Assembly language

Incomputer programming,assembly language(alternativelyassembler languageorsymbolic machine code),often referred to simply asassemblyand commonly abbreviated asASMorasm,is anylow-level programming languagewith a very strong correspondence between the instructions in the language and thearchitecture'smachine codeinstructions.Assembly language usually has onestatementper machine instruction (1:1), but constants,comments,assemblerdirectives,symboliclabelsof, e.g.,memory locations,registers,andmacrosare generally also supported.

Assembly language

Typicalsecondary outputfrom an assembler—showing original assembly language (right) for theMotorolaMC6800and the assembled form
Paradigm	Imperative,unstructured,oftenmetaprogramming(throughmacros), certain assemblers arestructuredorobject-oriented
First appeared	1947(1947)
Typing discipline	None
Filename extensions	.asm,.s,.inc,.wla,.SRCand several others depending on the assembler

The first assembly code in which a language is used to represent machine code instructions is found inKathleenandAndrew Donald Booth's 1947 work,Coding for A.R.C..Assembly code is converted into executable machine code by autility programreferred to as anassembler.The term "assembler" is generally attributed toWilkes,WheelerandGillin their 1951 bookThe Preparation of Programs for an Electronic Digital Computer,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 asassembly,as inassemblingthesource code.The computational step when an assembler is processing a program is calledassembly time.

Because assembly depends on the machine code instructions, each assembly languageis specific to a particularcomputer architecture.

Sometimes there is more than one assembler for the same architecture, and sometimes an assembler is specific to anoperating systemor to particular operating systems. Most assembly languages do not provide specificsyntaxfor 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 theprocessor,upon which allsystem callmechanisms ultimately rest. In contrast to assembly languages, mosthigh-level programming languagesare generallyportableacross multiple architectures but requireinterpretingorcompiling,much more complicated tasks than assembling.

In the first decades of computing, it was commonplace for bothsystems programmingandapplication programmingto 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 "No Silver Bullet",Fred Brookssummarised 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 theLinux kernelsource code is written in assembly; more than 97% is written inC.

Assembly language syntax

Assembly language uses amnemonicto represent, e.g., each low-levelmachine instructionoropcode,eachdirective,typically also eacharchitectural register,flag,etc. Some of the mnemonics may be built in and some user defined. Many operations require one or moreoperandsin order to form a complete instruction. Most assemblers permit named constants, registers, andlabelsfor program and memory locations, and can calculateexpressionsfor operands. Thus, 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 oraddressing modesusingoffsetsor other data as well as fixed addresses. Many assemblers offer additional mechanisms to facilitate program development, to control the assembly process, and to aiddebugging.

Some are column oriented, with specific fields in specific columns; this was very common for machines usingpunched cardsin the 1950s and early 1960s. Some assemblers have free-form syntax, with fields separated by delimiters, e.g., punctuation,white space.Some assemblers are hybrid, with, e.g., labels, in a specific column and other fields separated by delimiters; this became more common than column oriented syntax in the 1960s.

Terminology

• Amacro assembleris an assembler that includes amacroinstructionfacility 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.
  • Open coderefers to any assembler input outside of a macro definition.
• Across assembler(see alsocross compiler) is an assembler that is run on a computer oroperating system(thehostsystem) of a different type from the system on which the resulting code is to run (thetarget system). Cross-assembling facilitates the development of programs for systems that do not have the resources to support software development, such as anembedded systemor amicrocontroller.In such a case, the resultingobject codemust be transferred to the target system, viaread-only memory(ROM,EPROM,etc.), aprogrammer(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 asIntel hexorMotorola S-record).
• Ahigh-level assembleris 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.
• Amicroassembleris a program that helps prepare amicroprogramto control the low level operation of a computer.
• Ameta-assembleris "a program that accepts the syntactic and semantic description of an assembly language, and generates an assembler for that language",or that accepts an assembler source file along with such a description and assembles the source file in accordance with that description. "Meta-Symbol" assemblers for theSDS 9 SeriesandSDS Sigma seriesof computers are meta-assemblers.Sperry Univacalso provided a Meta-Assembler for theUNIVAC 1100/2200 series.
• inline assembler(orembedded 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

Anassemblerprogram createsobject codebytranslatingcombinations ofmnemonicsandsyntaxfor operations and addressing modes into their numerical equivalents. This representation typically includes anoperation code( "opcode") as well as other controlbitsand data. The assembler also calculates constant expressions and resolvessymbolic namesfor 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 includemacrofacilities for performing textual substitution – e.g., to generate common short sequences of instructions asinline,instead ofcalledsubroutines.

