Computer software

"Software" redirects here.

File:Packagedsoftwareatretail.jpg

Starting in the 1980s, application software has been sold in mass-produced packages through retailers.

Software fundamentally is the unique image or representation of physical or material alignment that constitutes configuration to or functional identity of a machine, usually a computer. As a content of memory, software in principle can be changed without the adjustment to the static paradigm of the hardware thus without the remanufacturing thereof. Commonly software is of an algorithmic form which translates into being to a sequence of machine instructions. Some software, however, is of a relational form which translates into being the map of a realization network (see VHDL).

Software is a program that enables a computer to perform a specific task, as opposed to the physical components of the system (hardware). This includes application software such as a word processor, which enables a user to perform a task, and system software such as an operating system, which enables other software to run properly, by interfacing with hardware and with other software.

The term "software" was first used in this sense by John W. Tukey in 1957. In computer science and software engineering, computer software is all computer programs. The concept of reading different sequences of instructions into the memory of a device to control computations was invented by Charles Babbage as part of his difference engine. The theory that is the basis for most modern software was first proposed by Alan Turing in his 1935 essay Computable numbers with an application to the Entscheidungsproblem.

Relationship to hardware

Computer software is so called in contrast to computer hardware, which encompasses the physical interconnections and devices required to store and execute (or run) the software. In computers, software is loaded into RAM and executed in the central processing unit. At the lowest level, software consists of a machine language specific to an individual processor. A machine language consists of groups of binary values signifying processor instructions (object code), which change the state of the computer from its preceding state. Software is an ordered sequence of instructions for changing the state of the computer hardware in a particular sequence. It is usually written in high-level programming languages that are easier and more efficient for humans to use (closer to natural language) than machine language. High-level languages are compiled or interpreted into machine language object code. Software may also be written in an assembly language, essentially, a mnemonic representation of a machine language using a natural language alphabet. Assembly language must be assembled into object code via an assembler.

Relationship to data

Software has historically been considered an intermediary between electronic hardware and data, which latter entity the hardware processes according to the sequence of instructions defined by the software. As computational math becomes increasingly complex, the distinction between software and data becomes less precise. Data has generally been considered as either the output or input of executed software. However, data is not the only possible output or input. For example, (system) configuration information may also be considered input, although not necessarily considered data (and certainly not applications data). The output of a particular piece of executed software may be the input for another executed piece of software. Therefore, software may be considered an interface between hardware, data, and/or (other) software.

Types

Practical computer systems divide software into three major classes: system software, programming software and application software, although the distinction is arbitrary and often blurred.

• System software helps run the computer hardware and computer system. It includes operating systems, device drivers, diagnostic tools, servers, windowing systems, utilities and more. The purpose of systems software is to insulate the applications programmer as much as possible from the details of the particular computer complex being use, especially memory and other hardware features, and such accessory devices as communications, printers, readers, displays, keyboards, etc.
• Programming software usually provides tools to assist a programmer in writing computer programs and software using different programming languages in a more convenient way. The tools include text editors, compilers, interpreters, linkers, debuggers, and so on. An Integrated development environment (IDE) merges those tools into a software bundle, and a programmer may not need to type multiple commands for compiling, interpreter, debugging, tracing, and etc., because the IDE usually has an advanced graphical user interface, or GUI.
• Application software allows humans to accomplish one or more specific (non-computer related) tasks. Typical applications include industrial automation, business software, educational software, medical software, databases and computer games. Businesses are probably the biggest users of application software, but almost every field of human activity now uses some form of application software. It is used to automate all sorts of functions.

Program and library

A program may not be sufficiently complete for execution by a computer. In particular, it may require additional software from a software library in order to be complete. Such a library may include software components used by stand-alone programs, but which cannot be executed on their own. Thus, programs may include standard routines that are common to many programs, extracted from these libraries. Libraries may also include 'stand-alone' programs which are activated by some computer event and/or perform some function (e.g., of computer 'housekeeping') but do not return data to their activating program. Programs may be called by other programs and/or may call other programs.vb

Three layers

Users often see things differently than programmers. People who use modern general purpose computers (as opposed to embedded systems, analog computers, supercomputers, etc.) usually see three layers of software performing a variety of tasks: platform, application, and user software.

