Hardware abstraction layer
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A hardware abstraction layer (HAL) is a layer between the physical hardware of a computer and the software that runs on that computer. The function is to hide differences in hardware and therefore provide a consistent platform to run applications on.
The best example of an HAL can be found in the AS/400 architecture. The implementation of the LIC, or Licensed Internal Code, was so successful that software written on the predecessor, the S/38, runs without modifications on an AS/400. That, regardless of the underlying hardware having been changed dramatically; at least 3 different types of processors have been in use.
BSD, Linux and the Windows NT based operating systems, also have a HAL. These operating systems have different subsystems for particular functions; e.g. sound and vision.
Operating systems having a defined HAL are easily portable across different hardware. This is especially important for embedded systems that run on dozens of different microcontrollers.
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