Some assemblers may also be able to perform some simple types ofinstruction set-specificoptimizations.One concrete example of this may be the ubiquitousx86assemblers from various vendors. Calledjump-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 forRISC architecturesthat can help optimize a sensibleinstruction schedulingto exploit theCPU pipelineas efficiently as possible.

Assemblers have been available since the 1950s, as the first step above machine language and beforehigh-level programming languagessuch asFortran,Algol,COBOLandLisp.There have also been several classes of translators and semi-automaticcode generatorswith properties similar to both assembly and high-level languages, withSpeedcodeas perhaps one of the better-known examples.

There may be several assemblers with differentsyntaxfor a particularCPUorinstruction set architecture.For instance, an instruction to add memory data to a register in ax86-family processor might beadd eax,[ebx],in originalIntel syntax,whereas this would be writtenaddl (%ebx),%eaxin theAT&T syntaxused by theGNU Assembler.Despite different appearances, different syntactic forms generally generate the same numericmachine code.A single assembler may also have different modes in order to support variations in syntactic forms as well as their exact semantic interpretations (such asFASM-syntax,TASM-syntax, ideal mode, etc., in the special case ofx86 assemblyprogramming).

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 assemblersprocess the source code once. For symbols used before they are defined, the assembler will emit"errata"after the eventual definition, telling thelinkeror the loader to patch the locations where the as yet undefined symbols had been used.
• Multi-pass assemblerscreate 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 withpeephole optimization,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 handleforward references), rewinding and rereading the program source ontape,or rereading a deck ofcardsorpunched 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 thelinking process(or theprogram loadif 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 referenceBKWDwhen assembling statementS2,but would not be able to determine the address of the forward referenceFWDwhen assembling the branch statementS1;indeed,FWDmay be undefined. A two-pass assembler would determine both addresses in pass 1, so they would be known when generating code in pass 2.

S1BFWD
...
FWDEQU *
...
BKWDEQU *
...
S2BBKWD

High-level assemblers

More sophisticatedhigh-level assemblersprovide 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., theIBM 700 seriesandIBM 7000 series,and since the 1960s forIBM System/360(S/360), amongst other machines)
• Object-oriented programmingfeatures such asclasses,objects,abstraction,polymorphism,andinheritance

SeeLanguage designbelow for more details.

Assembly language

A program written in assembly language consists of a series ofmnemonicprocessor instructions and meta-statements (known variously as declarative operations, directives, pseudo-instructions, pseudo-operations and pseudo-ops), comments and data. Assembly language instructions usually consist of anopcodemnemonic followed by anoperand,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 anassemblerintomachine languageinstructions that can be loaded into memory and executed.

For example, the instruction below tells anx86/IA-32processor to move animmediate 8-bit valueinto aregister.Thebinary codefor this instruction is 10110 followed by a 3-bit identifier for which register to use. The identifier for theALregister is 000, so the followingmachine codeloads theALregister with the data 01100001.

10110000 01100001

This binary computer code can be made more human-readable by expressing it inhexadecimalas follows.

B0 61

Here,B0means "Move a copy of the following value intoAL",and61is a hexadecimal representation of the value 01100001, which is 97 indecimal.Assembly language for the 8086 family provides themnemonicMOV(an abbreviation ofmove) 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.

MOVAL,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. the61hin this example), depending on the operands that follow the mnemonic. For example, for the x86/IA-32 CPUs, the Intel assembly language syntaxMOV AL, AHrepresents an instruction that moves the contents of registerAHinto registerAL.Thehexadecimal 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 isAH,and the destination isAL.

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 operand61his a valid hexadecimal numeric constant and is not a valid register name, so only theB0instruction can be applicable. In the second example, the operandAHis a valid register name and not a valid numeric constant (hexadecimal, decimal, octal, or binary), so only the88instruction can be applicable.

Assembly languages are always designed so that this sort of lack of ambiguity 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 as0Ahor0AH,notAH,specifically so that it cannot appear to be the name of registerAH.(The same rule also prevents ambiguity with the names of registersBH,CH,andDH,as well as with any user-defined symbol that ends with the letterHand 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 theALregister, 10110001 (B1) moves it intoCLand 10110010 (B2) does so intoDL.Assembly language examples for these follow.

MOVAL,1h;Load AL with immediate value 1
MOVCL,2h;Load CL with immediate value 2
MOVDL,3h;Load DL with immediate value 3

The syntax of MOV can also be more complex as the following examples show.

