In Operating Systems
A hardware abstraction layer (HAL) is an abstraction layer, implemented in software, between the physical hardware of a computer and the software that runs on that computer. Its function is to hide differences in hardware from most of the operating system kernel, so that most of the kernel-mode code does not need to be changed to run on systems with different hardware. On a PC, HAL can basically be considered to be the driver for the motherboard and allows instructions from higher level computer languages to communicate with lower level components, such as directly with hardware.
BSD, Mac OS X, Linux, CP/M, DOS, Solaris, and some other portable operating systems also have a HAL, even if it is not explicitly designated as such. Some operating systems, such as Linux, have the ability to insert one while running, like Adeos. The NetBSD operating system is widely known as having a clean hardware abstraction layer which allows it to be highly portable. As part of this system are uvm(9)/pmap(9), bus_space(9), bus_dma(9) and other subsystems. Popular buses which are used on more than one architecture are also abstracted, such as ISA, EISA, PCI, PCI-E, etc., allowing drivers to also be highly portable with a minimum of code modification.
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 platforms.
Read more about this topic: Hardware Abstraction
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