See the terminology section below for information regarding inconsistent use of the terms assembly and assembler.

An assembly language is a low-level language for programming computers. In Computer science, a low-level programming language is a language that provides little or no abstraction from a computer's microprocessor. It implements a symbolic representation of the numeric machine codes and other constants needed to program a particular CPU architecture. Machine code or machine language is a system of instructions and data executed directly by a Computer 's Central processing unit. This representation is usually defined by the hardware manufacturer, and is based on abbreviations (called mnemonics) that help the programmer remember individual instructions, registers, etc. A mnemonic device (nəˈmɒnɪk is a Memory aid Commonly met mnemonics are often verbal something such as a very short poem or a special word used to help a person remember In Computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage An assembly language is thus specific to a certain physical or virtual computer architecture (as opposed to most high-level languages, which are portable). In computing a high-level programming language is a Programming language with strong abstraction from the details of the computer See also Software portability In Computer science, porting is the process of adapting software so that an executable program can be created

Assembly languages were first developed in the 1950s, when they were referred to as second generation programming languages. Second-generation programming language is a generational way to categorize Assembly languages The term was coined to provide a distinction from higher level Third-generation They eliminated much of the error-prone and time-consuming first-generation programming needed with the earliest computers, freeing the programmer from tedium such as remembering numeric codes and calculating addresses. A first-generation Programming language is a machine-level programming language They were once widely used for all sorts of programming. However, by the 1980s (1990s on small computers), their use had largely been supplanted by high-level languages, in the search for improved programming productivity. In computing a high-level programming language is a Programming language with strong abstraction from the details of the computer Programming productivity refers to a variety of software development issues and methodologies affecting the quantity and quality of code produced by an individual or team Today, assembly language is used primarily for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues. Typical uses are device drivers, low-level embedded systems, and real-time systems. In computing a device driver or software driver is a Computer program allowing higher-level computer programs to interact with a Hardware device An embedded system is a special-purpose Computer system designed to perform one or a few dedicated functions often with Real-time computing constraints In Computer science, real-time computing (RTC is the study of hardware and software systems that are subject to a "real-time constraint"—i

A utility program called an assembler is used to translate assembly language statements into the target computer's machine code. Utility software (also known as service program, service routine, tool, or utility routine) is a type of Computer software. The assembler performs a more or less isomorphic translation (a one-to-one mapping) from mnemonic statements into machine instructions and data. In Abstract algebra, an isomorphism ( Greek: ἴσος isos "equal" and μορφή morphe "shape" is a bijective A mnemonic device (nəˈmɒnɪk is a Memory aid Commonly met mnemonics are often verbal something such as a very short poem or a special word used to help a person remember (This is in contrast with high-level languages, in which a single statement generally results in many machine instructions. In computing a high-level programming language is a Programming language with strong abstraction from the details of the computer A compiler, analogous to an assembler, is used to translate high-level language statements into machine code; or an interpreter executes statements directly. A compiler is a Computer program (or set of programs that translates text written in a computer language (the source language) into another In Computer science, an interpreter normally means a Computer program that executes, i )

Many sophisticated assemblers offer additional mechanisms to facilitate program development, control the assembly process, and aid debugging. In particular, most modern assemblers include a macro facility (described below), and are called macro assemblers. A macro (from the Greek 'μάκρο' for long or far in Computer science is a rule or Pattern that specifies how a certain input sequence (often a sequence

Contents 1 Key concepts 1.1 Assembler 1.2 Assembly language 2 Language design 2.1 Basic elements 2.2 Syntax 2.3 Macros 2.4 Support for structured programming 3 Use of assembly language 3.1 Historical perspective 3.2 Current usage 3.3 Typical applications 4 Related terminology 5 Further details 6 Example listing of assembly language source code 7 See also 8 References 9 Books 10 External links 10.1 References 10.2 Software

Key concepts

Assembler

Compare with: Microassembler. A microassembler (sometimes called a meta-assembler is a Computer program that helps prepare a microprogram to control the low level operation of a computer in much

Typically a modern assembler creates object code by translating assembly instruction mnemonics into opcodes, and by resolving symbolic names for memory locations and other entities. In Computer science, object code, or an object file, is the representation of code that a Compiler or Assembler generates by processing In computer technology an opcode ( op eration code) is the portion of a Machine language instruction that specifies the operation to be performed In Computer science, Identifiers ( IDs) are lexical tokens that name entities. ^[1] 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 facilities for performing textual substitution—e. A macro (from the Greek 'μάκρο' for long or far in Computer science is a rule or Pattern that specifies how a certain input sequence (often a sequence g. , to generate common short sequences of instructions to run inline, instead of in a subroutine. In Computing, inline expansion, or inlining, is a Compiler optimization that replaces a function Call site with the body of the Callee In Computer science, a subroutine ( function, method, procedure, or subprogram) is a portion of code within a larger

