Compiler in the context of Just-in-time compilation

Software consists of computer programs that instruct the execution of a computer. Software also includes design documents and specifications.

The history of software is closely tied to the development of digital computers in the mid-20th century. Early programs were written in the machine language specific to the hardware. The introduction of high-level programming languages in 1958 allowed for more human-readable instructions, making software development easier and more portable across different computer architectures. Software in a programming language is run through a compiler or interpreter to execute on the architecture's hardware. Over time, software has become complex, owing to developments in networking, operating systems, and databases.

In mathematics, the Strahler number or Horton–Strahler number of a mathematical tree is a numerical measure of its branching complexity.

These numbers were first developed in hydrology, as a way of measuring the complexity of rivers and streams, by Robert E. Horton (1945) and Arthur Newell Strahler (1952, 1957). In this application, they are referred to as the Strahler stream order and are used to define stream size based on a hierarchy of tributaries.The same numbers also arise in the analysis of L-systems and of hierarchical biological structures such as (biological) trees and animal respiratory and circulatory systems, in register allocation for compilation of high-level programming languages and in the analysis of social networks.

A computer scientist is a scientist who specializes in the academic study of computer science and technology.

Computer scientists typically work on the theoretical side of computation. Although computer scientists can also focus their work and research on specific areas (such as algorithm and data structure development and design, software engineering, information theory, database theory, theoretical computer science, numerical analysis, programming language theory, compiler, computer graphics, computer vision, robotics, computer architecture, operating system), their foundation is the theoretical study of computing from which these other fields derive.

In the context of hardware and software systems, formal verification is the act of proving or disproving the correctness of a system with respect to a certain formal specification or property, using formal methods of mathematics. Formal verification is a key incentive for formal specification of systems, and is at the core of formal methods.It represents an important dimension of analysis and verification in electronic design automation and is one approach to software verification. The use of formal verification enables the highest Evaluation Assurance Level (EAL7) in the framework of common criteria for computer security certification.

Formal verification can be helpful in proving the correctness of systems such as: cryptographic protocols, combinational circuits, digital circuits with internal memory, and software expressed as source code in a programming language. Prominent examples of verified software systems include the CompCert verified C compiler and the seL4 high-assurance operating system kernel.

Edsger Wybe Dijkstra (/ˈdaɪkstrə/ DYKE-strə; Dutch: [ˈɛtsxər ˈʋibə ˈdɛikstraː] ; 11 May 1930 – 6 August 2002) was a Dutch computer scientist, programmer, mathematician, and science essayist.

Born in Rotterdam in the Netherlands, Dijkstra studied mathematics and physics and then theoretical physics at the University of Leiden. Adriaan van Wijngaarden offered him a job as the first computer programmer in the Netherlands at the Mathematical Centre in Amsterdam, where he worked from 1952 until 1962. He formulated and solved the shortest path problem in 1956, and in 1960 developed the first compiler for the programming language ALGOL 60 in conjunction with colleague Jaap A. Zonneveld. In 1962 he moved to Eindhoven, and later to Nuenen, where he became a professor in the Mathematics Department at the Technische Hogeschool Eindhoven. In the late 1960s he built the THE multiprogramming system, which influenced the designs of subsequent systems through its use of software-based paged virtual memory. Dijkstra joined Burroughs Corporation as its sole research fellow in August 1973. The Burroughs years saw him at his most prolific in output of research articles. He wrote nearly 500 documents in the "EWD" series, most of them technical reports, for private circulation within a select group.

A computer program is a sequence or set of instructions in a programming language for a computer to execute. It is one component of software, which also includes documentation and other intangible components.

A computer program in its human-readable form is called source code. Source code needs another computer program to execute because computers can only execute their native machine instructions. Therefore, source code may be translated to machine instructions using a compiler written for the language. (Assembly language programs are translated using an assembler.) The resulting file is called an executable. Alternatively, source code may execute within an interpreter written for the language.

In computing, source code, or simply code or source, is human readable plain text that can eventually result in controlling the behavior of a computer. In order to control a computer, it must be processed by a computer program – either executed directly via an interpreter or translated into a more computer-consumable form such as via a compiler. Sometimes, code is compiled directly to machine code so that it can be run in the native language of the computer without further processing. But, many modern environments involve compiling to an intermediate representation such as bytecode that can either run via an interpreter or be compiled on-demand to machine code via just-in-time compilation.

The syntax of computer source code is code structured and ordered restricted to computer language rules. Like a natural language, a computer language (i.e. a programming language) defines the syntax that is valid for that language. A syntax error occurs when syntactically invalid source code is processed by an tool such as a compiler or interpreter.

The most commonly used languages are text-based with syntax based on strings. Alternatively, the syntax of a visual programming language is based on relationships between graphical elements.

C is a general-purpose programming language. It was created in the 1970s by Dennis Ritchie and remains widely used and influential. By design, C gives the programmer relatively direct access to the features of the typical CPU architecture, customized for the target instruction set. It has been and continues to be used to implement operating systems (especially kernels), device drivers, and protocol stacks, but its use in application software has been decreasing. C is used on computers that range from the largest supercomputers to the smallest microcontrollers and embedded systems.