MOVEAX,[EBX];Move the 4 bytes in memory at the address contained in EBX into EAX
MOV[ESI+EAX],CL;Move the contents of CL into the byte at address ESI+EAX
MOVDS,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 adisassembler.Unlikehigh-level languages,there is aone-to-one correspondencebetween many simple assembly statements and machine language instructions. However, in some cases, an assembler may providepseudoinstructions(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 richmacrolanguage (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.

Eachcomputer architecturehas 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 ofmnemonicsor 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. TheZilog Z80CPU, an enhancement of theIntel 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 mnemonicsMOV,MVI,LDA,STA,LXI,LDAX,STAX,LHLD,andSHLDfor various data transfer instructions, the Z80 assembly language uses the mnemonicLDfor all of them. A similar case is theNEC V20andV30CPUs, 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 asAMDandCyrixrepublished 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 andPig 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:

• Opcodemnemonics
• Data definitions
• Assembly directives

Opcode mnemonics and extended mnemonics

Instructions (statements) in assembly language are generally very simple, unlike those inhigh-level languages.Generally, a mnemonic is a symbolic name for a single executable machine language instruction (anopcode), and there is at least one opcode mnemonic defined for each machine language instruction. Each instruction typically consists of anoperationoropcodeplus zero or moreoperands.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 mnemonicsare often used to specify a combination of an opcode with a specific operand, e.g., the System/360 assemblers useBas an extended mnemonic forBCwith a mask of 15 andNOP( "NO OPeration" – do nothing for one step) forBCwith a mask of 0.

Extended mnemonicsare 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 instructionxchgax,axis used fornop,withnopbeing a pseudo-opcode to encode the instructionxchgax,ax.Some disassemblers recognize this and will decode thexchgax,axinstruction asnop.Similarly, IBM assemblers forSystem/360andSystem/370use the extended mnemonicsNOPandNOPRforBCandBCRwith zero masks. For the SPARC architecture, these are known assynthetic instructions.

Some assemblers also support simple built-in macro-instructions that generate two or more machine instructions. For instance, with some Z80 assemblers the instructionld hl,bcis recognized to generateld l,cfollowed byld h,b.These are sometimes known aspseudo-opcodes.

Mnemonics are arbitrary symbols; in 1985 theIEEEpublished 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 thealignmentof 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 termpseudo-opcodeis 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 (labelsorsymbols) with memory locations and various constants. Usually, every constant and variable is given a name so instructions can reference those locations by name, thus promotingself-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,GOTOdestinations are given labels. Some assemblers supportlocal symbolswhich are often lexically distinct from normal symbols (e.g., the use of "10$" as a GOTO destination).

Some assemblers, such asNASM,provide flexible symbol management, letting programmers manage differentnamespaces,automatically calculate offsets withindata 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 programsource codethat 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 supportpredefined macros,and others supportprogrammer-defined(and repeatedly re-definable) macros involving sequences of text lines in which variables and constants are embedded. The macro definition is most commonlya 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 IBMautocodersof the 1950s.

Macro assemblerstypically have directives to, e.g., define macros, define variables, set variables to the result of an arithmetic, logical or string expression, iterate, conditionally generate code. Some of those directives may be restricted to use within a macro definition, e.g.,MEXITinHLASM,while others may be permitted within open code (outside macro definitions), e.g.,AIFandCOPYin HLASM.

In assembly language, the term "macro" represents a more comprehensive concept than it does in some other contexts, such as thepre-processorin theC programming language,where its #define directive typically is used to create short single line macros. Assembler macro instructions, like macros inPL/Iand 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 takeparameters.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 "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 earlyvirtual machineinSNOBOL4(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 amacro 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 withIBM's Conversational Monitor System / Virtual Machine (VM/CMS) and with IBM's "real time transaction processing" add-ons, Customer Information Control SystemCICS,andACP/TPF,the airline/financial system that began in the 1970s and still runs many largecomputer 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 inCOBOLusing a pure macro assembler program containing lines of COBOL code inside assembly time operators instructing the assembler to generate arbitrary code. IBMOS/360uses macros to performsystem generation.The user specifies options by coding a series of assembler macros. Assembling these macros generates ajob streamto build the system, includingjob control languageandutilitycontrol 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 notTuring-completebecause 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 beingC,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 parametera-c,the macro expansion ofload a-c*boccurs. 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 providingstructured programmingelements to encode execution flow. The earliest example of this approach was in theConcept-14 macro set,originally proposed byHarlan 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 ofGOTOoperations in assembly code, one of the main factors causingspaghetti codein assembly language. This approach was widely accepted in the early 1980s (the latter days of large-scale assembly language use). IBM's High Level Assembler Toolkitincludes such a macro package.

