HOME

TheInfoList




In
computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, generating algorithms, Profilin ...
, assembly language (or assembler language), often abbreviated asm, is any
low-level programming language A low-level programming language is a programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages ar ...
in which there is a very strong correspondence between the instructions in the language and the architecture's
machine code In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, ge ...
instructions Instruction or instructions may refer to: Computing * Instruction, one operation of a processor within a computer architecture instruction set * Computer program, a collection of instructions Music * Instruction (band), a 2002 rock band from New Y ...
. Because assembly depends on the machine code instructions, every assembly language is designed for exactly one specific computer architecture. Assembly language may also be called ''symbolic machine code''. Assembly code is converted into executable machine code by a
utility program Utility software is software designed to help to analyze, configure, optimize or maintain a computer. It is used to support the computer infrastructure - in contrast to application software, which is aimed at directly performing tasks that benefit o ...
referred to as an ''
assembler Assembler may refer to: Arts and media * Nobukazu Takemura Nobukazu Takemura (竹村延和 ''Takemura Nobukazu'') is a Japanese musician and artist. Born in Hirakata, Osaka in August 1968, he became interested in music at a young age by listenin ...
''. The conversion process is referred to as ''assembly'', as in ''assembling'' the
source code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and ...

source code
. Assembly language usually has one statement per machine instruction (1:1), but comments and statements that are assembler
directives Directive may refer to: * Directive (European Union), a legislative act of the European Union * Directive (programming), a computer language construct that specifies how a compiler should process input * Directive (poem), "Directive" (poem), a poem ...
, macros, and symbolic labels of program and
memory locations Memory is the faculty of the brain by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If Foresight (psychology), past events cou ...
are often also supported. The term "assembler" is generally attributed to Wilkes,
Wheeler Wheeler may refer to: Places United States * Wheeler, Alabama Wheeler (also known as Wheeler Station) is an unincorporated area, unincorporated community in Lawrence County, Alabama, Lawrence County, Alabama, United States. Wheeler had a United St ...
and
Gill A gill () is a respiratory organ that many aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent ...
in their 1951 book ''
The Preparation of Programs for an Electronic Digital Computer ''The Preparation of Programs for an Electronic Digital Computer'' (sometimes called ''WWG'', after its authors' initials) was the first book on computer programming Computer programming is the process of designing and building an executable ...
'', who, however, used the term to mean "a program that assembles another program consisting of several sections into a single program". Each assembly language is specific to a particular
computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. The architecture of a system refers to its structure in terms of separately specifi ...
and sometimes to an
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
. However, some assembly languages do not provide specific
syntax In linguistics Linguistics is the scientific study of language, meaning that it is a comprehensive, systematic, objective, and precise study of language. Linguistics encompasses the analysis of every aspect of language, as well as the ...
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 processor, upon which all
system call In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
mechanisms ultimately rest. In contrast to assembly languages, most
high-level programming language In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , ...
s are generally
portable Portable may refer to: General * Portable building, a manufactured structure that is built off site and moved in upon completion of site and utility work * Portable classroom, a temporary building installed on the grounds of a school to provide ad ...
across multiple architectures but require
interpreting Interpreting is a translational activity in which one produces a first and final translation on the basis of a one-time exposure to an expression in a source language. The most common two modes of interpreting are simultaneous interpreting, wh ...
or
compiling In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest com ...

compiling
, a much more complicated task than assembling. The computational step when an assembler is processing a program is called ''assembly time''.


Assembly language syntax

Assembly language uses a
mnemonic A mnemonic () device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory Memory is the faculty of the by which or is , stored, and retrieved when needed. It is the ...

mnemonic
to represent each low-level
machine instruction In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, ge ...
or
opcode In computing, an opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine code, machine language instruction (computer sc ...
, typically also each ,
flag A flag is a piece of fabric A textile is a flexible material made by creating an interlocking network of yarn Yarn is a long continuous length of interlocked fibres, suitable for use in the production of textiles, sewing, crochetin ...
, etc. Many operations require one or more
operands In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
in order to form a complete instruction. Most assemblers permit named constants, registers, and
labels A label (as distinct from signage Signage is the design or use of signs and symbols to communicate a message. A signage also means signs ''collectively'' or being considered as a group. The term ''signage'' is documented to have been popula ...
for program and memory locations, and can calculate
expressions Expression may refer to: Linguistics * Expression (linguistics), a word, phrase, or sentence * Fixed expression, a form of words with a specific meaning * Idiom, a type of fixed expression * Metaphor#Common types, Metaphorical expression, a parti ...
for operands. Thus, the programmers are freed from tedious repetitive calculations and assembler programs are much more readable than machine code. Depending on the architecture, these elements may also be combined for specific instructions or
addressing mode Addressing modes are an aspect of the instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described b ...
s using
offset Offset or Off-Set may refer to: Arts, entertainment, and media * "Off-Set", a song by T.I. and Young Thug from the ''Furious 7: Original Motion Picture Soundtrack'' * Offset (EP), ''Offset'' (EP), a 2018 EP by singer Kim Chung-ha * Offset (film), ...
s or other data as well as fixed addresses. Many assemblers offer additional mechanisms to facilitate program development, to control the assembly process, and to aid
debugging In computer programming Computer programming is the process of designing and building an executable In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, caus ...

debugging
.


