Computer software, or simply software, is a part of a computer system that consists of data or computer instructions, in contrast to the physical hardware from which the system is built. In computer science and software engineering, computer software is all information processed by computer systems, programs and data. Computer software includes computer programs, libraries and related non-executable data, such as online documentation or digital media. Computer hardware and software require each other and neither can be realistically used on its own.
At the lowest level, executable code consists of machine language instructions specific to an individual processor--typically a central processing unit (CPU). A machine language consists of groups of binary values signifying processor instructions that change the state of the computer from its preceding state. For example, an instruction may change the value stored in a particular storage location in the computer--an effect that is not directly observable to the user. An instruction may also (indirectly) cause something to appear on a display of the computer system--a state change which should be visible to the user. The processor carries out the instructions in the order they are provided, unless it is instructed to "jump" to a different instruction, or is interrupted (by now multi-core processors are dominant, where each core can run instructions in order; then, however, each application software runs only on one core by default, but some software has been made to run on many).
The majority of software is written in high-level programming languages that are easier and more efficient for programmers to use because they are closer than machine languages to natural languages. High-level languages are translated into machine language using a compiler or an interpreter or a combination of the two. Software may also be written in a low-level assembly language, which has strong correspondence to the computer's machine language instructions and is translated into machine language using an assembler.
An outline (algorithm) for what would have been the first piece of software was written by Ada Lovelace in the 19th century, for the planned Analytical Engine. However, neither the Analytical Engine nor any software for it were ever created.
The first theory about software--prior to creation of computers as we know them today--was proposed by Alan Turing in his 1935 essay Computable numbers with an application to the Entscheidungsproblem (decision problem).
This eventually led to the creation of the twin academic fields of computer science and software engineering, which both study software and its creation. Computer science is more theoretical (Turing's essay is an example of computer science), where as software engineering focuses on more practical concerns.
However, prior to 1946, software as we now understand it--programs stored in the memory of stored-program digital computers--did not yet exist. The first electronic computing devices were instead rewired in order to "reprogram" them.
On virtually all computer platforms, software can be grouped into a few broad categories.
Based on the goal, computer software can be divided into:
Programming tools are also software in the form of programs or applications that software developers (also known as programmers, coders, hackers or software engineers) use to create, debug, maintain (i.e. improve or fix), or otherwise support software.
Software is written in one or more programming languages; there are many programming languages in existence, and each has at least one implementation, each of which consists of its own set of programming tools. These tools may be relatively self-contained programs such as compilers, debuggers, interpreters, linkers, and text editors, that can be combined together to accomplish a task; or they may form an integrated development environment (IDE), which combines much or all of the functionality of such self-contained tools. IDEs may do this by either invoking the relevant individual tools or by re-implementing their functionality in a new way. An IDE can make it easier to do specific tasks, such as searching in files in a particular project. Many programming language implementations provide the option of using both individual tools or an IDE.
Users often see things differently from programmers. People who use modern general purpose computers (as opposed to embedded systems, analog computers and supercomputers) usually see three layers of software performing a variety of tasks: platform, application, and user software.
Computer software has to be "loaded" into the computer's storage (such as the hard drive or memory). Once the software has loaded, the computer is able to execute the software. 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.
Software quality is very important, especially for commercial and system software like Microsoft Office, Microsoft Windows and Linux. If software is faulty (buggy), it can delete a person's work, crash the computer and do other unexpected things. Faults and errors are called "bugs" which are often discovered during alpha and beta testing. Software is often also a victim to what is known as software aging, the progressive performance degradation resulting from a combination of unseen bugs.