Platform software

Platform includes the basic input-output system (often described as firmware rather than software), device drivers, an operating system, and typically a graphical user interface which, in total, allow a user to interact with the computer and its peripherals (associated equipment). Platform software often comes bundled with the computer, and users may not realize that it exists or that they have a choice to use different platform software.

Application software

Application software or Applications are what most people think of when they think of software. Typical examples include office suites and video games. Application software is often purchased separately from computer hardware. Sometimes applications are bundled with the computer, but that does not change the fact that they run as independent applications. Applications are almost always independent programs from the operating system, though they are often tailored for specific platforms. Most users think of compilers, databases, and other "system software" as applications.

User-written software

User software tailors systems to meet the users specific needs. User software include spreadsheet templates, word processor macros, scientific simulations, graphics and animation scripts. Even email filters are a kind of user software. Users create this software themselves and often overlook how important it is. Depending on how competently the user-written software has been integrated into purchased application packages, many users may not be aware of the distinction between the purchased packages, and what has been added by fellow co-workers.

Software architecture

The software architecture of a system comprises its software components, their external properties, and their relationships with one another. The term also refers to documentation of a system's software architecture. Documenting software architecture facilitates communication between stakeholders, documents early decisions about high-level design, and allows reuse of design components and patterns between projects.

Background

The field of computer science has come across problems associated with complexity since its formation. Earlier problems of complexity were solved by developers by choosing the right data structures, developing algorithms, and by applying the concept of separation of concerns. Although the term “software architecture” is relatively new to the industry, the fundamental principles of the field have been applied sporadically by software engineering pioneers since mid 1980s. Early attempts to capture and explain software architecture of a system were imprecise and disorganized - often characterized by a set of box-and-line diagrams. During the 1990’s there was a concentrated effort to define and codify fundamental aspects of the discipline. Initial sets of design patterns, styles, best practices, description languages, and formal logic were developed during that time.

The software architecture discipline is centered on the idea of reducing complexity through abstraction and separation of concerns. To date there is still no agreement on the precise definition of the term “software architecture”.

As a maturing discipline with no clear rules on the right way to architect a system the action of architecting is still a composition of art and science. The “art” aspect of software architecture is due to the fact that a commercial software system supports some aspect of a business or a mission. How a system supports key business drivers is described via scenarios as non-functional requirements of a system, also known as quality attributes, determine how a system will behave. Every system is unique due to the nature of the business drivers it supports, as such the degree of quality attributes exhibited by a system such as fault-tolerance, backward compatibility, extensibility, reliability, maintainability, availability, security, usability, and such other –ilities will vary with each implementation.

To bring a software architecture user's perspective into the software architecture, it can be said that software architecture gives the direction to take steps and do the tasks involved in each such user's speciality area and interest e.g. the stake holders of software systems, the software developer, the software system operational support group, the software maintenance specialists, the deployer, the tester and also the business end userTemplate:Cite needed. In this sense software architecture is really the amalgamation of the multiple perspectives a system always embodies. The fact that those several different perspectives can be put together into a software architecture stands as the vindication of the need and justification of creation of software architecture before the software development in a project attains maturity.

History

The origins of software architecture as a concept was first identified in the research work of Edsger Dijkstra in 1968 and David Parnas in the early 1970’s. The scientists emphasized that the structure of a software system matters and getting the structure right is critical. The study of the field increased in popularity since the early 1990s with research work concentrating on architectural styles (patterns), architecture description languages, architecture documentation, and formal methods. Many research institutions such as Carnegie Mellon University and University of California, Irvine are doing a lot of research in the field of software architecture. Mary Shaw and David Garlan of Carnegie Mellon wrote a book titled Software Architecture: Perspectives on an Emerging Discipline in 1996, which brought forward the concepts in Software Architecture, such as components, connectors, styles and so on. UCI's Institute for Software Research's efforts in software architecture research is directed primarily in architectural styles, architecture description languages, and dynamic architectures.

