Hardware abstraction
Encyclopedia
Hardware abstractions are sets of routines in software
Computer software
Computer software, or just software, is a collection of computer programs and related data that provide the instructions for telling a computer what to do and how to do it....

 that emulate some platform-specific details, giving programs direct access to the hardware
Computer hardware
Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...

 resources.

They often allow programmers to write device
Computer hardware
Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...

-independent, high performance applications by avoiding standard Operating System
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

 (OS) calls to hardware. The process of abstracting pieces of hardware is often done from the perspective of a CPU. Each type of CPU has a specific instruction set architecture or ISA. The ISA represents the primitive operations of the machine that are available for use by assembly programmers and compiler writers. One of the main functions of a compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...

 is to allow a programmer to write an algorithm in a high-level language without having to care about CPU-specific instructions. Then it is the job of the compiler to generate a CPU-specific executable. The same type of abstraction is made in operating systems, but OS APIs now represent the primitive operations of the machine, rather than an ISA. This allows a programmer to use OS-level operations (i.e. task creation/deletion) in their programs while still remaining portable
Software portability
Portability in high-level computer programming is the usability of the same software in different environments. The prerequirement for portability is the generalized abstraction between the application logic and system interfaces...

 over a variety of different platforms.

Overview

Many early computer systems do not have any form of hardware abstraction. This means that anyone writing a program for such a system must know how each hardware device communicates. This becomes a significant challenge to software developers, who must know how every device available works to ensure it's compatible. With hardware abstraction, the program instead tells the operating system what the device should do, and the operating system gives the instruction. This means that programmers don't need to know how individual devices work, and their programs are compatible with any device.

An Example of this might be a "Joystick" abstraction. The joystick
Joystick
A joystick is an input device consisting of a stick that pivots on a base and reports its angle or direction to the device it is controlling. Joysticks, also known as 'control columns', are the principal control in the cockpit of many civilian and military aircraft, either as a center stick or...

 device, there are many physical implementations, is readable / writable through an API which many joystick-like devices might share. Most joystick-devices might report movement directions. Many joystick-devices might have sensitivity-settings that can be configured by an outside application. A Joystick abstraction hides details (e.g., register formats, I2C address) of the hardware so that a programmer using the abstracted API needn't understand the details of the device physical interface. This also allows code reuse
Code reuse
Code reuse, also called software reuse, is the use of existing software, or software knowledge, to build new software.-Overview:Ad hoc code reuse has been practiced from the earliest days of programming. Programmers have always reused sections of code, templates, functions, and procedures...

 since the same code can process standardised messages from any kind of implementation which supplies the "joystick" abstraction. A "nudge forward" can be from a potentiometer or from a capacitive touch sensor that recognises "swipe" gestures, as long as they both provide a signal related to "movement".

As physical limitations (e.g. resolution of sensor, temporal update frequency) may vary with hardware, an API can do little to hide that, other than by assuming a "least common denominator" model. Thus, certain deep architectural decisions from the implementation may become relevant to users of a particular instantiation of an abstraction.

A good metaphor is the abstraction of transportation. Both bicycling and driving a car are transportation. They both have commonalities (e.g., you must steer) and physical differences (e.g., use of feet). One can always specify the abstraction "drive to" and let the implementor decide whether bicycling or driving a car is best. The "wheeled terrestrial transport" function is abstracted and the details of "how to drive" are encapsulated.

Examples of "abstractions" on a PC include video input, printers, audio input and output, block devices (e.g. hard disk drives or USB FLASH drives), etc.

In certain computer science domains, such as Operating Systems or Embedded Systems, the abstractions have slightly different appearances (for instance, OSes tend to have more standardized interfaces), but the concept of abstraction and encapsulation of complexity are common, and deep.

Hardware abstraction layers are of an even lower level in computer languages than application programming interface
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

s (API) because they interact directly with hardware
Hardware
Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....

 instead of a system kernel, therefore HALs require less processing time than APIs. Higher level languages often use HALs and APIs to communicate with lower level components.

In Operating Systems

A hardware abstraction layer (HAL) is an abstraction layer
Abstraction layer
An abstraction layer is a way of hiding the implementation details of a particular set of functionality...

, implemented in software, between the physical hardware
Computer hardware
Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...

 of a computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

 and the software
Computer software
Computer software, or just software, is a collection of computer programs and related data that provide the instructions for telling a computer what to do and how to do it....

 that runs on that computer. Its function is to hide differences in hardware from most of the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer 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
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

, Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

, CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...

, DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...

, 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
NetBSD
NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...

 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 /, , and other subsystems. Popular buses which are used on more than one architecture are also abstracted, such as ISA
Industry Standard Architecture
Industry Standard Architecture is a computer bus standard for IBM PC compatible computers introduced with the IBM Personal Computer to support its Intel 8088 microprocessor's 8-bit external data bus and extended to 16 bits for the IBM Personal Computer/AT's Intel 80286 processor...

, EISA
Extended Industry Standard Architecture
The Extended Industry Standard Architecture is a bus standard for IBM PC compatible computers...

, PCI
Peripheral Component Interconnect
Conventional PCI is a computer bus for attaching hardware devices in a computer...

, 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 system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...

s that run on dozens of different platforms.

Microsoft Windows

The Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...

 operating system has a HAL in the kernel space, between hardware and kernel, drivers, executive services. This allows portability of the Windows NT kernel-mode code to a variety of processors, with different memory management unit
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...

 architectures, and a variety of systems with different I/O bus architectures; most of that code runs without change on those systems, when compiled for the instruction set
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...

 for those systems. For example, the SGI
Silicon Graphics
Silicon Graphics, Inc. was a manufacturer of high-performance computing solutions, including computer hardware and software, founded in 1981 by Jim Clark...

 Intel x86-based workstations were not IBM PC compatible
IBM PC compatible
IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to...

 workstations, but due to the HAL, Windows NT was able to run on them.

Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...

 and later (Windows Server 2008 and later for servers) automatically detect which hardware abstraction layer (HAL) should be used at boot time.

AS/400

An "extreme" example of a HAL can be found in the System/38
System/38
The System/38 was a midrange computer server platform manufactured and sold by the IBM Corporation. The system offered a number of innovative features, and was the brainchild of IBM engineer Dr. Frank Soltis...

 and AS/400 architecture. Most compilers for those systems generate an abstract machine code; the Licensed Internal Code, or LIC, translates this virtual machine code into native code for the processor on which it is running and executes the resulting native code. (The exceptions are compilers that generate the LIC itself; those compilers are not available outside IBM.) This was so successful that application software and operating system software above the LIC layer that were compiled on the original S/38 run without modification and without recompilation on the latest AS/400 systems, despite the fact that the underlying hardware has been changed dramatically; at least three different types of processors have been in use.
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