor Configurations
The following examples describe possible desktop and mobile system configurations with WDDM-capable graphics adapters in cases where a user wants to connect more than one display device:
Error Messages for Multimonitor Issues
In a system configured with heterogeneous graphics adapters (that is, adapters that use different drivers), Windows Vista will disable one of the two adapters and present an error message as shown here:
Device Manager will show the disabled device with an Error Code 43. Event Log Viewer will also have a message logged that contains the same text as in the error message displayed.
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