Terminology

* A macro assembler includes a macroinstruction facility so that (parameterized) assembly language text can be represented by a name, and that name can be used to insert the expanded text into other code. * A cross assembler (see also
cross compiler A cross compiler is a compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language) ...
) is an assembler that is run on a computer or
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
(the ''host'' system) of a different type from the system on which the resulting code is to run (the ''target system''). Cross-assembling facilitates the development of programs for systems that do not have the resources to support software development, such as an
embedded system An embedded system is a computer system A computer is a machine that can be programmed to carry out Sequence, sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Co ...
or a
microcontroller A microcontroller (MCU for ''microcontroller unit'') is a small computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets o ...

microcontroller
. In such a case, the resulting
object code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...
must be transferred to the target system, via
read-only memory Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
(ROM,
EPROM An EPROM (rarely EROM), or erasable programmable read-only memory, is a type of programmable read-only memory A programmable read-only memory (PROM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. (eF ...

EPROM
, etc.), a
programmer A computer programmer, sometimes called a software developer, a programmer or more recently a coder (especially in more informal contexts), is a person who creates computer software Software is a collection of Instruction (computer science) ...
(when the read-only memory is integrated in the device, as in microcontrollers), or a data link using either an exact bit-by-bit copy of the object code or a text-based representation of that code (such as
Intel hex Intel hexadecimal object file format, Intel hex format or Intellec Hex is a file format ogg-file: 154 kilobytes. A file format is a standard way that information is encoded for storage in a computer file. It specifies how bits are used to ...
or Motorola S-record). * A
high-level assembler High-level assemblers in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , ...
is a program that provides language abstractions more often associated with high-level languages, such as advanced control structures ( IF/THEN/ELSE, DO CASE, etc.) and high-level abstract data types, including structures/records, unions, classes, and sets. * A microassembler is a program that helps prepare a
microprogram Microcode is a processor design Processor design is the design engineerA design engineer is an engineer focused on the engineering design process in any of the various engineering disciplines (including civil engineer, civil, mechanical engineerin ...
, called ''firmware'', to control the low level operation of a computer. * A meta-assembler is "a program that accepts the syntactic and semantic description of an assembly language, and generates an assembler for that language". "Meta-Symbol" assemblers for the SDS 9 Series and
SDS Sigma series The SDS Sigma series is a series of third generation computers that were introduced by Scientific Data Systems of the United States in 1966. The first machines in the series are the 16-bit Sigma 2 and the 32-bit Sigma 7; the Sigma 7 was the first ...
of computers are meta-assemblers."Used as a meta-assembler, it enables the user to design his own programming languages and to generate processors for such languages with a minimum of effort."
Sperry Univac UNIVAC (Universal Automatic Computer) was a line of electronic digital stored-program computers starting with the products of the Eckert–Mauchly Computer Corporation. Later the name was applied to a division of the Remington Rand company and succe ...
also provided a Meta-Assembler for the
UNIVAC 1100/2200 series The UNIVAC 1100/2200 series is a series of compatible 36-bit 36-bit computers were popular in the early mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer A computer is a machine that can ...
. *
inline assemblerIn computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a specific task. Programming involves tasks such as: analysis, generatin ...
(or embedded assembler) is assembler code contained within a high-level language program. This is most often used in systems programs which need direct access to the hardware.


Key concepts


Assembler

An assembler program creates
object code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...
by translating combinations of
mnemonics A mnemonic (, the first "m" is not pronounced) device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory Memory is the faculty of the brain by which data or informat ...
and
syntax In linguistics Linguistics is the scientific study of language, meaning that it is a comprehensive, systematic, objective, and precise study of language. Linguistics encompasses the analysis of every aspect of language, as well as the ...

syntax
for operations and addressing modes into their numerical equivalents. This representation typically includes an ''operation code'' ("
opcode In computing, an opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine code, machine language instruction (computer sc ...
") as well as other control
bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algo ...
s and data. The assembler also calculates constant expressions and resolves symbolic names for memory locations and other entities. The use of symbolic references is a key feature of assemblers, saving tedious calculations and manual address updates after program modifications. Most assemblers also include
macro Macro (or MACRO) may refer to: Science and technology * Macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the o ...
facilities for performing textual substitution – e.g., to generate common short sequences of instructions as inline, instead of ''called''
subroutine In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, generat ...
s. Some assemblers may also be able to perform some simple types of
instruction set In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of co ...
-specific
optimization File:Nelder-Mead Simionescu.gif, Nelder-Mead minimum search of Test functions for optimization, Simionescu's function. Simplex vertices are ordered by their values, with 1 having the lowest ( best) value., alt= Mathematical optimization (alter ...
s. One concrete example of this may be the ubiquitous
x86 x86 is a family of instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, ...

x86
assemblers from various vendors. Called jump-sizing, most of them are able to perform jump-instruction replacements (long jumps replaced by short or relative jumps) in any number of passes, on request. Others may even do simple rearrangement or insertion of instructions, such as some assemblers for
RISC In computer engineering Computer engineering (CoE or CpE) is a branch of engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, v ...
architecture upright=1.45, alt=Plan d'exécution du second étage de l'hôtel de Brionne (dessin) De Cotte 2503c – Gallica 2011 (adjusted), Plan of the second floor (attic storey) of the Hôtel de Brionne in Paris – 1734. Architecture (Latin ''archi ...
s that can help optimize a sensible
instruction scheduling In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algorit ...
to exploit the CPU pipeline as efficiently as possible. Assemblers have been available since the 1950s, as the first step above machine language and before
high-level programming language In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , ...
s such as
Fortran Fortran (; formerly FORTRAN) is a general-purpose, compiled language, compiled imperative programming, imperative programming language that is especially suited to numerical analysis, numeric computation and computational science, scientific com ...

