Hardware Abstraction Layer
The Windows NT hardware abstraction layer, or HAL, is a layer between the physical hardware of the computer and the rest of the operating system. It was designed to hide differences in hardware and therefore provide a consistent platform on which the kernel is run. The HAL includes hardware-specific code that controls I/O interfaces, interrupt controllers and multiple processors.
However, despite its purpose and designated place within the architecture, the HAL isn't a layer that sits entirely below the kernel, the way the kernel sits below the Executive: all known HAL implementations depend in some measure on the kernel, or even the Executive. In practice, this means that kernel and HAL variants come in matching sets that are specifically engineered to work together.
In particular hardware abstraction does not involve abstracting the instruction set, which generally falls under the wider concept of portability. Abstracting the instruction set, when necessary (such as for handling the several revisions to the x86 instruction set, or emulating a missing math coprocessor), is performed by the kernel, or via platform virtualization.
Read more about this topic: Windows NT Architecture, Kernel Mode
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