MultiMonitor Support and Windows Vista

Updated: November 8, 2006

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Graphics Multimonitor Concepts
Multimonitor Support before WDDM
Multimonitor Support in WDDM
WDDM and Multimonitor Configurations
Error Messages for Multimonitor Issues
Resources

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.

Dual View

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.

Homogeneous Multi-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:

Two identical cards from the same graphics hardware vendor - for example, two PCIe ATI Radeon x600 cards, each in an x16 PCIe slot.
Two different cards from the same graphics hardware vendor - for example, one PCIe NVIDIA GeForce 7600 in an x16 slot and another PCIe NVIDIA GeForce 6600 in a second x16 slot.

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.

Heterogeneous Multi-adapter

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.

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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.

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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:

All graphics adapters in a system must use the same display driver model. That is, all of them should either be running XPDM or WDDM. The driver models are mutually exclusive, and Windows Vista does not allow the simultaneous loading of both an XPDM driver and a WDDM driver.
If a system has one graphics adapter with a XPDM driver and another with a WDDM driver, then Windows Vista will choose the POST device, which is the one with VGA resources. This is commonly referred to as the "VGA adapter."
If multiple graphics adapters are present in a system, all of them must use the same WDDM driver. If there are two graphics adapters with WDDM drivers from two different manufacturers, then Windows will disable one of them. The VGA adapter will be enabled, and the second device will be disabled.
Notice that XPDM drivers still support heterogeneous multi-adapter as they did in Windows XP. A user who has such a configuration working fine in Windows XP will encounter a problem when upgrading to Windows Vista. An external monitor connected to one of the graphics adapters will have no video signal, because it is disabled. An error message will appear on system boot, as described later in this article.
The solution for this problem could be as follows:
- A user could force the installation of a XPDM driver for each of these devices, and therefore get heterogeneous multi-adapter multi-monitor to work as in Windows XP.
  -Or-
- The user could change the graphics hardware configuration by choosing multiple graphics adapters that use the same WDDM driver. Graphics adapters from the same ASIC family generally have the same graphics driver. In late 2006, each of the major graphics vendors had a single WDDM driver for all supported WDDM graphics adapters. Please consult the graphics vendor's Web site for details on their driver support.

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.

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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:

Two monitors or dual-view: Single graphics adapter with two outputs, for example, DVI and VGA.
Two, three, or more monitors:
- Graphics adapter A with two outputs in an x16 PCIe slot, plus another graphics adapter B with one or more outputs in another x16 PCIe slot.
  Both A and B use the same WDDM driver.
- Graphics adapter A with two outputs in an x16 PCIe slot, plus another graphics adapter B with one or more outputs in an x1 PCIe slot.
  Both A and B use the same WDDM driver.
- Graphics adapter A with two outputs in an x16 PCIe slot, plus another graphics adapter B with one or more outputs in a conventional PCI slot.
  Both A and B use the same WDDM driver.
- Graphics adapter A with two outputs in an AGP slot, plus another graphics adapter B with one or more outputs in a conventional PCI slot.
  Both A and B use the same WDDM driver.
- A mobile PC with an internal graphics adapter A, connected to a docking station that includes an external graphics adapter B.
  Both the internal graphics adapter A and external graphics adapter B use the same WDDM driver.

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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:

Error Code 43

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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