Assemblers are generally simpler to write than compilers for high-level languages, and have been available since the 1950s. A compiler is a Computer program (or set of programs that translates text written in a computer language (the source language) into another In computing a high-level programming language is a Programming language with strong abstraction from the details of the computer The 1950s Decade refers to the years of 1950 to 1959 inclusive Modern assemblers, especially for RISC based architectures, such as MIPS, Sun SPARC and HP PA-RISC, optimize instruction scheduling to exploit the CPU pipeline efficiently. MIPS (originally an acronym for Microprocessor without Interlocked Pipeline Stages) is a RISC microprocessor architecture developed by MIPS Technologies SPARC (from Scalable Processor Architecture is a RISC Microprocessor Instruction set architecture originally PA-RISC is a Microprocessor architecture developed by Hewlett-Packard 's Systems & VLSI Technology Operation. In Computer science, instruction scheduling is a Compiler optimization used to improve instruction-level parallelism which improves performance on machines with In Computing, a pipeline is a set of data processing elements connected in series so that the output of one element is the input of the next one

More sophisticated high-level assemblers provide language abstractions such as:

• Advanced control structures
• High-level procedure/function declarations and invocations
• High-level abstract data types, including structures/records, unions, classes, and sets
• Sophisticated macro processing
• Object-Oriented features such as encapsulation, polymorphism, inheritance, interfaces

See Language design below for more details. High-level assemblers are Assembly language translators that incorporate features found in modern High-level programming languages into an assembler

Note that, in normal professional usage, the term assembler is often used ambiguously: It is frequently used to refer to an assembly language itself, rather than to the assembler utility. Thus: "CP/CMS was written in S/360 assembler" as opposed to "ASM-H was a widely-used S/370 assembler. The IBM System/360 ( S/360) is a Mainframe computer system family announced by IBM on April 7, 1964. The IBM System/370 (often S/370) was a model range of IBM mainframes announced on June 30, 1970 "

Assembly language

A program written in assembly language consists of a series of instructions--mnemonics that correspond to a stream of executable instructions, when translated by an assembler, that can be loaded into memory and executed. See the terminology section below for information regarding inconsistent use of the terms assembly and assembler

For example, an x86/IA-32 processor can execute the following binary instruction as expressed in machine language:

• Binary: 10110000 01100001 (Hexadecimal: B0 61)

The equivalent assembly language representation is easier to remember (example in Intel syntax, more mnemonic):

• mov al, 61h

This instruction means:

• Move the value 61h (or 97 decimal; the h-suffix means hexadecimal) into the processor register named "al". See also X86 assembly language The generic term x86 refers to the most commercially successful Instruction set architecture in the history of Personal IA-32 ( Intel Architecture 32-bit) often generically called X86 or x86-32, is the Instruction set architecture of Intel Machine code or machine language is a system of instructions and data executed directly by a Computer 's Central processing unit. A mnemonic device (nəˈmɒnɪk is a Memory aid Commonly met mnemonics are often verbal something such as a very short poem or a special word used to help a person remember The decimal ( base ten or occasionally denary) Numeral system has ten as its base. In Mathematics and Computer science, hexadecimal (also base -, hexa, or hex) is a Numeral system with a In Computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage

The mnemonic "mov" represents the opcode 1011 which moves the value in the second operand into the register indicated by the first operand. The mnemonic was chosen by the instruction set designer to abbreviate "move", making it easier for the programmer to remember. A comma-separated list of arguments or parameters follows the opcode; this is a typical assembly language statement.

In practice many programmers drop the word mnemonic and, technically incorrectly, call "mov" an opcode. When they do this they are referring to the underlying binary code which it represents. To put it another way, a mnemonic such as "mov" is not an opcode, but as it symbolizes an opcode, one might refer to "the opcode mov" for example when one intends to refer to the binary opcode it symbolizes rather than to the symbol--the mnemonic--itself. As few modern programmers have need to be mindful of actually what binary patterns are the opcodes for specific instructions, the distinction has in practice become a bit blurred among programmers but not among processor designers.

Transforming assembly into machine language is accomplished by an assembler, and the reverse by a disassembler. See the terminology section below for information regarding inconsistent use of the terms assembly and assembler A disassembler is a Computer program that translates Machine language into Assembly language —the inverse operation to that of an assembler. Unlike in high-level languages, there is usually a one-to-one correspondence between simple assembly statements and machine language instructions. In computing a high-level programming language is a Programming language with strong abstraction from the details of the computer In Mathematics, a bijection, or a bijective function is a function f from a set X to a set Y with the property However, in some cases, an assembler may provide pseudoinstructions 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 language (discussed below) which is used by vendors and programmers to generate more complex code and data sequences. A macro (from the Greek 'μάκρο' for long or far in Computer science is a rule or Pattern that specifies how a certain input sequence (often a sequence

Every computer architecture has its own machine language. In Computer engineering, computer architecture is the conceptual design and fundamental operational structure of a Computer system On this level, each instruction is simple enough to be executed using a relatively small number of electronic circuits. Computers differ by the number and type of operations they support. For example, a new 64-bit machine would have different circuitry from a 32-bit machine. They may also have different sizes and numbers of registers, and different representations of data types 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 mnemonics or assembly-language syntax may exist for a single instruction set, typically instantiated in different assembler programs. A mnemonic device (nəˈmɒnɪk is a Memory aid Commonly met mnemonics are often verbal something such as a very short poem or a special word used to help a person remember In these cases, the most popular one is usually that supplied by the manufacturer and used in its documentation.

