"Hardware Abstraction Layer" redirects here. For the UNIX-like operating system subsystem, see
HAL (software)
.
Sets of routines in software
Hardware abstractions
are sets of routines in
software
that provide programs with access to
hardware
resources through programming interfaces. The programming interface allows all devices in a particular class
C
of hardware devices to be accessed through identical interfaces even though
C
may contain different subclasses of devices that each provide a different hardware interface.
Hardware abstractions often allow programmers to write
device
-independent, high performance applications by providing standard
operating 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
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 (e.g. task creation/deletion) in their programs while retaining
portability
over a variety of different platforms.
Overview
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Many early computer systems did not have any form of hardware abstraction. This meant that anyone writing a program for such a system would have to know how each hardware device communicated with the rest of the system. This was a significant challenge to software developers since they then had to know how every hardware device in a system worked to ensure the software's
compatibility
. With hardware abstraction, rather than the program communicating directly with the hardware device, it communicates to the operating system what the device should do, which then generates a hardware-dependent instruction to the device. This meant programmers didn't need to know how specific devices worked, making their programs compatible with any device.
An example of this might be a "Joystick" abstraction. The
joystick
device, of which 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 format,
I2C
address) of the hardware so that a programmer using the abstracted API, does not need to understand the details of the device's physical interface. This also allows
code reuse
since the same code can process standardized 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 drive
), etc.
In certain computer science domains, such as operating systems or embedded systems, the abstractions have slightly different appearances (for instance, Operating Systems tend to have more standardized interfaces), but the concept of abstraction and encapsulation of complexity are common, and deep.
The hardware abstraction layer reside below the
application programming interface
(API) in a software stack, whereas the application layer (often written in a high level language) resides above the API and communicates with the hardware by calling functions in the API.
In operating systems
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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 Microsoft Windows, 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, but prevents direct access to the hardware.
CP/M
(
CP/M BIOS
),
DOS
(
DOS BIOS
),
Solaris
,
Linux
,
BSD
,
macOS
, 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.
[1]
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
,
PCIe
, etc., allowing drivers to also be highly portable with a minimum of code modification.
Operating systems having a defined HAL are more easily portable across different hardware. This is especially important for
embedded systems
that run on dozens of different platforms.
Microsoft Windows
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The
Windows NT
kernel has a HAL in the kernel space between hardware and the executive services that are contained in the file
NTOSKRNL.EXE
[2]
[3]
under
%WINDOWS%\system32\hal.dll
. This allows portability of the Windows NT kernel-mode code to a variety of processors, with different
memory management unit
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
applicable to those systems. For example, the
SGI
Intel x86-based workstations were not
IBM PC compatible
workstations, but due to the HAL,
Windows 2000
was able to run on them.
[4]
[
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]
Since
Windows Vista
and
Windows Server 2008
, the HAL used is automatically determined during
startup
.
[5]
AS/400
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An "extreme" example of a HAL can be found in the
System/38
and
AS/400
architectures, currently implemented in the
IBM i
operating system. 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.
[6]
(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.
[6]
Android
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]
Android
introduced a HAL known as the "vendor interface" (codenamed "Project Treble") on
version 8.0 "Oreo"
. It abstracts low-level code from the Android OS framework, and they must be made
forward compatible
to support future versions of Android to ease the development of firmware updates.
[7]
Before Project Treble Android relied on various non-standardized legacy HALs.
[8]
Halium
is an Android-based HAL used by several mobile operating systems such as
Ubuntu Touch
and
LuneOS
to run on smartphones with Android pre-installed.
See also
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References
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- ^
"Portability and supported hardware platforms"
. The NetBSD Foundation
. Retrieved
12 May
2009
.
- ^
"Windows NT Hardware Abstraction Layer (HAL)"
.
Microsoft
. 31 October 2006
. Retrieved
25 August
2007
.
- ^
Custer, Helen (1993),
Inside Windows NT
,
Microsoft Press
,
Bibcode
:
1993iwn..book.....C
- ^
"Changing hardware abstraction layer in Windows 2000 / XP ? Smallvoid.com"
. 15 January 2001
. Retrieved
18 September
2020
.
- ^
Russinovich, Mark E.; Solomon, David A.; Ionescu, Alex (2008).
Windows Internals: Including Windows Server 2008 and Windows Vista
(5 ed.). Redmond, Washington, USA:
Microsoft Press
. p. 65.
ISBN
978-0-7356-2530-3
.
- ^
a
b
Soltis, Frank G. (1997).
Inside the AS/400: Featuring the AS/400e Series
(2 ed.). Loveland, Colorado, USA: Duke Press.
ISBN
978-1-882419-66-1
.
- ^
"Google's "Project Treble" solves one of Android's many update roadblocks"
.
Ars Technica
. 12 May 2017
. Retrieved
12 May
2017
.
- ^
"Conventional & legacy HALs"
.
Android Open Source Project
.
Further reading
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]