Fortran
,
Algol Algol , designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright multiple star in the constellation A constellation is an area on the celestial s ...
,
COBOL COBOL (; an acronym An acronym is a word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning (lingui ...

COBOL
and
Lisp Lisp (historically LISP) is a family of programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbo ...
. There have also been several classes of translators and semi-automatic
code generator In computing, code generation is the process by which a compiler's code generator converts some intermediate representation of source code into a form (e.g., machine code) that can be readily executed by a machine. Sophisticated compilers typicall ...
s with properties similar to both assembly and high-level languages, with Speedcode as perhaps one of the better-known examples. There may be several assemblers with different
syntax In linguistics Linguistics is the scientific study of language, meaning that it is a comprehensive, systematic, objective, and precise study of language. Linguistics encompasses the analysis of every aspect of language, as well as the ...
for a particular
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of ...

CPU
or
instruction set architecture In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , ...
. For instance, an instruction to add memory data to a register in a
x86 x86 is a family of instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, ...

x86
-family processor might be add eax,/code>, in original ''
Intel syntax x86 assembly language is a family of backward-compatible assembly language In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or ...
'', whereas this would be written addl (%ebx),%eax in the '' AT&T syntax'' used by the
GNU Assembler The GNU Assembler, commonly known as gas or simply as, its executable name, is the assembler used by the GNU Project The GNU Project () is a free software, mass collaboration project that Richard Stallman announced on September 27, 1983. Its ...
. Despite different appearances, different syntactic forms generally generate the same numeric
machine code In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, ge ...
. A single assembler may also have different modes in order to support variations in syntactic forms as well as their exact semantic interpretations (such as
FASM FASM (''flat assembler'') is an Assembly language assembler, assembler for x86 processors. It supports x86 assembly language, Intel-style assembly language on the IA-32 and x86-64 computer architectures. It claims high speed, size optimizations, ...
-syntax,
TASM Turbo Assembler (sometimes shortened to the name of the executable, TASM) is an Assembly language#Assembler, assembler for software development published by Borland in 1989. It runs on and produces code for 16- or 32-bit x86 MS-DOS and compatibl ...
-syntax, ideal mode, etc., in the special case of
x86 assembly x86 assembly language is a family of backward-compatible assembly languages, which provide some level of compatibility all the way back to the Intel 8008 introduced in April 1972. x86 assembly languages are used to produce object code for the x8 ...
programming).


Number of passes

There are two types of assemblers based on how many passes through the source are needed (how many times the assembler reads the source) to produce the object file. * One-pass assemblers go through the source code once. Any symbol used before it is defined will require "errata" at the end of the object code (or, at least, no earlier than the point where the symbol is defined) telling the
linker Linker or linkers may refer to: Computing * Linker (computing) In computing, a linker or link editor is a computer System software, system program that takes one or more object files (generated by a compiler or an assembler (computing), asse ...
or the loader to "go back" and overwrite a placeholder which had been left where the as yet undefined symbol was used. * Multi-pass assemblers create a table with all symbols and their values in the first passes, then use the table in later passes to generate code. In both cases, the assembler must be able to determine the size of each instruction on the initial passes in order to calculate the addresses of subsequent symbols. This means that if the size of an operation referring to an operand defined later depends on the type or distance of the operand, the assembler will make a pessimistic estimate when first encountering the operation, and if necessary, pad it with one or more " no-operation" instructions in a later pass or the errata. In an assembler with
peephole optimization Peephole optimization is an Optimizing_compiler, optimization technique performed on a small set of compiler-generated instructions; the small set is known as the peephole or window. Peephole optimization involves changing the small set of instru ...
, addresses may be recalculated between passes to allow replacing pessimistic code with code tailored to the exact distance from the target. The original reason for the use of one-pass assemblers was memory size and speed of assembly – often a second pass would require storing the symbol table in memory (to handle
forward referenceIn computer programming, a forward declaration is a declaration (computer science), declaration of an Identifier (computer programming), identifier (denoting an entity such as a type, a variable, a constant, or a function) for which the programmer ha ...
s), rewinding and rereading the program source on tape, or rereading a deck of
cards {{Redirect, CARDS, other uses, Cards (disambiguation){{!Cards The CARDS programme, of Community Assistance for Reconstruction, Development and Stabilisation, is the EU's main instrument of financial assistance to the Western Balkans, covering spec ...

cards
or
punched paper tape file:PaperTapes-5and8Hole.jpg, Five- and eight-hole punched paper tape file:Harwell-dekatron-witch-10.jpg, Paper tape reader on the Harwell computer with a small piece of five-hole tape connected in a circle – creating a physical program loop P ...

punched paper tape
. Later computers with much larger memories (especially disc storage), had the space to perform all necessary processing without such re-reading. The advantage of the multi-pass assembler is that the absence of errata makes the linking process (or the program load if the assembler directly produces executable code) faster. Example: in the following code snippet, a one-pass assembler would be able to determine the address of the backward reference BKWD when assembling statement S2, but would not be able to determine the address of the forward reference FWD when assembling the branch statement S1; indeed, FWD may be undefined. A two-pass assembler would determine both addresses in pass 1, so they would be known when generating code in pass 2. B ... EQU * ... EQU * ... B