Language design

Basic elements

Instructions (statements) in assembly language are generally very simple, unlike those in high-level languages. In computing a high-level programming language is a Programming language with strong abstraction from the details of the computer Each instruction typically consists of an operation or opcode plus zero or more operands. In Mathematics, an operand is one of the inputs (arguments of an Operator. Most instructions refer to a single value, or a pair of values. Generally, an opcode is a symbolic name for a single executable machine language instruction. Operands can be either immediate (typically one byte values, coded in the instruction itself) 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.

Most assemblers also support pseudo-operations, which are directives obeyed by the assembler at assembly time instead of the CPU at run time. (For example, pseudo-ops would be used to reserve storage areas and optionally set their initial contents. ) The names of pseudo-ops often start with a dot to distinguish them from machine instructions.

Some assemblers also support pseudo-instructions, which generate two or more machine instructions.

Symbolic assemblers allow programmers to associate arbitrary names (labels or symbols) with memory locations. Usually, every constant and variable is given a name so instructions can reference those locations by name, thus promoting self-documenting code. In Computer programming, self-documenting (or self-describing) is a common descriptor for Source code that follows certain loosely-defined conventions for 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. GOTO is a statement found in many computer Programming languages It is a combination of the English words go and to Some assemblers support local symbols which are lexically distinct from normal symbols (e. g. , the use of "10$" as a GOTO destination).

Most assemblers provide flexible symbol management, allowing programmers to 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. A namespace is an abstract container or environment created to hold a logical grouping of unique identifiers (i A data structure in Computer science is a way of storing Data in a computer so that it can be used efficiently 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 assembly source code that are ignored by the assembler. In Computer science, source code (commonly just source or code) is any sequence of statements or declarations written in some Human-readable Good use of comments is even more important with assembly code than with higher-level languages, as the meaning of a sequence of instructions is harder to decipher from the code itself.

Wise use of these facilities can greatly simplify the problems of coding and maintaining low-level code. Raw assembly source code as generated by compilers or disassemblers — code without any comments, meaningful symbols, or data definitions — is quite difficult to read when changes must be made.

Syntax

Beside there are many dialects and different assemblers available, there are two main language branches, especially in x86 assembly. See also X86 assembly language The generic term x86 refers to the most commercially successful Instruction set architecture in the history of Personal The so called AT&T syntax and the so called Intel syntax, original used for the documentation of x86 CPU platform (therefore Intel syntax). Before proposing a merge request please see Talk and see if the merger you propose has recently been made and The Intel syntax is dominant in the Windows world, in the Unix/Linux world both are in use (because of the AT&T syntax only support of GCC in former times). Here is a summarized list of the main differences between Intel and AT&T syntax:

• in AT&T the source comes before the destination, opposite to the Intel syntax
• in AT&T the opcodes are suffixed with a letter indicating the size of the operands (e. g. `l' for dword, `w' for word, `b' for byte).
• in AT&T immediate values must be prefixed with a `$', and registers must be prefixed with a `%'.
• in AT&T effective addresses use the general syntax DISP(BASE,INDEX,SCALE). In the Intel syntax effective addresses it is variable and has to be in square brackets. Additionally, size keywords like 'byte', 'word' or 'dword' have to be used. A concrete example would be:
  • AT&T: movl mem_location(%ebx,%ecx,4), %eax
  • equivalent to the following in intel syntax: mov eax, dword [eax + ecx*4 + mem_location]

Assemblers which support Intel syntax include MASM, TASM, NASM, FASM and YASM. The Microsoft Macro Assembler (abbreviated MASM) is an x86 High-level assembler for DOS and Microsoft Windows. The Turbo Assembler ( TASM) mainly PC -targeted assembler package was Borland 's offering in the X86 assembler programming tool The Netwide Assembler ( NASM) is an assembler and Disassembler for the Intel x86 architecture FASM ( Flat Assembler) is a free and Open source Intel-style assembler supporting the IA-32 and X86-64 architectures Yasm is a software program that attempts to be a complete rewrite of the NASM assembler. The GNU Assembler (part of GCC) supports both syntaxes now since version 2. The GNU Assembler, commonly known as Gas, is the assembler used by the GNU Project. 1 via the intel_syntax directive. ^[2]

Macros

Many assemblers support macros, programmer-defined symbols that stand for some sequence of text lines. This sequence of text lines may include a sequence of instructions, or a sequence of data storage pseudo-ops. Once a macro has been defined using the appropriate pseudo-op, its name may be used in place of an mnemonic. When the assembler processes such a statement, it replaces the statement with the text lines associated with that macro, then processes them just as though they had appeared in the source code file all along (including, in better assemblers, expansion of any macros appearing in the replacement text).