A curious design wasA-Natural,a "stream-oriented" assembler for 8080/Z80,processorsfromWhitesmiths Ltd.(developers of theUnix-likeIdrisoperating system, and what was reported to be the first commercialCcompiler). The language was classified as an assembler because it worked with raw machine elements such asopcodes,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:
REPEAT20
switchrv(nrandom,9);generate a number between 0 and 8
movecx,7
case0
print"case 0"
caseecx;in contrast to most other programming languages,
print"case 7";the Masm32 switch allows "variable cases"
case1..3
.ifeax==1
print"case 1"
.elseifeax==2
print"case 2"
.else
print"cases 1 to 3: other"
.endif
case4,6,8
print"cases 4, 6 or 8"
default
movebx,19;print 20 stars
.Repeat
print"*"
decebx
.UntilSign?;loop until the sign flag is set
endsw
printchr$(13,10)
ENDM
exit
enddemomain

Use of assembly language

Historical perspective

Assembly languages were not available at the time when thestored-program computerwas introduced.Kathleen Booth"is credited with inventing assembly language"based on theoretical work she began in 1947, while working on theARC2atBirkbeck, University of Londonfollowing consultation byAndrew Booth(later her husband) with mathematicianJohn von Neumannand physicistHerman Goldstineat theInstitute for Advanced Study.

In late 1948, theElectronic Delay Storage Automatic Calculator(EDSAC) had an assembler (named "initial orders" ) integrated into itsbootstrapprogram. It used one-letter mnemonics developed byDavid 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 theIBM 650computer written by Stan Poley in 1955.

Assembly languages eliminate much of the error-prone, tedious, and time-consumingfirst-generationprogramming 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. However, by the late 1950s,their use had largely been supplanted by higher-level languages, in the search for improvedprogramming productivity.Today, assembly language is still used for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues.Typical uses aredevice drivers,low-levelembedded systems,andreal-timesystems (see§ Current usage).

Numerous programs have been written entirely in assembly language. TheBurroughs MCP(1961) was the first computer for which an operating system was not developed entirely in assembly language; it was written inExecutive Systems Problem Oriented Language(ESPOL), an Algol dialect. Many commercial applications were written in assembly language as well, including a large amount of theIBM mainframesoftware written by large corporations.COBOL,FORTRANand some PL/I eventually displaced much of this work, although a number of large organizations retained assembly-language application infrastructures well into the 1990s.

Assembly language has long been the primary development language for 8-bit home computers such as theApple II,Atari 8-bit computers,MSX,ZX Spectrum,andCommodore 64.InterpretedBASICdialects on these systems offer insufficient execution speed and insufficient facilities to take full advantage of the available hardware. These systems have severe resource constraints, idiosyncratic memory and display architectures, and provide limited system services. There are also few high-level language compilers suitable for microcomputer use. Similarly, assembly language is the default choice for 8-bit consoles such as theAtari 2600andNintendo Entertainment System.

Key software forIBM PC compatibleswas written in assembly language, such asMS-DOS,Turbo Pascal,and theLotus 1-2-3spreadsheet. As computer speed grew exponentially, assembly language became a tool for speeding up parts of programs, such as the rendering ofDoom,rather than a dominant development language. In the 1990s, assembly language was used to get performance out of systems such as theSega Saturnand as the primary language for arcade hardware based on theTMS34010integrated CPU/GPU such asMortal KombatandNBA Jam.

Current usage

There has been debate 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.