High-level assemblers

More sophisticated
high-level assembler High-level assemblers in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , ...
s provide language abstractions such as: * High-level procedure/function declarations and invocations * Advanced control structures (IF/THEN/ELSE, SWITCH) * High-level abstract data types, including structures/records, unions, classes, and sets * Sophisticated macro processing (although available on ordinary assemblers since the late 1950s for, e.g., the IBM 700 series and
IBM 7000 series The IBM 700/7000 series is a series of large-scale (mainframe A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications, bulk data processing (such as the ...
, and since the 1960s for
IBM System/360 The IBM System/360 (S/360) is a family of mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or ...
(S/360), amongst other machines) *
Object-oriented programming Object-oriented programming (OOP) is a programming paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mai ...
features such as
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
es,
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Entity, something that is tangible and within the grasp of the senses ** Object (abstract), an object which does not exist at any particular time or pl ...
s,
abstraction Abstraction in its main sense is a conceptual process where general rules Rule or ruling may refer to: Human activity * The exercise of political Politics (from , ) is the set of activities that are associated with Decision-making, mak ...
, polymorphism, and
inheritance Inheritance is the practice of passing on private property Private property is a legal designation for the ownership of property by non-governmental legal entities. Private property is distinguishable from public property Public property i ...
See
Language design A language is a structured system of communication used by humans, including speech (spoken language A spoken language is a language A language is a structured system of communication used by humans, including speech (spoken language), g ...
below for more details.


Assembly language

A program written in assembly language consists of a series of
mnemonic A mnemonic () device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory Memory is the faculty of the by which or is , stored, and retrieved when needed. It is the ...

mnemonic
processor instructions and meta-statements (known variously as directives, pseudo-instructions, and pseudo-ops), comments and data. Assembly language instructions usually consist of an
opcode In computing, an opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine code, machine language instruction (computer sc ...
mnemonic followed by an
operand In mathematics an operand is the object of a mathematical operation, i.e., it is the object or quantity that is operated on. Example The following arithmetic expression shows an example of operators and operands: :3 + 6 = 9 In the above example, ...
, which might be a list of data, arguments or parameters. Some instructions may be "implied," which means the data upon which the instruction operates is implicitly defined by the instruction itself—such an instruction does not take an operand. The resulting statement is translated by an
assembler Assembler may refer to: Arts and media * Nobukazu Takemura Nobukazu Takemura (竹村延和 ''Takemura Nobukazu'') is a Japanese musician and artist. Born in Hirakata, Osaka in August 1968, he became interested in music at a young age by listenin ...
into
machine language In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, ...
instructions that can be loaded into memory and executed. For example, the instruction below tells an
x86 x86 is a family of instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, ...

x86
/
IA-32 IA-32 (short for "Intel Architecture, 32-bit", sometimes also called i386) is the 32-bit 32-bit microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It incl ...
processor to move an immediate 8-bit value into a
register A register is an authoritative list of one kind of information. Register or registration may refer to: Arts entertainment, and media Music * Register (music), the relative "height" or range of a note, melody, part, instrument, etc. * ''Regis ...
. The
binary code A binary code represents text Text may refer to: Written word * Text (literary theory), any object that can be read, including: **Religious text, a writing that a religious tradition considers to be sacred **Text, a verse or passage from script ...

binary code
for this instruction is 10110 followed by a 3-bit identifier for which register to use. The identifier for the ''AL'' register is 000, so the following
machine code In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, ge ...
loads the ''AL'' register with the data 01100001. 10110000 01100001 This binary computer code can be made more human-readable by expressing it in
hexadecimal In mathematics and computing, the hexadecimal (also base 16 or hex) numeral system is a Numeral system#Positional systems in detail, positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system repres ...
as follows. B0 61 Here, B0 means 'Move a copy of the following value into ''AL'', and 61 is a hexadecimal representation of the value 01100001, which is 97 in
decimal The decimal numeral system A numeral system (or system of numeration) is a writing system A writing system is a method of visually representing verbal communication Communication (from Latin ''communicare'', meaning "to share") is t ...
. Assembly language for the 8086 family provides the
mnemonic A mnemonic () device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory Memory is the faculty of the by which or is , stored, and retrieved when needed. It is the ...