Since macros can have short names but expand to several lines of code, they can be used to make assembly language programs much shorter. They can also be used to add higher levels of structure to assembly programs.

Many assemblers have built-in macros for system calls and other special code sequences.

Macro assemblers often allow macros to take parameters. In Computer programming, a parameter is a variable which takes on the meaning of a corresponding Argument (computer science is same article--> argument 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 macros, and allowing macros to save context or exchange information. Thus a macro might emit a large number of 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. An organization using assembly language that has been heavily extended using such a macro suite can be considered to be working in a (slightly) higher-level language such programmers are not working with a computer's lowest-level conceptual elements.

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 (CMS/VM) and with its "real time transaction processing" add-on, Customer Information Control System, CICS.

It was also possible to use solely the macro processing capabilities of an assembler to generate code written in completely different languages, for example, to generate a version of a program in Cobol using a pure macro assembler program containing lines of Cobol code inside assembly time operators instructing the assembler to generate arbitrary code.

This was because, as was realized in the 1970s, 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 in fact appeared in and appears in the C programming language, which supports "preprocessor instructions" to set variables, and make conditional tests on their values. Note that unlike certain previous macro processors inside assemblers, the C preprocessor was not Turing-complete because it lacked the ability to either loop or "go to", the latter allowing the programmer to loop. In computability theory, several closely-related terms are used to describe the "computational power" of a computational system (such as an Abstract machine or

Despite the power of macro processing, it fell into disuse in high level languages while remaining a perennial for assemblers.

This was because many programmers were rather confused by macro parameter substitution and did not disambiguate macro processing from assembly and execution.

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, an unexpected macro expansion occurs.

To avoid this, users of macro processors learned to religiously parenthesize formal parameters inside macro definitions, and callers had to do the same to their "actual" parameters.

PL/I and C feature macros, but this facility was underused or dangerous when used because they can only manipulate text. On the other hand, homoiconic languages, such as Lisp and Prolog, retain the power of assembly language macros because they are able to manipulate their own code as data. Lisp (or LISP) is a family of Computer Programming languages with a long history and a distinctive fully parenthesized syntax Prolog is a Logic programming language It is a general purpose language often associated with Artificial intelligence and Computational linguistics

Support for structured programming

Some assemblers have incorporated structured programming elements to encode execution flow. Structured programming can be seen as a subset or subdiscipline of Procedural programming, one of the major Programming paradigms It is most famous for removing or The earliest example of this approach was in the Concept-14 macro set developed by Marvin Zloof at IBM's Thomas Watson Research Center, which extended the S/370 macro assembler with IF/ELSE/ENDIF and similar control flow blocks. Thomas J Watson Research Center is the headquarters for the IBM Research Division 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. GOTO is a statement found in many computer Programming languages It is a combination of the English words go and to Spaghetti code is a Pejorative term for Source code which has a complex and tangled Control structure, especially one using many GOTOs exceptions This approach was widely accepted in the early 80s (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 Ltd. (developers of the Unix-like Idris Operating System, and what was reported to be the first commercial C compiler). The Zilog Z80 is an 8-bit Microprocessor designed and sold by Zilog from July 1976 onwards Whitesmiths Ltd was a Software company based in Westford Massachusetts. Unix (officially trademarked as UNIX, sometimes also written as Unix with Small caps) is a computer Idris is a multi-tasking Unix-like, multi-user Real-time operating system released by Whitesmiths, of Westford Massachusetts. tags please moot on the talk page first! --> In Computing, C is a general-purpose cross-platform block structured A compiler is a Computer program (or set of programs that translates text written in a computer language (the source language) into another The language was classified as an assembler, because it worked with raw machine elements such as opcodes, 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. ^[3]

Use of assembly language

Historical perspective

Historically, a large number of programs have been written entirely in assembly language. Operating systems were almost exclusively written in assembly language until the widespread acceptance of C in the 1970s and early 1980s. tags please moot on the talk page first! --> In Computing, C is a general-purpose cross-platform block structured Many commercial applications were written in assembly language as well, including a large amount of the IBM mainframe software written by large corporations. COBOL and FORTRAN eventually displaced much of this work, although a number of large organizations retained assembly-language application infrastructures well into the 90s. COBOL (ˈkoʊbɒl is one of the oldest programming languages still in active use Fortran (previously FORTRAN) is a general-purpose, procedural, imperative Programming language that is especially suited to

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 size (and hence speed), and reliability: the writers of Cardbox-Plus said simply "we use assembler because then all the bugs are ours". This held true for 8-bit versions of the program, which had no bugs at all, but ironically it turned out to be false with 16 bits: Cardbox-Plus 2. 0 had to be upgraded to Cardbox-Plus 2. 1 because a bug in Microsoft's macro assembler caused Cardbox-Plus to index the number "-0" differently from the number "0".

