When the hardware does not support a feature through the hardware abstraction layer (HAL), DirectDraw attempts to emulate it. This emulated functionality is provided through the hardware-emulation layer (HEL). The HEL presents its capabilities to DirectDraw just as the HAL would. And, as with the HAL, applications never work directly with the HEL. The result is transparent support for almost all major features, regardless of whether a given feature is supported by hardware or through the HEL.
Obviously, software emulation cannot equal the performance that hardware features provide. You can query for the features the hardware supports by using the IDirectDraw2::GetCaps method. By examining these capabilities during application initialization, you can adjust application parameters to provide optimum performance over varying levels of hardware performance.
In some cases, certain combinations of hardware supported features and emulation can result in slower performance than emulation alone. For example, if the display device driver supports DirectDraw but not stretch blitting, noticeable performance losses will occur when stretch blitting from video memory surfaces. This happens because video memory is often slower than system memory, forcing the CPU to wait when accessing video memory surfaces. If your application uses a feature that isn't supported by the hardware, it is sometimes best to create surfaces in system memory, thereby avoiding performance losses created when the CPU accesses video memory.
For more information, see Hardware Abstraction Layer (HAL).