mnemonic
MOV (an abbreviation of ''move'') for instructions such as this, so the machine code above can be written as follows in assembly language, complete with an explanatory comment if required, after the semicolon. This is much easier to read and to remember. MOV AL, 61h ; Load AL with 97 decimal (61 hex) In some assembly languages (including this one) the same mnemonic, such as MOV, may be used for a family of related instructions for loading, copying and moving data, whether these are immediate values, values in registers, or memory locations pointed to by values in registers or by immediate (a/k/a direct) addresses. Other assemblers may use separate opcode mnemonics such as L for "move memory to register", ST for "move register to memory", LR for "move register to register", MVI for "move immediate operand to memory", etc. If the same mnemonic is used for different instructions, that means that the mnemonic corresponds to several different binary instruction codes, excluding data (e.g. the 61h in this example), depending on the operands that follow the mnemonic. For example, for the x86/IA-32 CPUs, the Intel assembly language syntax MOV AL, AH represents an instruction that moves the contents of register ''AH'' into register ''AL''. The hexadecimal form of this instruction is: 88 E0 The first byte, 88h, identifies a move between a byte-sized register and either another register or memory, and the second byte, E0h, is encoded (with three bit-fields) to specify that both operands are registers, the source is ''AH'', and the destination is ''AL''. In a case like this where the same mnemonic can represent more than one binary instruction, the assembler determines which instruction to generate by examining the operands. In the first example, the operand 61h is a valid hexadecimal numeric constant and is not a valid register name, so only the B0 instruction can be applicable. In the second example, the operand AH is a valid register name and not a valid numeric constant (hexadecimal, decimal, octal, or binary), so only the 88 instruction can be applicable. Assembly languages are always designed so that this sort of unambiguousness is universally enforced by their syntax. For example, in the Intel x86 assembly language, a hexadecimal constant must start with a numeral digit, so that the hexadecimal number 'A' (equal to decimal ten) would be written as 0Ah or 0AH, not AH, specifically so that it cannot appear to be the name of register ''AH''. (The same rule also prevents ambiguity with the names of registers ''BH'', ''CH'', and ''DH'', as well as with any user-defined symbol that ends with the letter ''H'' and otherwise contains only characters that are hexadecimal digits, such as the word "BEACH".) Returning to the original example, while the x86 opcode 10110000 (B0) copies an 8-bit value into the ''AL'' register, 10110001 (B1) moves it into ''CL'' and 10110010 (B2) does so into ''DL''. Assembly language examples for these follow. MOV AL, 1h ; Load AL with immediate value 1 MOV CL, 2h ; Load CL with immediate value 2 MOV DL, 3h ; Load DL with immediate value 3 The syntax of MOV can also be more complex as the following examples show. MOV EAX, ; Move the 4 bytes in memory at the address contained in EBX into EAX MOV SI+EAX CL ; Move the contents of CL into the byte at address ESI+EAX MOV DS, DX ; Move the contents of DX into segment register DS In each case, the MOV mnemonic is translated directly into one of the opcodes 88-8C, 8E, A0-A3, B0-BF, C6 or C7 by an assembler, and the programmer normally does not have to know or remember which. Transforming assembly language into machine code is the job of an assembler, and the reverse can at least partially be achieved by a disassembler. Unlike high-level languages, there is a one-to-one correspondence between many simple assembly statements and machine language instructions. However, in some cases, an assembler may provide ''pseudoinstructions'' (essentially macros) which expand into several machine language instructions to provide commonly needed functionality. For example, for a machine that lacks a "branch if greater or equal" instruction, an assembler may provide a pseudoinstruction that expands to the machine's "set if less than" and "branch if zero (on the result of the set instruction)". Most full-featured assemblers also provide a rich macro (computer science), macro language (discussed below) which is used by vendors and programmers to generate more complex code and data sequences. Since the information about pseudoinstructions and macros defined in the assembler environment is not present in the object program, a disassembler cannot reconstruct the macro and pseudoinstruction invocations but can only disassemble the actual machine instructions that the assembler generated from those abstract assembly-language entities. Likewise, since comments in the assembly language source file are ignored by the assembler and have no effect on the object code it generates, a disassembler is always completely unable to recover source comments. Each
computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. The architecture of a system refers to its structure in terms of separately specifi ...
has its own machine language. Computers differ in the number and type of operations they support, in the different sizes and numbers of registers, and in the representations of data in storage. While most general-purpose computers are able to carry out essentially the same functionality, the ways they do so differ; the corresponding assembly languages reflect these differences. Multiple sets of
mnemonic A mnemonic () device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory Memory is the faculty of the by which or is , stored, and retrieved when needed. It is the ...

mnemonic
s or assembly-language syntax may exist for a single instruction set, typically instantiated in different assembler programs. In these cases, the most popular one is usually that supplied by the CPU manufacturer and used in its documentation. Two examples of CPUs that have two different sets of mnemonics are the Intel 8080 family and the Intel 8086/8088. Because Intel claimed copyright on its assembly language mnemonics (on each page of their documentation published in the 1970s and early 1980s, at least), some companies that independently produced CPUs compatible with Intel instruction sets invented their own mnemonics. The Zilog Z80 CPU, an enhancement of the Intel 8080A, supports all the 8080A instructions plus many more; Zilog invented an entirely new assembly language, not only for the new instructions but also for all of the 8080A instructions. For example, where Intel uses the mnemonics ''MOV'', ''MVI'', ''LDA'', ''STA'', ''LXI'', ''LDAX'', ''STAX'', ''LHLD'', and ''SHLD'' for various data transfer instructions, the Z80 assembly language uses the mnemonic ''LD'' for all of them. A similar case is the NEC V20 and NEC V30, V30 CPUs, enhanced copies of the Intel 8086 and 8088, respectively. Like Zilog with the Z80, NEC invented new mnemonics for all of the 8086 and 8088 instructions, to avoid accusations of infringement of Intel's copyright. (It is questionable whether such copyrights can be valid, and later CPU companies such as Advanced Micro Devices, AMD and Cyrix republished Intel's x86/IA-32 instruction mnemonics exactly with neither permission nor legal penalty.) It is doubtful whether in practice many people who programmed the V20 and V30 actually wrote in NEC's assembly language rather than Intel's; since any two assembly languages for the same instruction set architecture are isomorphic (somewhat like English and Pig Latin), there is no requirement to use a manufacturer's own published assembly language with that manufacturer's products.