Typical examples of large assembly language programs from this time are the MS-DOS operating system, the early IBM PC spreadsheet program Lotus 1-2-3, and almost all popular games for the Atari 800 family of home computers. MS-DOS (short for M icro' s' oft D isk O perating S ystem is an Operating system commercialized by Microsoft. A spreadsheet is a Computer application that simulates a paper worksheet Lotus 1-2-3 is a Spreadsheet program from Lotus Software (now part of IBM) The Atari 8-bit family is a series of 8-bit Home computers manufactured from 1979 to 1992 Even into the 1990s, most console video games were written in assembly, including most games for the Mega Drive/Genesis and the Super Nintendo Entertainment System. The is a 16-bit Video game console released by Sega in Japan in 1988 North America in 1989 and the PAL region in 1990 The Super Nintendo Entertainment System or Super NES (also called SNES and Super Nintendo) is a 16-bit Video game console that was The popular arcade game NBA Jam (1993) is another example. NBA Jam is a Basketball Arcade game developed by Midway in 1993. On the Commodore 64, Amiga, Atari ST, as well as ZX Spectrum home computers, assembler has long been the primary development language. This was in large part due to the fact that 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, most notably Amiga, even have IDEs with highly advanced debugging and macro facilities, such as the freeware ASM-One assembler, comparable to that of Microsoft Visual Studio facilities (ASM-One predates Microsoft Visual Studio).

The smallest symbolic assembler ever written was The Assembler for the unexpanded VIC-20, written by Don French and published by French Silk. The VIC-20 ( Germany: VC-20; Japan: VIC-1001) is an 8-bit Home computer which was sold by Commodore Business Machines It was only 1639 bytes in length but was packed with features for its size. The Assembler supported the usual symbolic addressing and the definition of character strings and hex strings, but it also allowed address expressions of any number of terms combined with +-*/, logical AND, logical OR, and exponentiation operators.

Current usage

There have always been debates over the usefulness and performance of assembly language relative to high-level languages, though this gets less attention today. Assembly language has specific niche uses where it is important; see below. But in general, modern optimizing compilers are claimed to render high-level languages into code that can run as fast as hand-written assembly, despite some counter-examples that can be created. The complexity of modern processors makes effective hand-optimization increasingly difficult. Moreover, and to the dismay of efficiency lovers, increasing processor performance has meant that most CPUs sit idle most of the time, with delays caused by predictable bottlenecks such as I/O operations and paging. In Computer Operating systems that have their Main memory divided into pages, paging (sometimes called swapping) is a transfer This has made raw code execution speed a non-issue for most programmers.

There are really only a handful of situations where today's expert practitioners would choose assembly language:

• When a stand-alone binary executable is required, i. e. one that must execute without recourse to the run-time components or libraries associated with a high-level language; this is perhaps the most common situation. These are embedded programs that store only a small amount of memory and the device is intended to do single purpose tasks. Such examples consist of telephones, automobile fuel and ignition systems, air-conditioning control systems, security systems, and sensors.
• When interacting directly with the hardware.
• When using processor-specific instructions not exploited by or available to the compiler. A common example is the bitwise rotation instruction at the core of many encryption algorithms. In combinatorial Mathematics, a circular shift is a Permutation of the entries in a Tuple where the last element becomes the first element
• Embedded systems.
• When extreme optimization is required, e. g. , in an inner loop in a processor-intensive algorithm. Some game programmers are experts at writing code that takes advantage of the capabilities of hardware features in systems enabling the games to run faster.
• When a system with severe resource constraints (e. g. , an embedded system) must be hand-coded to maximize the use of limited resources; but this is becoming less common as processor price/performance improves
• When no high-level language exists, e. An embedded system is a special-purpose Computer system designed to perform one or a few dedicated functions often with Real-time computing constraints g. , on a new or specialized processor
• Real-time programs that need precise timing and responses, such as simulations, flight navigation systems, and medical equipment. In Computer science, real-time computing (RTC is the study of hardware and software systems that are subject to a "real-time constraint"—i (For example, in a fly-by-wire system, telemetry must be interpreted and acted upon within strict time constraints. Aircraft flight control systems consist of Flight control surfaces, the respective cockpit controls connecting linkages and the necessary operating mechanisms to control Such systems must eliminate sources of unpredictable delays – such as may be created by interpreted languages, automatic garbage collection, paging operations, or preemptive multitasking. In Computer science, garbage collection ( GC) is a form of automatic Memory management. Pre-emption or preemption in Computing is the act of temporarily interrupting a task being carried out by a computer system, without requiring 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 the programmer greater visibility and control over processing details. )
• When complete control over the environment is required (for example in extremely high security situations, where nothing can be taken for granted). A backdoor in a Computer system (or Cryptosystem or Algorithm) is a method of bypassing normal Authentication, securing remote access to a computer
• When writing computer viruses, bootloaders, certain device drivers, or other items very close to the hardware or low-level operating system.
• When reverse-engineering existing binaries, which may or may not have originally been written in a high-level language.
• Reverse engineering and modification of video games (known as ROM Hacking), commonly done to games for Nintendo hardware such as the SNES and NES, is possible with a range of techniques, of which the most widely employed is altering the program code at the assembly language level. Reverse engineering (RE is the process of discovering the technological principles of a device object or system through analysis of its structure function and operation A video game is a Game that involves interaction with a User interface to generate visual feedback on a video device. ROM hacking is is a Multinational corporation headquartered in Kyoto Japan founded on The Super Nintendo Entertainment System or Super NES (also called SNES and Super Nintendo) is a 16-bit Video game console that was The Nintendo Entertainment System (often abbreviated as NES or simply Nintendo) is an 8-bit Video game console that was released by
• Assembly language is still used for writing games and other software for graphics calculators. For example, see http://tifreakware.net/tutorials/89/a/calc/fargoii.htm
• Finally, compiler writers usually write software that generates assembly code, and should therefore be expert assembly language programmers themselves

