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Hardware abstractions are sets of routines in software that emulate some platform-specific details, giving programs direct access to the hardware resources. They 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== 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, 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'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, 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 interfaces (API) because they interact directly with hardware 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hardware abstraction」の詳細全文を読む スポンサード リンク
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