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The Windows NT hardware abstraction layer (HAL) refers to a layer of software that deals directly with your computer hardware. Because the HAL operates at a level between the hardware and the Windows NT executive services, applications and device drivers need not be aware of any hardware-specific information. The HAL provides routines that enable a single device driver to support a device on different hardware platforms, making device driver development much easier. It hides hardware dependent details such as I/O interfaces, interrupt controllers, and multiprocessor communication mechanisms. Applications and device drivers are no longer allowed to deal with hardware directly and must make calls to HAL routines to determine hardware specific information. Thus, through the filter provided by the HAL, different hardware configurations can be accessed in the same manner.
Microsoft Windows NT "Resource Guide"
"Inside Windows NT," Helen Custer, Microsoft Press, 1993