Nevertheless, 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 very important. Such fundamental topics as binary arithmetic, memory allocation, 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. The binary numeral system, or base-2 number system, is a Numeral system that represents numeric values using two symbols usually 0 and 1. In Computer science, dynamic memory allocation is the allocation of memory storage for use in a Computer program during the Runtime of that program In Computer science, a stack is an Abstract data type and Data structure based on the principle of Last In First Out (LIFO A character encoding consists of a code that pairs a sequence of characters from a given character set (sometimes incorrectly referred to as Code page In Computing, an interrupt is an asynchronous signal from hardware indicating the need for attention or a synchronous event in software indicating the need for a change A compiler is a Computer program (or set of programs that translates text written in a computer language (the source language) into another 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. ^[4]

Typical applications

Hard-coded assembly language is typically used in a system's boot ROM (BIOS on IBM-compatible PC systems). In Computing, booting ( booting up) is a bootstrapping process that starts Operating systems when the user turns on a Computer system In Computing, the BIOS (ˈbaɪoʊs A personal computer ( PC) is any Computer whose original sales price size and capabilities make it useful for individuals and which is intended to be operated This low-level code is used, among other things, to initialize and test the system hardware prior to booting the OS, and is stored in ROM. Once a certain level of hardware initialization has taken place, execution transfers to other code, typically written in higher level languages; but the code running immediately after power is applied is usually written in assembly language. The same is true of most boot loaders. In Computing, booting ( booting up) is a bootstrapping process that starts Operating systems when the user turns on a Computer system

Many compilers render high-level languages into assembly first before fully compiling, allowing the assembly code to be viewed for debugging and optimization purposes. Relatively low-level languages, such as C, often provide special syntax to embed assembly language directly in the source code. tags please moot on the talk page first! --> In Computing, C is a general-purpose cross-platform block structured Programs using such facilities, such as the Linux kernel, can then construct abstractions utilizing different assembly language on each hardware platform. Linux is an operating system kernel used by a family of Unix-like Operating systems These are popularly termed Linux operating systems and The system's portable code can then utilize these processor-specific components through a uniform interface.

Assembly language is also valuable in reverse engineering, since many programs are distributed only in machine code form, and machine code is usually easy to translate into assembly language and carefully examine in this form, but very difficult to translate into a higher-level language. Reverse engineering (RE is the process of discovering the technological principles of a device object or system through analysis of its structure function and operation Tools such as the Interactive Disassembler make extensive use of disassembly for such a purpose. The Interactive Disassembler, more commonly known as simply IDA, is a commercial Disassembler widely used for Reverse engineering.

A particular niche that makes use of the assembly language is the demoscene. The demoscene is a Computer art Subculture that specializes in producing demos, which are non-interactive audio-visual presentations that run in Certain competitions require the contestants to restrict their creations to a very small size (e. g. , 1, 4 or 64 KiB), and assembly language is the language of choice to achieve this goal. When resources, particularly CPU-processing constrained systems, like the Amiga and the Commodore 64, are a concern, assembler coding is a must: optimized assembler code is written "by hand" and instructions are sequenced manually by the coder or coders in an attempt to reduce the number of CPU cycles to a minimum; the CPU constraints are so great that every CPU cycle counts. However, using such techniques have enabled systems like the Commodore 64 to produce real-time 3D graphics with advanced effects, a feat which would be considered unlikely or even impossible for a system with a 0. 99 MHz processor.