A successor to the programming language B, C was originally developed at Bell Labs by Ritchie between 1972 and 1973 to construct utilities running on Unix. It was applied to re-implementing the kernel of the Unix operating system. During the 1980s, C gradually gained popularity. It has become one of the most widely used programming languages, with C compilers available for practically all modern computer architectures and operating systems. The book The C Programming Language, co-authored by the original language designer, served for many years as the de facto standard for the language. C has been standardized since 1989 by the American National Standards Institute (ANSI) and, subsequently, jointly by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC).

In computer programming, a comment is text embedded in source code that a translator (compiler or interpreter) ignores. Generally, a comment is an annotation intended to make the code easier for a programmer to understand – often explaining an aspect that is not readily apparent in the program (non-comment) code. For this article, comment refers to the same concept in a programming language, markup language, configuration file and any similar context. Some development tools, other than a source code translator, do parse comments to provide capabilities such as API document generation, static analysis, and version control integration. The syntax of comments varies by programming language yet there are repeating patterns in the syntax among languages as well as similar aspects related to comment content.

The flexibility supported by comments allows for a wide degree of content style variability. To promote uniformity, style conventions are commonly part of a programming style guide. But, best practices are disputed and contradictory.

A "Hello, World!" program is usually a simple computer program that emits (or displays) to the screen (often the console) a message similar to "Hello, World!". A small piece of code in most general-purpose programming languages, this program is used to illustrate a language's basic syntax. Such a program is often the first written by a student of a new programming language, but it can also be used as a sanity check to ensure that the computer software intended to compile or run source code is correctly installed, and that its operator understands how to use it.

The THE multiprogramming system (THE OS) was a computer operating system designed by a team led by Edsger W. Dijkstra, described in monographs in 1965-66 and published in 1968.Dijkstra never named the system; "THE" is simply the abbreviation of "Technische Hogeschool Eindhoven", then the name (in Dutch) of the Eindhoven University of Technology of the Netherlands. The THE system was primarily a batch system that supported multitasking; it was not designed as a multi-user operating system. It was much like the SDS 940, but "the set of processes in the THE system was static".

The THE system apparently introduced the first forms of software-based paged virtual memory (the Electrologica X8 did not support hardware-based memory management), freeing programs from being forced to use physical locations on the drum memory. It did this by using a modified ALGOL compiler (the only programming language supported by Dijkstra's system) to "automatically generate calls to system routines, which made sure the requested information was in memory, swapping if necessary". Paged virtual memory was also used for buffering input/output (I/O) device data, and for a significant portion of the operating system code, and nearly all the ALGOL 60 compiler. In this system, semaphores were used as a programming construct for the first time.

A translator or programming language processor is a computer program that converts the programming instructions written in human convenient form into machine language codes that the computers understand and process. It is a generic term that can refer to a compiler, assembler, or interpreter—anything that converts code from one computer language into another. These include translations between high-level and human-readable computer languages such as C++ and Java, intermediate-level languages such as Java bytecode, low-level languages such as the assembly language and machine code, and between similar levels of language on different computing platforms, as well as from any of these to any other of these.

Software and hardware represent different levels of abstraction in computing. Software is typically written in high-level programming languages, which are easier for humans to understand and manipulate, while hardware implementations involve low-level descriptions of physical components and their interconnections. Translator computing facilitates the conversion between these abstraction levels. Overall, translator computing plays a crucial role in bridging the gap between software and hardware implementations, enabling developers to leverage the strengths of each platform and optimize performance, power efficiency, and other metrics according to the specific requirements of the application.

An intermediate representation (IR) is the data structure or code used internally by a compiler or virtual machine to represent source code. An IR is designed to be conducive to further processing, such as optimization and translation. A "good" IR must be accurate – capable of representing the source code without loss of information – and independent of any particular source or target language. An IR may take one of several forms: an in-memory data structure, or a special tuple- or stack-based code readable by the program. In the latter case it is also called an intermediate language.

A canonical example is found in most modern compilers. For example, the CPython interpreter transforms the linear human-readable text representing a program into an intermediate graph structure that allows flow analysis and re-arrangement before execution. Use of an intermediate representation such as this allows compiler systems like the GNU Compiler Collection and LLVM to be used by many different source languages to generate code for many different target architectures.

Bytecode (also called portable code or p-code) is an intermediate representation form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (normally numeric addresses) that encode the result of compiler parsing and performing semantic analysis of things like type, scope, and nesting depths of program objects.

The name bytecode stems from instruction sets that have one-byte opcodes followed by optional parameters. Intermediate representations such as bytecode may be output by programming language implementations to ease interpretation, or it may be used to reduce hardware and operating system dependence by allowing the same code to run cross-platform, on different devices. Bytecode may often be either directly executed on a virtual machine (a p-code machine, i.e., interpreter), or it may be further compiled into machine code for better performance.

A disassembler is a computer program that translates machine language into assembly language—the inverse operation to that of an assembler. The output of disassembly is typically formatted for human-readability rather than for input to an assembler, making disassemblers primarily a reverse-engineering tool. Common uses include analyzing the output of high-level programming language compilers and their optimizations, recovering source code when the original is lost, performing malware analysis, modifying software (such as binary patching), and software cracking.

A disassembler differs from a decompiler, which targets a high-level language rather than an assembly language. A fundamental method of software analysis is disassembly. Unlike decompilers, which make attempts at recreating high-level human readable structures using binaries, disassemblers are aimed at generating a symbolic assembly, meaning it's attempting to reconstruct the assembly closest to its executions. Disassembled code is hence normally more accurate but also lower level and less abstract than decompiled code and thus it can be much more easily analyzed.