Language design


Basic elements

There is a large degree of diversity in the way the authors of assemblers categorize statements and in the nomenclature that they use. In particular, some describe anything other than a machine mnemonic or extended mnemonic as a pseudo-operation (pseudo-op). A typical assembly language consists of 3 types of instruction statements that are used to define program operations: * Opcode mnemonics * Data definitions * Assembly directives


Opcode mnemonics and extended mnemonics

Instructions (statements) in assembly language are generally very simple, unlike those in high-level programming language, high-level languages. Generally, a mnemonic is a symbolic name for a single executable machine language instruction (an
opcode In computing, an opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine code, machine language instruction (computer sc ...
), and there is at least one opcode mnemonic defined for each machine language instruction. Each instruction typically consists of an ''operation'' or ''opcode'' plus zero or more ''operands''. Most instructions refer to a single value or a pair of values. Operands can be immediate (value coded in the instruction itself), registers specified in the instruction or implied, or the addresses of data located elsewhere in storage. This is determined by the underlying processor architecture: the assembler merely reflects how this architecture works. ''Extended mnemonics'' are often used to specify a combination of an opcode with a specific operand, e.g., the System/360 assemblers use as an extended mnemonic for with a mask of 15 and ("NO OPeration" – do nothing for one step) for with a mask of 0. ''Extended mnemonics'' are often used to support specialized uses of instructions, often for purposes not obvious from the instruction name. For example, many CPU's do not have an explicit NOP instruction, but do have instructions that can be used for the purpose. In 8086 CPUs the instruction is used for , with being a pseudo-opcode to encode the instruction . Some disassemblers recognize this and will decode the instruction as . Similarly, IBM assemblers for IBM System/360, System/360 and IBM System/370, System/370 use the extended mnemonics and for and with zero masks. For the SPARC architecture, these are known as ''synthetic instructions''. Some assemblers also support simple built-in macro-instructions that generate two or more machine instructions. For instance, with some Z80 assemblers the instruction is recognized to generate followed by . These are sometimes known as ''pseudo-opcodes''. Mnemonics are arbitrary symbols; in 1985 the IEEE published Standard 694 for a uniform set of mnemonics to be used by all assemblers. The standard has since been withdrawn.


Data directives

There are instructions used to define data elements to hold data and variables. They define the type of data, the length and the data structure alignment, alignment of data. These instructions can also define whether the data is available to outside programs (programs assembled separately) or only to the program in which the data section is defined. Some assemblers classify these as pseudo-ops.


Assembly directives

Assembly directives, also called pseudo-opcodes, pseudo-operations or pseudo-ops, are commands given to an assembler "directing it to perform operations other than assembling instructions". Directives affect how the assembler operates and "may affect the object code, the symbol table, the listing file, and the values of internal assembler parameters". Sometimes the term ''pseudo-opcode'' is reserved for directives that generate object code, such as those that generate data. The names of pseudo-ops often start with a dot to distinguish them from machine instructions. Pseudo-ops can make the assembly of the program dependent on parameters input by a programmer, so that one program can be assembled in different ways, perhaps for different applications. Or, a pseudo-op can be used to manipulate presentation of a program to make it easier to read and maintain. Another common use of pseudo-ops is to reserve storage areas for run-time data and optionally initialize their contents to known values. Symbolic assemblers let programmers associate arbitrary names (''label (programming language), labels'' or ''symbols'') with memory locations and various constants. Usually, every constant and variable is given a name so instructions can reference those locations by name, thus promoting self-documenting code. In executable code, the name of each subroutine is associated with its entry point, so any calls to a subroutine can use its name. Inside subroutines, GOTO destinations are given labels. Some assemblers support ''local symbols'' which are often lexically distinct from normal symbols (e.g., the use of "10$" as a GOTO destination). Some assemblers, such as Netwide Assembler, NASM, provide flexible symbol management, letting programmers manage different namespaces, automatically calculate offsets within data structures, and assign labels that refer to literal values or the result of simple computations performed by the assembler. Labels can also be used to initialize constants and variables with relocatable addresses. Assembly languages, like most other computer languages, allow comments to be added to program
source code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and ...

source code
that will be ignored during assembly. Judicious commenting is essential in assembly language programs, as the meaning and purpose of a sequence of binary machine instructions can be difficult to determine. The "raw" (uncommented) assembly language generated by compilers or disassemblers is quite difficult to read when changes must be made.


Macros

Many assemblers support ''predefined macros'', and others support ''programmer-defined'' (and repeatedly re-definable) macros involving sequences of text lines in which variables and constants are embedded. The macro definition is most commonly a mixture of assembler statements, e.g., directives, symbolic machine instructions, and templates for assembler statements. This sequence of text lines may include opcodes or directives. Once a macro has been defined its name may be used in place of a mnemonic. When the assembler processes such a statement, it replaces the statement with the text lines associated with that macro, then processes them as if they existed in the source code file (including, in some assemblers, expansion of any macros existing in the replacement text). Macros in this sense date to IBM autocoders of the 1950s. In assembly language, the term "macro" represents a more comprehensive concept than it does in some other contexts, such as the pre-processor in the C (programming language), C programming language, where its #define directive typically is used to create short single line macros. Assembler macro instructions, like macros in PL/I and some other languages, can be lengthy "programs" by themselves, executed by interpretation by the assembler during assembly. Since macros can have 'short' names but expand to several or indeed many lines of code, they can be used to make assembly language programs appear to be far shorter, requiring fewer lines of source code, as with higher level languages. They can also be used to add higher levels of structure to assembly programs, optionally introduce embedded debugging code via parameters and other similar features. Macro assemblers often allow macros to take parameter (computer science), parameters. Some assemblers include quite sophisticated macro languages, incorporating such high-level language elements as optional parameters, symbolic variables, conditionals, string manipulation, and arithmetic operations, all usable during the execution of a given macro, and allowing macros to save context or exchange information. Thus a macro might generate numerous assembly language instructions or data definitions, based on the macro arguments. This could be used to generate record-style data structures or "loop unrolling, unrolled" loops, for example, or could generate entire algorithms based on complex parameters. For instance, a "sort" macro could accept the specification of a complex sort key and generate code crafted for that specific key, not needing the run-time tests that would be required for a general procedure interpreting the specification. An organization using assembly language that has been heavily extended using such a macro suite can be considered to be working in a higher-level language since such programmers are not working with a computer's lowest-level conceptual elements. Underlining this point, macros were used to implement an early virtual machine in SNOBOL4 (1967), which was written in the SNOBOL Implementation Language (SIL), an assembly language for a virtual machine. The target machine would translate this to its native code using a macro assembler. This allowed a high degree of portability for the time. Macros were used to customize large scale software systems for specific customers in the mainframe era and were also used by customer personnel to satisfy their employers' needs by making specific versions of manufacturer operating systems. This was done, for example, by systems programmers working with IBM's Conversational Monitor System / Virtual Machine (VM/CMS) and with IBM's "real time transaction processing" add-ons, Customer Information Control System CICS, and Airline Control Program, ACP/Transaction Processing Facility, TPF, the airline/financial system that began in the 1970s and still runs many large computer reservation systems (CRS) and credit card systems today. It is also possible to use solely the macro processing abilities of an assembler to generate code written in completely different languages, for example, to generate a version of a program in
COBOL COBOL (; an acronym An acronym is a word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning (lingui ...