Related terminology

• Assembly language or assembler language is commonly called assembly, assembler, ASM, or symbolic machine code. A generation of IBM mainframe programmers called it BAL for Basic Assembly Language.

Note: Calling the language assembler is of course potentially confusing and ambiguous, since this is also the name of the utility program that translates assembly language statements into machine code. Some may regard this as imprecision or error. However, this usage has been common among professionals and in the literature for decades. ^[5] Similarly, some early computers called their assembler its assembly program. ^[6])

• The computational step where an assembler is run, including all macro processing, is known as assembly time.

• The use of the word assembly dates from the early years of computers (cf. short code, speedcode). Short Code was one of the first higher-level languages ever developed for an Electronic computer. Speedcoding or Speedcode was the first higher-level language created for an IBM computer.

• A cross assembler (see cross compiler) produces code for one type of processor, but runs on another. A cross compiler is a Compiler capable of creating Executable code for a platform other than the one on which the compiler is run This technology is particularly important when developing software for new processors, or when developing for embedded systems. This allows, for instance, a 32-bit x86 processor to assemble code to run on a 64-bit x64 processor.

• An assembler directive is a command given to an assembler. These directives may do anything from telling the assembler to include other source files, to telling it to allocate memory for constant data.

Further details

For any given personal computer, mainframe, embedded system, and game console, both past and present, at least one--possibly dozens--of assemblers have been written. For some examples, see the list of assemblers. See also Assembly language This is a list of assemblers software programs that translate assembly language Source code into binary programs

On Unix systems, the assembler is traditionally called as, although it is not a single body of code, being typically written anew for each port. Unix (officially trademarked as UNIX, sometimes also written as Unix with Small caps) is a computer as is a generic name for an assembler on Unix. The GNU Project 's assembler is named Gas A number of Unix variants use GAS. The GNU Assembler, commonly known as Gas, is the assembler used by the GNU Project.

Within processor groups, each assembler has its own dialect. Sometimes, some assemblers can read another assembler's dialect, for example, TASM can read old MASM code, but not the reverse. The Turbo Assembler ( TASM) mainly PC -targeted assembler package was Borland 's offering in the X86 assembler programming tool The Microsoft Macro Assembler (abbreviated MASM) is an x86 High-level assembler for DOS and Microsoft Windows. FASM and NASM have similar syntax, but each support different macros that could make them difficult to translate to each other. FASM ( Flat Assembler) is a free and Open source Intel-style assembler supporting the IA-32 and X86-64 architectures The Netwide Assembler ( NASM) is an assembler and Disassembler for the Intel x86 architecture The basics are all the same, but the advanced features will differ. ^[7]

Also, assembly can sometimes be portable across different operating systems on the same type of CPU. Calling conventions between operating systems often differ slightly or not at all, and with care it is possible to gain some portability in assembly language, usually by linking with a C library that does not change between operating systems. In Computer science, a calling convention is a standardized method for a program to pass parameters to a function and receive a result value back from it tags please moot on the talk page first! --> In Computing, C is a general-purpose cross-platform block structured

For example, many things in libc depend on the preprocessor to do OS-specific, C-specific things to the program before compiling. The C standard library (also known as libc) is a now-standardized collection of Header files and library routines used to implement common operations such In fact, some functions and symbols are not even guaranteed to exist outside of the preprocessor. Worse, the size and field order of structs, as well as the size of certain typedefs such as off_t, are entirely unavailable in assembly language without help from a configure script, and differ even between versions of Linux, making it impossible to portably call functions in libc other than ones that only take simple integers and pointers as parameters. typedef is a keyword in the C and C++ Programming languages It is used to give a Data type a new name Configure scripts are an automated method of modifying Source code before compilation in order to produce code tailored to the system on which a binary Executable Linux (commonly pronounced ˈlɪnəks To address this issue, FASMLIB project provides portable assembly library for Win32 and Linux platforms, but it is yet very incomplete. FASMLIB is a portable general-purpose library for the X86 series of processors written in 32 bit Assembly language. ^[8]

Some higher level computer languages, such as C and Borland Pascal, support inline assembly where relatively brief sections of assembly code can be embedded into the high level language code. tags please moot on the talk page first! --> In Computing, C is a general-purpose cross-platform block structured Turbo Pascal is a complete software development system that includes a Compiler and an Integrated Development Environment (IDE for the Pascal programming language In Computer programming, the inline assembler is a feature of some Compilers that allows very low level code written in assembly to be embedded in a The Forth programming language commonly contains an assembler used in CODE words. Forth is a structured, imperative, stack-based, computer Programming language and programming environment

