computer science

HAL (software)

HAL (software) is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand HAL (software) rather than just read about it. In short: HAL (Hardware Abstraction Layer or rather Hardware Annotation Library) is a software subsystem for UNIX-like operating systems providing hardware abstraction. HAL is now deprecated on most Linux distributions and on FreeBSD.

Key takeaways

HAL (software) belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect HAL (software) to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of HAL (software) from memory before moving on to harder problems.

Reference excerpt

HAL (Hardware Abstraction Layer or rather Hardware Annotation Library) is a software subsystem for UNIX-like operating systems providing hardware abstraction. HAL is now deprecated on most Linux distributions and on FreeBSD. Functionality is being merged into udev on Linux as of 2008–2010 and devd on FreeBSD. Previously, HAL was built on top of udev. Some other operating systems which don't have an alternative like udev or devd still use HAL. The purpose of the hardware abstraction layer was to allow desktop applications to discover and use the hardware of the host system through a simple, portable and abstract API, regardless of the type of the underlying hardware. HAL for Linux OS was originally envisioned by Havoc Pennington. It became a freedesktop.org project, and was a key part of the software stack of the GNOME and KDE desktop environments. It is free software, dual-licensed under both the GNU General Public License and the Academic Free License. HAL is unrelated to the concept of Windows NT kernel HALs, which handle some platform-specific core functionality within the kernel, such as interrupt routing.

Rationale Traditionally, the operating system kernel was responsible for providing an abstract interface to the hardware the system ran on. Applications used the system call interface, or performed file I/O on device nodes in order to communicate with hardware through these abstractions. This sufficed for the simple hardware of early desktop computing. Computer hardware, however, has increased in complexity and the abstractions provided by Unix kernels have not kept pace with the proliferating number of device and peripheral types now common on both server and desktop computers. Most modern buses have also become hotplug-capable and can have non-trivial topologies. As a result, devices are discovered or change state in ways which can be difficult to track through the system call interface or Unix IPC. The complexity of doing so forces application authors to re-implement hardware support logic. Some devices also require privileged helper programs to prepare them for use. These must often be invoked in ways that can be awkward to express with the Unix permissions model (for example, allowing users to join wireless networks only if they are logged into the video console). Application authors resort to using setuid binaries or run service daemons to provide their own access control and privilege separation, potentially introducing security holes each time.

Design HAL is a single daemon responsible for discovering, enumerating and mediating access to most of the hardware on the host computer. Applications communicate with HAL through the D-Bus IPC mechanism, which abstracts the hardware behind an object-based RPC mechanism. Each logical hardware device is represented as a D-Bus object, and its bus address is used as a unique identifier. Devices include abstractions like disk partitions and visible wireless networks. The device's functionality is exposed through D-Bus interfaces, and its state accessed through properties, a set of key-value pairs. HAL broadcasts hardware events as signals on these objects; applications can listen for signals and react to the hardware events that they signify—events such as a digital camera being plugged in, an optical disc spinning up or a laptop computer closing its lid.

Implementations and obsolescence On Linux, HAL uses /sys (a virtual file system for Linux systems) to discover hardware and listen for kernel hotplug events. Some Linux distributions also provide a udev rule to allow the udev daemon to notify HAL whenever new device nodes appear.

Deprecated As of 2011, Linux distributions such as Ubuntu, Debian, and Fedora and on FreeBSD, and projects such as KDE, GNOME and X.org are in the process of deprecating HAL as it has "become a large monolithic unmaintainable mess". The process is largely complete, but some use of HAL remains – Debian squeeze (Feb 2011) and Ubuntu version 10.04 remove HAL from the basic system and boot process. In Linux, it is in the process of being merged into udev (main udev, libudev, and udev-extras) and existing udev and kernel functionality. The replacement for non-Linux systems such as FreeBSD is devd. Initially a new daemon DeviceKit was planned to replace certain aspects of HAL, but in March 2009, DeviceKit was deprecated in favor of adding the same code to udev as package udev-extras, and some functions have now moved to udev proper.

Worked examples

Example 1 — a first encounter with HAL (software)

Start with the simplest possible case. Write down what HAL (software) claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to HAL (software) before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about HAL (software) ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of HAL (software)

In research: HAL (software) appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses HAL (software) in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: HAL (software) is common in secondary-school and first-year university syllabi. It links to neighbouring topics Applications using D-Bus, Device drivers, Free system software, so understanding it makes those chapters shorter.
In everyday life: Look for HAL (software) outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study HAL (software) in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what HAL (software) means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain HAL (software) out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is HAL (software) in simple terms?

Why does HAL (software) matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study HAL (software)?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on HAL (software).