COBOL
using a pure macro assembler program containing lines of COBOL code inside assembly time operators instructing the assembler to generate arbitrary code. IBM OS/360 uses macros to perform system generation. The user specifies options by coding a series of assembler macros. Assembling these macros generates a job stream to build the system, including job control language and IBM mainframe utility programs, utility control statements. This is because, as was realized in the 1960s, the concept of "macro processing" is independent of the concept of "assembly", the former being in modern terms more word processing, text processing, than generating object code. The concept of macro processing appeared, and appears, in the C programming language, which supports "preprocessor instructions" to set variables, and make conditional tests on their values. Unlike certain previous macro processors inside assemblers, the C preprocessor is not Turing-complete because it lacks the ability to either loop or "go to", the latter allowing programs to loop. Despite the power of macro processing, it fell into disuse in many high level languages (major exceptions being C (programming language), C, C++ and PL/I) while remaining a perennial for assemblers. Macro parameter substitution is strictly by name: at macro processing time, the value of a parameter is textually substituted for its name. The most famous class of bugs resulting was the use of a parameter that itself was an expression and not a simple name when the macro writer expected a name. In the macro: foo: macro a load a*b the intention was that the caller would provide the name of a variable, and the "global" variable or constant b would be used to multiply "a". If foo is called with the parameter a-c, the macro expansion of load a-c*b occurs. To avoid any possible ambiguity, users of macro processors can parenthesize formal parameters inside macro definitions, or callers can parenthesize the input parameters.


Support for structured programming

Packages of macros have been written providing structured programming elements to encode execution flow. The earliest example of this approach was in the Concept-14 macro set, originally proposed by Harlan Mills (March 1970), and implemented by Marvin Kessler at IBM's Federal Systems Division, which provided IF/ELSE/ENDIF and similar control flow blocks for OS/360 assembler programs. This was a way to reduce or eliminate the use of GOTO operations in assembly code, one of the main factors causing spaghetti code in assembly language. This approach was widely accepted in the early 1980s (the latter days of large-scale assembly language use). A curious design was A-natural, a "stream-oriented" assembler for 8080/Z80, processors from Whitesmiths, Whitesmiths Ltd. (developers of the Unix-like Idris (operating system), Idris operating system, and what was reported to be the first commercial C (programming language), C compiler). The language was classified as an assembler because it worked with raw machine elements such as opcodes, processor register, registers, and memory references; but it incorporated an expression syntax to indicate execution order. Parentheses and other special symbols, along with block-oriented structured programming constructs, controlled the sequence of the generated instructions. A-natural was built as the object language of a C compiler, rather than for hand-coding, but its logical syntax won some fans. There has been little apparent demand for more sophisticated assemblers since the decline of large-scale assembly language development. In spite of that, they are still being developed and applied in cases where resource constraints or peculiarities in the target system's architecture prevent the effective use of higher-level languages. Assemblers with a strong macro engine allow structured programming via macros, such as the switch macro provided with the Masm32 package (this code is a complete program): include \masm32\include\masm32rt.inc ; use the Masm32 library .code demomain: REPEAT 20 switch rv(nrandom, 9) ; generate a number between 0 and 8 mov ecx, 7 case 0 print "case 0" case ecx ; in contrast to most other programming languages, print "case 7" ; the Masm32 switch allows "variable cases" case 1 .. 3 .if eax

1 print "case 1" .elseif eax

2 print "case 2" .else print "cases 1 to 3: other" .endif case 4, 6, 8 print "cases 4, 6 or 8" default mov ebx, 19 ; print 20 stars .Repeat print "*" dec ebx .Until Sign? ; loop until the sign flag is set endsw print chr$(13, 10) ENDM exit end demomain


Use of assembly language


Historical perspective

Assembly languages were not available at the time when the stored-program computer was introduced. Kathleen Booth "is credited with inventing assembly language" 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. In late 1948, the 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". Reports on the EDSAC introduced the term "assembly" for the process of combining fields into an instruction word. SOAP (Symbolic Optimal Assembly Program) was an assembly language for the IBM 650 computer written by Stan Poley in 1955. Assembly languages eliminate 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. Assembly languages were once widely used for all sorts of programming. However, by the 1980s (1990s on microcomputers), their use had largely been supplanted by higher-level languages, in the search for improved programming productivity. Today, assembly language is still used for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues. Typical uses are device drivers, low-level
embedded system An embedded system is a computer system A computer is a machine that can be programmed to carry out Sequence, sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Co ...
s, and real-time computing, real-time systems. Historically, numerous programs have been 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 written by large corporations.
COBOL COBOL (; an acronym An acronym is a word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning (lingui ...