Many people use an emulator to debug assembly-language programs. An emulator duplicates (provides an emulation of the functions of one System using a different system so that the second system behaves like (and appears to

Example listing of assembly language source code

Address	Label	Instruction (AT&T syntax)	Object code[9]
		. begin
		. org 2048
	a_start	. equ 3000
2048		ld length,%
2064		be done	00000010 10000000 00000000 00000110
2068		addcc %r1,-4,%r1	10000010 10000000 01111111 11111100
2072		addcc %r1,%r2,%r4	10001000 10000000 01000000 00000010
2076		ld %r4,%r5	11001010 00000001 00000000 00000000
2080		ba loop	00010000 10111111 11111111 11111011
2084		addcc %r3,%r5,%r3	10000110 10000000 11000000 00000101
2088	done:	jmpl %r15+4,%r0	10000001 11000011 11100000 00000100
2092	length:	20	00000000 00000000 00000000 00010100
2096	address:	a_start	00000000 00000000 00001011 10111000
		. org a_start
3000	a:

Example of a selection of instructions (for a virtual computer^[10]) with the corresponding address in memory where each instruction will be placed. A Universal Virtual Computer (UVC is much like a Virtual machine (VM in computing by means that it creates a layer between the underlying Computer platform In Computer science, a memory address is an identifier for a memory location at which a Computer program or a hardware device can store a piece of data These addresses are not static, see memory management. Memory management is the act of managing Computer memory. In its simpler forms this involves providing ways to allocate portions of memory to programs at their request Accompanying each instruction is the generated (by the assembler) object code that coincides with the virtual computer's architecture (or ISA). In Computer science, object code, or an object file, is the representation of code that a Compiler or Assembler generates by processing An instruction set is a list of all the instructions and all their variations that a processor can execute

See also

• Little man computer - an educational computer model with a base-10 assembly language
• x86 assembly language - the assembly language for common Intel 80x86 microprocessors
• Compiler
• Disassembler
• List of assemblers
• Instruction set
• Microassembler

References

Books

• Dominic Sweetman: See MIPS Run. Morgan Kaufmann Publishers, 1999. ISBN 1-55860-410-3
• John Waldron: Introduction to RISC Assembly Language Programming. Addison Wesley, 1998. ISBN 0-201-39828-1
• Jeff Duntemann: Assembly Language Step-by-Step. Wiley, 2000. ISBN 0-471-37523-3
• Paul Carter: PC Assembly Language. Free ebook, 2001.
  Website
• Robert Britton: MIPS Assembly Language Programming. Prentice Hall, 2003. ISBN 0-13-142044-5
• Randall Hyde: The Art of Assembly Language. No Starch Press, 2003. ISBN 1-886411-97-2
  Draft versions available online as PDF and HTML
• Jonathan Bartlett: Programming from the Ground Up. Bartlett Publishing, 2004. ISBN 0-9752838-4-7
  Available online as PDF and as HTML
• ASM Community Wiki Book "A Wiki-based book full of helpful ASM info, tutorials and code examples" by the ASM Community

External links

References

• The ASM Community, a great ASM programming resource including a Messageboard and an ASM Wiki Book
• Intel Assembly 80x86 CodeTable (a cheat sheet reference)
• Unix Assembly Language Programming
• PPR: Learning Assembly Language
• Assembly Language Programming Examples
• Typed Assembly Language (TAL)
• Authoring Windows Applications In Assembly Language
• Information on Linux assembly programming
• Terse: Algebraic Assembly Language for x86
• Iczelion's Win32 Assembly Tutorial
• IBM z/Architecture Principles of Operation IBM manuals on mainframe machine language and internals. Mainframes (often colloquially referred to as Big Iron) are Computers used mainly by large organizations for critical applications typically bulk data
• IBM High Level Assembler IBM manuals on mainframe assembler language.
• Assembly Optimization Tips by Mark Larson
• Mainframe Assembler Forum
• NASM Manual
• Build yourself an assembler (eniAsm project) and various assembly articles and tutorials
• Encoding Intel x86/IA-32 Assembler Instructions

Software

• MenuetOS - hobby Operating System for the PC written entirely in 64-bit assembly language
• SB-Assembler for most 8-bit processors/controllers
• GNU lightning, a library that generates assembly language code at run-time which is useful for Just-In-Time compilers
• WinAsm Studio, The Assembly IDE - Free Downloads, Source Code , a free Assembly IDE, a lot of open source programs to download and a popular Board
• nasm, the Netwide Assembler
• GoAsm - a component of the free "Go" tools: 32-bit and 64-bit Windows programming for x86 and x86-64

MenuetOS (also known as MeOS) is an Operating system with a monolithic preemptive, real-time kernel, including video drivers See also X86 assembly language The generic term x86 refers to the most commercially successful Instruction set architecture in the history of Personal x86-64 is a Superset of the x86 instruction set architecture.

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