As of July 2017,theTIOBE indexof programming language popularity ranks assembly language at 11, ahead ofVisual Basic,for example.Assembler can be used to optimize for speed or optimize for size. In the case of speed optimization, modernoptimizing compilersare claimedto 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/Ooperations andpaging.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 witholder processorsthat have limited high-level language options such as theAtari 2600,Commodore 64,andgraphing calculators.Programs for these computers of the 1970s and 1980s are often written in the context ofdemosceneorretrogamingsubcultures.
• Code that must interact directly with the hardware, for example indevice driversandinterrupt handlers.
• In an embedded processor orDSP,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 thebitwise rotationinstruction 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 therun-timecomponents orlibrariesassociated 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 algebrawithBLASordiscrete cosine transformation(e.g.SIMDassembly version fromx264).
• Programs that create vectorized functions for programs in higher-level languages such as C. In the higher-level language this is sometimes aided by compilerintrinsic functionswhich map directly to SIMD mnemonics, but nevertheless result in a one-to-one assembly conversion specific for the given vector processor.
• Real-timeprograms such as simulations, flight navigation systems, and medical equipment. For example, in afly-by-wiresystem, 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, automaticgarbage collection,paging operations, orpreemptive multitasking.However, some higher-level languages incorporate run-time components and operating system interfaces that can introduce such delays. Choosing assembly orlower-level languagesfor such systems gives programmers greater visibility and control over processing details.
• Cryptographic algorithms that must always take strictly the same time to execute, preventingtiming attacks.
• Video encoders and decoders such as rav1e (an encoder forAV1)and dav1d (the reference decoder for AV1)contain assembly to leverageAVX2andARM Neoninstructions when available.
• Modify and extend legacy code written for IBM mainframe computers.
• Situations where complete control over the environment is required, in extremely high-security situations wherenothing can be taken for granted.
• Computer viruses,bootloaders,certaindevice drivers,or other items very close to the hardware or low-level operating system.
• Instruction set simulatorsfor monitoring, tracing anddebuggingwhere additional overhead is kept to a minimum.
• Situations where no high-level language exists, on a new or specialized processor for which nocross compileris available.
• Reverse engineeringand modifying program files such as:
  • existingbinariesthat 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 termedROM 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 mostcomputer scienceandelectronic engineeringprograms. Although few programmers today regularly work with assembly language as a tool, the underlying concepts remain important. Such fundamental topics asbinary arithmetic,memory allocation,stack processing,character setencoding,interruptprocessing, andcompilerdesign 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'sbootcode, the low-level code that initializes and tests the system hardware prior to booting the operating system and is often stored inROM.(BIOSonIBM-compatible PCsystems andCP/Mis an example.)
• Assembly language is often used for low-level code, for instance foroperating 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 fordebuggingand optimization purposes.
• Some compilers for relatively low-level languages, such asPascalorC,allow the programmer to embed assembly language directly in the source code (so calledinline assembly). Programs using such facilities can then construct abstractions using different assembly language on each hardware platform. The system'sportable codecan then use these processor-specific components through a uniform interface.
• Assembly language is useful inreverse engineering.Many programs are distributed only in machine code form which is straightforward to translate into assembly language by adisassembler,but more difficult to translate into a higher-level language through adecompiler.Tools such as theInteractive Disassemblermake 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

Further reading

• Bartlett, Jonathan (2004).Programming from the Ground Up - An introduction to programming using linux assembly language.Bartlett Publishing.ISBN0-9752838-4-7.Archivedfrom the original on 2020-03-24.Retrieved2020-03-24.[4]
• Britton, Robert (2003).MIPS Assembly Language Programming.Prentice Hall.ISBN0-13-142044-5.
• Calingaert, Peter (1979) [1978-11-05]. Written atUniversity of North Carolina at Chapel Hill.Horowitz, Ellis(ed.).Assemblers, Compilers, and Program Translation.Computer software engineering series (1st printing, 1st ed.). Potomac, Maryland, US:Computer Science Press, Inc.ISBN0-914894-23-4.ISSN0888-2088.LCCN78-21905.Retrieved2020-03-20.(2+xiv+270+6 pages)
• Duntemann, Jeff (2000).Assembly Language Step-by-Step.Wiley.ISBN0-471-37523-3.
• Hyde, Randall (2010).The Art of Assembly Language(2nd ed.). No Starch Press.ISBN978-1593272074.
• Jorgensen, Ed."x86-64 Assembly Language Programming with Ubuntu"(PDF).
• Kann, Charles W. (2015)."Introduction to MIPS Assembly Language Programming".Archivedfrom the original on 2020-03-24.Retrieved2020-03-24.
• Kann, Charles W. (2021)."Introduction to Assembly Language Programming: From Soup to Nuts: ARM Edition".Open Educational Resources.
• Norton, Peter;Socha, John(1986).Peter Norton's Assembly Language Book for the IBM PC.New York, US: Brady Books.
• Singer, Michael (1980).PDP-11. Assembler Language Programming and Machine Organization.New York, US:John Wiley & Sons.
• Sweetman, Dominic (1999).See MIPS Run.Morgan Kaufmann Publishers.ISBN1-55860-410-3.
• Waldron, John (1998).Introduction to RISC Assembly Language Programming.Addison Wesley.ISBN0-201-39828-1.
• "ASM Community Book".2009. Archived fromthe originalon 2013-05-30.Retrieved2013-05-30.( "An online book full of helpful ASM info, tutorials and code examples" by the ASM Community, archived at the internet archive.)

External links

• Assembly languageatCurlie
• Unix Assembly Language ProgrammingArchived2020-02-17 at theWayback Machine
• Linux Assembly
• PPR: Learning Assembly Language
• NASM – The Netwide AssemblerBSD-licensed x86 assembler
• Assembly Language Programming Examples
• Authoring Windows Applications In Assembly Language
• Assembly Optimization Tipsby Mark Larson