The Hardware Abstraction Layer (HAL) is a kernel-mode library of hardware-manipulating routines provided by Microsoft or by the hardware manufacturer. It lies at the lowest level of the Windows NT Executive between the hardware and the rest of the operating system.
This software layer hides, or abstracts, the characteristics of the platform behind standard entry points so that all platforms and architectures look alike to the operating system. It enables the same operating system to run on different platforms with different processors. For example, it allows Windows NT to run on single or multiprocessor computers and enables higher-level device drivers, such as graphics display drivers, to format data for different kind of monitors.
A HAL is installed during setup. Different processor configurations often use different HAL drivers. The HAL routines are called by other components of the Windows NT Executive, including the Microkernel, and by higher-level device drivers.