ANSI/IEEE 1471-2000: Recommended Practice for Architecture Description of Software-Intensive Systems is the first formal standard in the area of software architecture, and was recently adopted by ISO as ISO/IEC DIS 25961.

Describing architectures

Architecture description languages

Architecture description languages (ADLs) are used to describe a Software Architecture. Several different ADLs have been developed by different organizations, including Wright (developed by Carnegie Mellon), Acme (developed by Carnegie Mellon), xADL (developed by UCI), Darwin (developed by Imperial College London), DAOP-ADL (developed by University of Málaga). Common elements of an ADL are component, connector and configuration.

Views

Software architecture is commonly organized in views, which are analogous to the different types of blueprints made in building architecture. Within the ontology established by ANSI/IEEE 1471-2000, views are instances of viewpoints, where a viewpoint exists to describe the architecture in question from the perspective of a given set of stakeholders and their concerns.

Some possible views (actually, viewpoints in the 1471 ontology) are:

• Functional/logic view
• Code view
• Development/structural view
• Concurrency/process/thread view
• Physical/deployment view
• User action/feedback view

Several languages for describing software architectures have been devised, but no consensus has yet been reached on which symbol-set and view-system should be adopted. Some believe that UML will establish a standard for software architecture viewsTemplate:Cite needed. Others believe that effective development of software relies on understanding unique constraints of each problem, and so universal notations are doomed because each provides a notational bias that necessarily makes the notation useless or dangerous for some set of tasksTemplate:Cite needed. They point to the proliferation of programming languages and a succession of failed attempts to impose a single 'universal language' on programmers, as proof that software thrives on diversity and not on standards.

Architecture examples

There are many common ways of designing computer software modules and their communications, among them:

• Client-server
• Distributed computing
• Peer-to-peer
• Blackboard
• Implicit invocation
• Pipes and filters
• Plugin
• Monolithic system
• Three-tier model
• Structured (module-based but usually monolithic within modules)
• Software componentry (strictly module-based, usually object-oriented programming within modules, slightly less monolithic)
• Service-oriented architecture

Operation

Computer software has to be "loaded" into the computer's storage (also known as memory and RAM).

Once the software is loaded, the computer is able to execute the software. Computers operate by executing the computer program. This involves passing instructions from the application software, through the system software, to the hardware which ultimately receives the instruction as machine code. Each instruction causes the computer to carry out an operation — moving data, carrying out a computation, or altering the control flow of instructions.

Data movement is typically from one place in memory to another. Sometimes it involves moving data between memory and registers which enable high-speed data access in the CPU. Moving data, especially large amounts of it, can be costly. So, this is sometimes avoided by using "pointers" to data instead. Computations include simple operations such as incrementing the value of a variable data element. More complex computations may involve many operations and data elements together.

Instructions may be performed sequentially, conditionally, or iteratively. Sequential instructions are those operations that are performed one after another. Conditional instructions are performed such that different sets of instructions execute depending on the value(s) of some data. In some languages this is know as an "if" statement. Iterative instructions are performed repetitively and may depend on some data value. This is sometimes called a "loop." Often, one instruction may "call" another set of instructions that are defined in some other program or module. When more than one computer processor is used, instructions may be executed simultaneously.

A simple example of the way software operates is what happens when a user selects an entry such as "Copy" from a menu. In this case, a conditional instruction is executed to copy text from data in a 'document' area residing in memory, perhaps to an intermediate storage area known as a 'clipboard' data area. If a different menu entry such as "Paste" is chosen, the software may execute the instructions to copy the text from the clipboard data area to a specific location in the same or another document in memory.

Depending on the application, even the example above could become complicated. The field of software engineering endeavors to manage the complexity of how software operates. This is especially true for software that operates in the context of a large or powerful computer system.

