Compatibility
Various releases of Windows have included and supported various versions of DirectX, allowing newer versions of the operating system to continue running applications designed for earlier versions of DirectX until those versions can be gradually phased out in favor of newer APIs, drivers, and hardware.
APIs such as Direct3D and DirectSound need to interact with hardware, and they do this through a device driver. Hardware manufacturers have to write these drivers for a particular DirectX version's device driver interface (or DDI), and test each individual piece of hardware to make them DirectX compatible. Some hardware devices have only DirectX compatible drivers (in other words, one must install DirectX in order to use that hardware). Early versions of DirectX included an up-to-date library of all of the DirectX compatible drivers currently available. This practice was stopped however, in favor of the web-based Windows Update driver-update system, which allowed users to download only the drivers relevant to their hardware, rather than the entire library.
Prior to DirectX 10, DirectX runtime was designed to be backward compatible with older drivers, meaning that newer versions of the APIs were designed to interoperate with older drivers written against a previous version's DDI. The application programmer had to query the available hardware capabilities using a complex system of "cap bits" each tied to a particular hardware feature. Direct3D 7 and earlier would work on any version of the DDI, Direct3D 8 requires a minimum DDI level of 6 and Direct3D 9 requires a minimum DDI level of 7. However, the Direct3D 10 runtime in Windows Vista cannot run on older hardware drivers due to the significantly updated DDI, which requires a unified feature set and abandons the use of "cap bits".
Direct3D 11 runtime introduces Direct3D 9, 10, and 10.1 "feature levels", compatibility modes which allow use of only the hardware features defined in the specified version of Direct3D. For Direct3D 9 hardware, there are three different feature levels, grouped by common capabilities of "low", "med" and "high-end" video cards; the runtime directly uses Direct3D 9 DDI provided in all WDDM drivers.
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