Editing Assembly language (section)

==Use of assembly language==
When the [[stored-program computer]] was introduced, programs were written in machine code, and loaded into the computer from punched paper tape or toggled directly into memory from console switches.{{cn|date=August 2024}} [[Kathleen Booth]] "is credited with inventing assembly language"<ref name="Dufresne_2018"/><ref name="Booth_1947"/> based on theoretical work she began in 1947, while working on the [[APEXC|ARC2]] at [[Birkbeck, University of London]], following consultation by [[Andrew Donald Booth|Andrew Booth]] (later her husband) with mathematician [[John von Neumann]] and physicist [[Herman Goldstine]] at the [[Institute for Advanced Study]].<ref name="Booth_1947"/><ref name="Campbell-Kelly_1982"/>

In late 1948, the [[EDSAC|Electronic Delay Storage Automatic Calculator]] (EDSAC) had an assembler (named "initial orders") integrated into its [[booting|bootstrap]] program. It used one-letter mnemonics developed by [[David Wheeler (computer scientist)|David Wheeler]], who is credited by the IEEE Computer Society as the creator of the first "assembler".<ref name="Salomon_1992"/><ref name="Campbell-Kelly_1980"/><ref name="Wheeler_1985"/> Reports on the EDSAC introduced the term "assembly" for the process of combining fields into an instruction word.<ref name="Wilkes_1949"/> SOAP ([[Symbolic Optimal Assembly Program]]) was an assembly language for the [[IBM 650]] computer written by Stan Poley in 1955.<ref name="Cruz_2019"/>

Assembly languages eliminated much of the error-prone, tedious, and time-consuming [[first-generation language|first-generation]] programming needed with the earliest computers, freeing programmers from tedium such as remembering numeric codes and calculating addresses. They were once widely used for all sorts of programming. By the late 1950s their use had largely been supplanted by higher-level languages in the search for improved [[programming productivity]].<ref>{{Cite magazine |last=Abell |first=John C. |title=Oct. 15, 1956: Fortran Forever Changes Computing's Fortunes |url=https://www.wired.com/2009/10/1015fortran-launch/ |access-date=2024-03-02 |magazine=Wired |language=en-US |issn=1059-1028}}</ref> Today, assembly language is still used for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues.<ref>{{cite journal|title=The Origins of Informatics|last=Collen|first=Morris F.|journal=[[Journal of the American Medical Informatics Association]]|date=March–April 1994|volume=1|issue=2|pages=96–97|doi=10.1136/jamia.1994.95236152| pmid=7719803 | pmc=116189 }}</ref> Typical uses are [[device driver]]s, low-level [[embedded system]]s, and [[real-time computing|real-time]] systems (see {{section link|#Current usage}}).

Numerous programs were written entirely in assembly language. The [[Burroughs MCP]] (1961) was the first computer for which an operating system was not developed entirely in assembly language; it was written in [[Executive Systems Problem Oriented Language]] (ESPOL), an Algol dialect. Many commercial applications were written in assembly language as well, including a large amount of the [[IBM mainframe]] software developed by large corporations. [[COBOL]], [[FORTRAN]] and some PL/I eventually displaced assembly language, although a number of large organizations retained assembly-language application infrastructures well into the 1990s.

Assembly language was the primary development language for 8-bit home computers such as the [[Apple II]], [[Atari 8-bit computers]], [[ZX Spectrum]], and [[Commodore 64]]. [[Interpreter (computing)|Interpreted]] [[BASIC]] on these systems did not offer maximum execution speed and full use of facilities to take full advantage of the available hardware. Assembly language was the default choice for programming 8-bit consoles such as the [[Atari 2600]] and [[Nintendo Entertainment System]].

Key software for [[IBM PC compatible]]s such as [[MS-DOS]], [[Turbo Pascal]], and the [[Lotus 1-2-3]] spreadsheet was written in assembly language. As computer speed grew exponentially, assembly language became a tool for speeding up parts of programs, such as the rendering of ''[[Doom (1993 video game)|Doom]]'', rather than a dominant development language. In the 1990s, assembly language was used to maximise performance from systems such as the [[Sega Saturn]],<ref name="Pettus_2008"/> and as the primary language for arcade hardware using the [[TMS34010]] integrated CPU/GPU such as ''[[Mortal Kombat (1992 video game)|Mortal Kombat]]'' and ''[[NBA Jam (1993 video game)|NBA Jam]]''.