COBOL
, FORTRAN and some PL/I eventually displaced much of this work, although a number of large organizations retained assembly-language application infrastructures well into the 1990s. Most early microcomputers relied on hand-coded assembly language, including most operating systems and large applications. This was because these systems had severe resource constraints, imposed idiosyncratic memory and display architectures, and provided limited, buggy system services. Perhaps more important was the lack of first-class high-level language compilers suitable for microcomputer use. A psychological factor may have also played a role: the first generation of microcomputer programmers retained a hobbyist, "wires and pliers" attitude. In a more commercial context, the biggest reasons for using assembly language were minimal bloat (size), minimal overhead, greater speed, and reliability. Typical examples of large assembly language programs from this time are IBM PC DOS operating systems, the Turbo Pascal compiler and early applications such as the spreadsheet program Lotus 1-2-3. Assembly language was used to get the best performance out of the Sega Saturn, a console that was notoriously challenging to develop and program games for. The 1993 arcade game ''NBA Jam (1993 video game), NBA Jam'' is another example. Assembly language has long been the primary development language for many popular home computers of the 1980s and 1990s (such as the MSX, Sinclair Research, Sinclair ZX Spectrum, Commodore 64, Commodore Amiga, and Atari ST). This was in large part because Interpreted language, interpreted BASIC dialects on these systems offered insufficient execution speed, as well as insufficient facilities to take full advantage of the available hardware on these systems. Some systems even have an integrated development environment (IDE) with highly advanced debugging and macro facilities. Some compilers available for the Radio Shack TRS-80 and its successors had the capability to combine inline assembly source with high-level program statements. Upon compilation, a built-in assembler produced inline machine code.


Current usage

There have always been debates over the usefulness and performance of assembly language relative to high-level languages. Although assembly language has specific niche uses where it is important (see below), there are other tools for optimization. , the TIOBE index of programming language popularity ranks assembly language at 11, ahead of Visual Basic, for example. Assembler can be used to optimize for speed or optimize for size. In the case of speed optimization, modern optimizing compilers are claimed to render high-level languages into code that can run as fast as hand-written assembly, despite the counter-examples that can be found. The complexity of modern processors and memory sub-systems makes effective optimization increasingly difficult for compilers, as well as for assembly programmers. Moreover, increasing processor performance has meant that most CPUs sit idle most of the time, with delays caused by predictable bottlenecks such as cache misses, I/O operations and paging. This has made raw code execution speed a non-issue for many programmers. There are some situations in which developers might choose to use assembly language: * Writing code for systems with older processors that have limited high-level language options such as the Atari 2600, Commodore 64, and graphing calculators. * Code that must interact directly with the hardware, for example in device drivers and interrupt handlers. * In an embedded processor or 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. * A stand-alone executable of compact size is required that must execute without recourse to the run-time system, run-time components or library (computing), libraries associated with a high-level language. Examples have included firmware for telephones, automobile fuel and ignition systems, air-conditioning control systems, security systems, and sensors. * 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 or DCT (math), discrete cosine transformation (e.g. SIMD assembly version from x264). * 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 functions 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 (some) interpreted languages, automatic garbage collection (computer science), garbage collection, paging operations, or preemptive multitasking. However, some higher-level languages incorporate run-time components and operating system interfaces that can introduce such delays. Choosing assembly or 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 attacks. * Modify and extend legacy code written for IBM mainframe computers. https://www-01.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R3sc236852/$file/idad500_v2r3.pdf * 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 viruses, bootloaders, certain device drivers, or other items very close to the hardware or low-level operating system. * Instruction set simulators for monitoring, tracing and
debugging In computer programming Computer programming is the process of designing and building an executable In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, caus ...

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 A cross compiler is a compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language) ...
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 games (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 allocation, stack (data structure), 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 behavior 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: I) the basic concepts; II) to recognize situations where the use of assembly language might be appropriate; and III) to see how efficient executable code can be created from high-level languages.


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 Personal Computer, PC systems and CP/M is an example.) * Assembly language is often used for low-level code, for instance for Kernel (operating system), operating system kernels, 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.


See also

* Compiler * Comparison of assemblers * Disassembler * Hexadecimal * Instruction set architecture * Little man computer – an educational computer model with a base-10 assembly language * Nibble * Typed assembly language


Notes


References


Further reading

*

* * (2+xiv+270+6 pages) * * * * * * * * ("An online book full of helpful ASM info, tutorials and code examples" by the ASM Community, archived at the internet archive.)


External links

*
Unix Assembly Language Programming

Linux Assembly

PPR: Learning Assembly Language

NASM – The Netwide Assembler (a popular assembly language)





Assembly Optimization Tips
by Mark Larson

{{DEFAULTSORT:Assembly Language Assembly languages, *Assembly language Computer-related introductions in 1949 Embedded systems Low-level programming languages Programming language implementation Programming languages created in 1949