Many bugs are discovered and eliminated (debugged) through software testing. However, software testing rarely--if ever--eliminates every bug; some programmers say that "every program has at least one more bug" (Lubarsky's Law). In the waterfall method of software development, separate testing teams are typically employed, but in newer approaches, collectively termed agile software development, developers often do all their own testing, and demonstrate the software to users/clients regularly to obtain feedback. Software can be tested through unit testing, regression testing and other methods, which are done manually, or most commonly, automatically, since the amount of code to be tested can be quite large. For instance, NASA has extremely rigorous software testing procedures for many operating systems and communication functions. Many NASA-based operations interact and identify each other through command programs. This enables many people who work at NASA to check and evaluate functional systems overall. Programs containing command software enable hardware engineering and system operations to function much easier together.
The software's license gives the user the right to use the software in the licensed environment, and in the case of free software licenses, also grants other rights such as the right to make copies.
Proprietary software can be divided into two types:
Software patents, like other types of patents, are theoretically supposed to give an inventor an exclusive, time-limited license for a detailed idea (e.g. an algorithm) on how to implement a piece of software, or a component of a piece of software. Ideas for useful things that software could do, and user requirements, are not supposed to be patentable, and concrete implementations (i.e. the actual software packages implementing the patent) are not supposed to be patentable either--the latter are already covered by copyright, generally automatically. So software patents are supposed to cover the middle area, between requirements and concrete implementation. In some countries, a requirement for the claimed invention to have an effect on the physical world may also be part of the requirements for a software patent to be held valid--although since all useful software has effects on the physical world, this requirement may be open to debate. Meanwhile, American copyright law was applied to various aspects of the writing of the software code.
Software patents are controversial in the software industry with many people holding different views about them. One of the sources of controversy is that the aforementioned split between initial ideas and patent does not seem to be honored in practice by patent lawyers--for example the patent for Aspect-Oriented Programming (AOP), which purported to claim rights over any programming tool implementing the idea of AOP, howsoever implemented. Another source of controversy is the effect on innovation, with many distinguished experts and companies arguing that software is such a fast-moving field that software patents merely create vast additional litigation costs and risks, and actually retard innovation. In the case of debates about software patents outside the United States, the argument has been made that large American corporations and patent lawyers are likely to be the primary beneficiaries of allowing or continue to allow software patents.
Design and implementation of software varies depending on the complexity of the software. For instance, the design and creation of Microsoft Word took much more time than designing and developing Microsoft Notepad because the latter has much more basic functionality.
Software is usually designed and created (aka coded/written/programmed) in integrated development environments (IDE) like Eclipse, IntelliJ and Microsoft Visual Studio that can simplify the process and compile the software (if applicable). As noted in a different section, software is usually created on top of existing software and the application programming interface (API) that the underlying software provides like GTK+, JavaBeans or Swing. Libraries (APIs) can be categorized by their purpose. For instance, the Spring Framework is used for implementing enterprise applications, the Windows Forms library is used for designing graphical user interface (GUI) applications like Microsoft Word, and Windows Communication Foundation is used for designing web services. When a program is designed, it relies upon the API. For instance, if a user is designing a Microsoft Windows desktop application, he or she might use the .NET Windows Forms library to design the desktop application and call its APIs like Form1.Close and Form1.Show to close or open the application, and write the additional operations him/herself that it needs to have. Without these APIs, the programmer needs to write these APIs him/herself. Companies like Oracle and Microsoft provide their own APIs so that many applications are written using their software libraries that usually have numerous APIs in them.
A person who creates software is called a programmer, software engineer or software developer, terms that all have a similar meaning. More informal terms for programmer also exist such as "coder" and "hacker" - although use of the latter word may cause confusion, because it is more often used to mean someone who illegally breaks into computer systems.
A great variety of software companies and programmers in the world comprise a software industry. Software can be quite a profitable industry: Bill Gates, the co-founder of Microsoft was the richest person in the world in 2009, largely due to his ownership of a significant number of shares in Microsoft, the company responsible for Microsoft Windows and Microsoft Office software products.
Non-profit software organizations include the Free Software Foundation, GNU Project and Mozilla Foundation. Software standard organizations like the W3C, IETF develop recommended software standards such as XML, HTTP and HTML, so that software can interoperate through these standards.