Currently, almost the only limitations on the use of computer software in applications is the ingenuity of the designer/programmer. Consequently, large areas of activities (such as playing grand master level chess) formerly assumed to incapable of software simulation are now routinely programmed. The only area that has so far proved reasonably secure from software simulation is the realm of human art— especially, pleasing music, and literature.

Kinds of software by operation: computer program as executable, source code or script, configuration.

Quality and reliability

Software reliability considers the errors, faults, and failures related to the creation and operation of software.

See Software auditing, Software quality, Software testing, and Software reliability.

License

Software license gives the user the right to use the software in the licensed environment, some software comes with the license when purchased off the shelf, or OEM license when bundled with hardware. Software can also be in the form of freeware or shareware.

Proprietary software

Proprietary software is software that has restrictions on using and copying it, usually enforced by a proprietor. The prevention of use, copying, or modification can be achieved by legal or technical means. Technical means include releasing machine-readable binaries only, and withholding the human-readable source code. Legal means can involve software licensing, copyright and patent law. Proprietary software can be sold for money as commercial software or available at zero-price as freeware. The monopoly provided by proprietary software allows a distributor of commercial copies to charge any price for those copies. Distributors of proprietary software have more control over what users can do with the software than non-proprietary software.

Free software

Free software, as defined by the Free Software Foundation, is software which can be used, copied, studied, modified and redistributed without restriction. Freedom from such restrictions is central to the concept, with the opposite of free software being proprietary software (a distinction unrelated to whether a fee is charged). The usual way for software to be distributed as free software is for the software to be licensed to the recipient with a free software license (or be in the public domain), and the source code of the software to be made available (for a compiled language).

To help distinguish libre (freedom) software from gratis (zero price) software, Richard Stallman, founder of the free software movement, developed the following explanation: "Free software is a matter of liberty, not price. To understand the concept, you should think of 'free' as in 'free speech', not as in 'free beer'". More specifically, free software means that computer users have the freedom to cooperate with whom they choose, and to control the software they use. The GNU Manifesto contains language that gives evidence of Stallman's initial confusion with the usage.

Most free software is distributed online without charge, or off-line at the marginal cost of distribution, but this is not required, and people may sell copies for any price. Thus, free software is entirely compatible with commercial software: a prohibition on selling the software would be a restriction failing the free software definition.

Patents

The issue of software patents is very controversial, since while patents protect the ideas of "inventors", they are widely believed to hinder software development. See Hacker ethic.

See also

free software Portal

• Algorithm
• Computer programming
• Software development process
• Software development tools
• Software engineering
• Software maintenance
• Software optimization
• Software brittleness
• Software licenses
  • Proprietary software
  • Free software
  • Open source software
  • Software piracy
  • Freeware
  • Shareware
  • Postcardware
  • Shovelware
  • Dual license
• Software as a Service
• Software Security Assurance
• List of open source software packages
• Software industry

Further reading

• Softword: Provenance for the Word 'Software', Paul Niquette, ISBN 1-58922-233-4, Internet Version at http://www.niquette.com/books/softword/tocsoft.html
• Len Bass, Paul Clements, Rick Kazman: Software Architecture in Practice, Second Edition. Addison Wesley, Reading 5/9/2003 ISBN 0321154959 (This book, now in second edition, eloquently covers the fundamental concepts of the discipline. The theme is centered around achieving quality attributes of a system.)
• Philippe Kruchten: Architectural Blueprints - the 4+1 View Model of Software Architecture. In: IEEE Software. 12 (6) November 1995, pp. 42-50 (also available online at the Rational website(PDF))

External links

• Software architecture definitions at Carnegie Mellon University Software Engineering Institute
• Software architecture vs. software design
• Worldwide Institute of Software Architects
• Grady Booch's Handbook of Software Architecture project
• SoftwareArchitectures.com Independent resource of information on the discipline
• International Association of Software Architects
• Microsoft Architecture Journal

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