===Current usage===
There has been debate over the usefulness and performance of assembly language relative to high-level languages.<ref name="Kauler_1997" />

Although assembly language has specific niche uses where it is important (see below), there are other tools for optimization.<ref name="Hsieh_2020"/>

{{As of|2017|07}}, the [[TIOBE index]] of programming language popularity ranks assembly language at 11, ahead of [[Visual Basic]], for example.<ref name="tiobe"/> Assembler can be used to optimize for speed or optimize for size. In the case of speed optimization, modern [[optimizing compiler]]s are claimed<ref name="Rusling_2012"/> to render high-level languages into code that can run as fast as hand-written assembly, despite some counter-examples.<ref name="Markoff_2005"/><ref name="bit-field"/><ref name="gcc-mess"/> The complexity of modern processors and memory sub-systems makes effective optimization increasingly difficult for compilers and assembly programmers alike.<ref name="Hyde_2008"/><ref name="compiler-fails1"/> Increasing processor performance has meant that most CPUs sit idle most of the time,<ref name="Click_2014"/> with delays caused by predictable bottlenecks such as cache misses, [[Input/output|I/O]] operations and [[Memory paging|paging]], making raw code execution speed a non-issue for many programmers.

There are still certain computer programming domains in which the use of assembly programming is more common:

* Writing code for systems with {{Clarify|date=June 2021|reason=Does this refer only to microprocessors or also to midrange and mainframe systems?|text=older processors}} that have limited high-level language options such as the [[Atari 2600]], [[Commodore 64]], and [[graphing calculator]]s.<ref name="Fargo_2008"/> Programs for these computers of the 1970s and 1980s are often written in the context of [[demoscene]] or [[retrogaming]] subcultures.
* Code that must interact directly with the hardware, for example in [[device driver]]s and [[interrupt handler]]s.
* In an embedded processor or [[Digital Signal Processor|DSP]], high-repetition interrupts require the shortest number of cycles per interrupt, such as an interrupt that occurs 1000 or 10000 times a second.
* Programs that need to use processor-specific instructions not implemented in a compiler. A common example is the [[circular shift|bitwise rotation]] instruction at the core of many encryption algorithms, as well as querying the parity of a byte or the 4-bit carry of an addition.
* Stand-alone executables that are required to execute without recourse to the [[run-time system|run-time]] components or [[library (computing)|libraries]] associated with a high-level language, such as the firmware for telephones, automobile fuel and ignition systems, air-conditioning control systems,and security systems.
* Programs with performance-sensitive inner loops, where assembly language provides optimization opportunities that are difficult to achieve in a high-level language. For example, [[linear algebra]] with [[Basic Linear Algebra Subprograms|BLAS]]<ref name="Markoff_2005"/><ref name="BLAS_2008"/> or [[Discrete cosine transform|discrete cosine transformation]] (e.g. [[Single instruction, multiple data|SIMD]] assembly version from [[x264]]<ref name="Videolan_2010"/>).
* Programs that create vectorized functions for programs in higher-level languages such as C.  In the higher-level language this is sometimes aided by compiler [[intrinsic function]]s which map directly to SIMD mnemonics, but nevertheless result in a one-to-one assembly conversion specific for the given vector processor.
* [[Real-time computing|Real-time]] programs such as simulations, flight navigation systems, and medical equipment. For example, in a [[fly-by-wire]] system, telemetry must be interpreted and acted upon within strict time constraints. Such systems must eliminate sources of unpredictable delays, which may be created by interpreted languages, automatic [[garbage collection (computer science)|garbage collection]], paging operations, or [[preemptive multitasking]]. Choosing assembly or [[low-level programming language|lower-level languages]] for such systems gives programmers greater visibility and control over processing details.
* Cryptographic algorithms that must always take strictly the same time to execute, preventing [[timing attack]]s.
* Video encoders and decoders such as rav1e (an encoder for [[AV1]])<ref>{{cite web |url=https://github.com/xiph/rav1e/blob/v0.6.3/README.md#features-1= |title=rav1e/README.md at v0.6.3 |website=[[GitHub]] |access-date=21 February 2023 |archive-date=22 February 2023 |archive-url=https://web.archive.org/web/20230222005925/https://github.com/xiph/rav1e/blob/v0.6.3/README.md |url-status=live}}</ref> and dav1d (the reference decoder for AV1)<ref>{{cite web |url=https://code.videolan.org/videolan/dav1d/-/blob/1.1.0/README.md |title=README.md · 1.1.0 · VideoLAN / dav1d |date=13 February 2023 |access-date=21 February 2023 |archive-date=22 February 2023 |archive-url=https://web.archive.org/web/20230222004317/https://code.videolan.org/videolan/dav1d/-/blob/1.1.0/README.md |url-status=live}}</ref> contain assembly to leverage [[AVX2]] and [[Neon (instruction set)|ARM Neon]] instructions when available.
* Modify and extend legacy code written for IBM mainframe computers.<ref name="Bosworth_2016"/><ref>{{cite web |url=https://www-01.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R3sc236852/$file/idad500_v2r3.pdf |title=z/OS Version 2 Release 3 DFSMS Macro Instructions for Data Sets |publisher=IBM |date=15 February 2019 |access-date=14 September 2021 |url-status=live|archive-url=https://web.archive.org/web/20210625140314/https://www-01.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R3sc236852/$file/idad500_v2r3.pdf |archive-date=25 June 2021 }}</ref>
* Situations where complete control over the environment is required, in extremely high-security situations where [[Trusting trust#Reflections on Trusting Trust|nothing can be taken for granted]].
* [[Computer virus]]es, [[bootloader]]s, certain [[device driver]]s, or other items very close to the hardware or low-level operating system.
* [[Instruction set simulator]]s for monitoring, tracing and [[debugging]] where additional overhead is kept to a minimum.
* Situations where no high-level language exists, on a new or specialized processor for which no [[cross compiler]] is available.
* [[Reverse engineering]] and modifying program files such as:
** existing [[binary file|binaries]] that may or may not have originally been written in a high-level language, for example when trying to recreate programs for which source code is not available or has been lost, or cracking copy protection of proprietary software.
** [[Video game]]s (also termed [[ROM hacking]]), which is possible via several methods. The most widely employed method is altering program code at the assembly language level.

