Assembly language

In <a href="/facts/Computer_programming/QmSsFnu0">computer programming</a>, assembly language (alternatively assembler language or symbolic machine code), often referred to simply as assembly and commonly abbreviated as ASM or asm, is any <a href="/facts/Low-level_programming_language/EEpKS6mb">low-level programming language</a> with a very strong correspondence between the instructions in the language and the <a href="/facts/Computer_architecture/tHtwxgR2">architecture's</a> <a href="/facts/Machine_code/bgdQ1Fxf">machine code</a> <a href="/facts/Instruction_set_architecture/8p8vwUeE">instructions</a>. Assembly language usually has one <a href="/facts/Statement_(computer_science)/bEkBufqX">statement</a> per machine instruction (1:1), but constants, <a href="/facts/Comment_(computer_programming)/Q8hHFq8h">comments</a>, assembler <a href="/facts/Directive_(programming)/RsozxGG0">directives</a>, symbolic <a href="/facts/Label_(computer_science)/DT6KU8R5">labels</a> of, e.g., <a href="/facts/Memory_location/7EsUmbea">memory locations</a>, <a href="/facts/Processor_register/32RvbUdz">registers</a>, and <a href="/facts/Macro_instruction/7abAsm1o">macros</a> are generally also supported.
The first assembly code in which a language is used to represent machine code instructions is found in <a href="/facts/Kathleen_Booth/EqozPTAr">Kathleen</a> and <a href="/facts/Andrew_Donald_Booth/5Rnt1oKI">Andrew Donald Booth</a>'s 1947 work, Coding for A.R.C.. Assembly code is converted into executable machine code by a <a href="/facts/Utility_software/Hn3XtISQ">utility program</a> referred to as an <a href="/facts/Assembler_(computing)/7XuV0cla">assembler</a>. The term "assembler" is generally attributed to <a href="/facts/Maurice_Wilkes/2RJ58EKc">Wilkes</a>, <a href="/facts/David_Wheeler_(computer_scientist)/eMm8hmrz">Wheeler</a> and <a href="/facts/Stanley_Gill/ABhUaW33">Gill</a> in their 1951 book <a href="/facts/The_Preparation_of_Programs_for_an_Electronic_Digital_Computer/YjlnESbJ">The Preparation of Programs for an Electronic Digital Computer</a>, who, however, used the term to mean "a program that assembles another program consisting of several sections into a single program". The conversion process is referred to as assembly, as in assembling the <a href="/facts/Source_code/o9J6Jv23">source code</a>. The computational step when an assembler is processing a program is called assembly time.
Because assembly depends on the machine code instructions, each assembly language is specific to a particular <a href="/facts/Computer_architecture/tHtwxgR2">computer architecture</a>.
Sometimes there is more than one assembler for the same architecture, and sometimes an assembler is specific to an <a href="/facts/Operating_system/8XTuvMh2">operating system</a> or to particular operating systems. Most assembly languages do not provide specific <a href="/facts/Syntax_(programming_languages)/zM5oInza">syntax</a> for operating system calls, and most assembly languages can be used universally with any operating system, as the language provides access to all the real capabilities of the <a href="/facts/Processor_(computing)/z3wPljrI">processor</a>, upon which all <a href="/facts/System_call/NW44AFJr">system call</a> mechanisms ultimately rest. In contrast to assembly languages, most <a href="/facts/High-level_programming_language/smjECUhx">high-level programming languages</a> are generally <a href="/facts/Porting/7s6tFs8B">portable</a> across multiple architectures but require <a href="/facts/Interpreter_(computing)/JB85fbyb">interpreting</a> or <a href="/facts/Compiler/WNfCDFJe">compiling</a>, much more complicated tasks than assembling.
In the first decades of computing, it was commonplace for both <a href="/facts/Systems_programming/urTteIsW">systems programming</a> and <a href="/facts/Application_programming/QmSsFnu0">application programming</a> to take place entirely in assembly language. While still irreplaceable for some purposes, the majority of programming is now conducted in higher-level interpreted and compiled languages. In "<a href="/facts/No_Silver_Bullet/JadE6YVc">No Silver Bullet</a>", <a href="/facts/Fred_Brooks/1FNtgMgE">Fred Brooks</a> summarised the effects of the switch away from assembly language programming: "Surely the most powerful stroke for software productivity, reliability, and simplicity has been the progressive use of high-level languages for programming. Most observers credit that development with at least a factor of five in productivity, and with concomitant gains in reliability, simplicity, and comprehensibility."
Today, it is typical to use small amounts of assembly language code within larger systems implemented in a higher-level language, for performance reasons or to interact directly with hardware in ways unsupported by the higher-level language. For instance, just under 2% of version 4.9 of the <a href="/facts/Linux_kernel/dlkduHuA">Linux kernel</a> source code is written in assembly; more than 97% is written in <a href="/facts/C_(programming_language)/Ky2No763">C</a>.

Assembly language open-in-new

Assembly language