Assembly language is still taught in most [[computer science]] and [[electronic engineering]] programs. Although few programmers today regularly work with assembly language as a tool, the underlying concepts remain important. Such fundamental topics as [[binary arithmetic]], [[memory management|memory allocation]], [[Stack (abstract data type)|stack processing]], [[character set]] encoding, [[interrupt]] processing, and [[compiler]] design would be hard to study in detail without a grasp of how a computer operates at the hardware level. Since a computer's behaviour is fundamentally defined by its instruction set, the logical way to learn such concepts is to study an assembly language. Most modern computers have similar instruction sets. Therefore, studying a single assembly language is sufficient to learn the basic concepts, recognize situations where the use of assembly language might be appropriate, and to see how efficient executable code can be created from high-level languages.<ref name="Hyde_2003"/>

===Typical applications===
* Assembly language is typically used in a system's [[booting|boot]] code, the low-level code that initializes and tests the system hardware prior to booting the operating system and is often stored in [[read-only memory|ROM]]. ([[BIOS]] on [[IBM-compatible PC]] systems and [[CP/M]] is an example.)
* Assembly language is often used for low-level code, for instance for [[operating system kernel]]s, which cannot rely on the availability of pre-existing system calls and must indeed implement them for the particular processor architecture on which the system will be running.
* Some compilers translate high-level languages into assembly first before fully compiling, allowing the assembly code to be viewed for [[debugging]] and optimization purposes.
* Some compilers for relatively low-level languages, such as [[Pascal (programming language)|Pascal]] or [[C (programming language)|C]], allow the programmer to embed assembly language directly in the source code (so called [[inline assembly]]). Programs using such facilities can then construct abstractions using different assembly language on each hardware platform. The system's [[software portability|portable code]] can then use these processor-specific components through a uniform interface.
* Assembly language is useful in [[reverse engineering]]. Many programs are distributed only in machine code form which is straightforward to translate into assembly language by a [[disassembler]], but more difficult to translate into a higher-level language through a [[decompiler]]. Tools such as the [[Interactive Disassembler]] make extensive use of disassembly for such a purpose. This technique is used by hackers to crack commercial software, and competitors to produce software with similar results from competing companies.
* Assembly language is used to enhance speed of execution, especially in early personal computers with limited processing power and RAM.
* Assemblers can be used to generate blocks of data, with no high-level language overhead, from formatted and commented source code, to be used by other code.<ref name="Paul_2001_NECPINW"/><ref name="Paul_2002_CPI"/>