88-634: In computer programming , assembly language (alternatively assembler language or symbolic machine code ), often referred to simply as assembly and commonly abbreviated as ASM or asm , is any low-level programming language with a very strong correspondence between the instructions in the language and the architecture's machine code instructions . Assembly language usually has one statement per machine instruction (1:1), but constants, comments , assembler directives , symbolic labels of, e.g., memory locations , registers , and macros are generally also supported. The first assembly code in which
176-476: A register . The binary code for this instruction is 10110 followed by a 3-bit identifier for which register to use. The identifier for the AL register is 000, so the following machine code loads the AL register with the data 01100001. This binary computer code can be made more human-readable by expressing it in hexadecimal as follows. Here, B0 means "Move a copy of the following value into AL ", and 61
264-402: A compiler can make it crash when parsing some large source file, a simplification of the test case that results in only few lines from the original source file can be sufficient to reproduce the same crash. Trial-and-error/divide-and-conquer is needed: the programmer will try to remove some parts of the original test case and check if the problem still exists. When debugging the problem in a GUI,
352-540: A few simple readability transformations made code shorter and drastically reduced the time to understand it. Following a consistent programming style often helps readability. However, readability is more than just programming style. Many factors, having little or nothing to do with the ability of the computer to efficiently compile and execute the code, contribute to readability. Some of these factors include: The presentation aspects of this (such as indents, line breaks, color highlighting, and so on) are often handled by
440-404: A general CPU or a more specialized processing unit), the opcodes are defined by the processor's instruction set architecture (ISA), and can be described by means of an opcode table . The types of operations may include arithmetic , data copying, logical operations , and program control, as well as special instructions (e.g., CPUID ). In addition to the opcode, many instructions also specify
528-515: A higher-level language, for performance reasons or to interact directly with hardware in ways unsupported by the higher-level language. For instance, just under 2% of version 4.9 of the Linux kernel source code is written in assembly; more than 97% is written in C . Assembly language uses a mnemonic to represent, e.g., each low-level machine instruction or opcode , each directive , typically also each architectural register , flag , etc. Some of
616-492: A language is used to represent machine code instructions is found in Kathleen and Andrew Donald Booth 's 1947 work, Coding for A.R.C. . Assembly code is converted into executable machine code by a utility program referred to as an assembler . The term "assembler" is generally attributed to Wilkes , Wheeler and Gill in their 1951 book The Preparation of Programs for an Electronic Digital Computer , who, however, used
704-477: A list of data, arguments or parameters. Some instructions may be "implied", which means the data upon which the instruction operates is implicitly defined by the instruction itself—such an instruction does not take an operand. The resulting statement is translated by an assembler into machine language instructions that can be loaded into memory and executed. For example, the instruction below tells an x86 / IA-32 processor to move an immediate 8-bit value into
792-717: A macro definition, e.g., MEXIT in HLASM , while others may be permitted within open code (outside macro definitions), e.g., AIF and COPY in HLASM. In assembly language, the term "macro" represents a more comprehensive concept than it does in some other contexts, such as the pre-processor in the C programming language , where its #define directive typically is used to create short single line macros. Assembler macro instructions, like macros in PL/I and some other languages, can be lengthy "programs" by themselves, executed by interpretation by
880-481: A mask of 0. Extended mnemonics are often used to support specialized uses of instructions, often for purposes not obvious from the instruction name. For example, many CPU's do not have an explicit NOP instruction, but do have instructions that can be used for the purpose. In 8086 CPUs the instruction xchg ax , ax is used for nop , with nop being a pseudo-opcode to encode the instruction xchg ax , ax . Some disassemblers recognize this and will decode
968-438: A mnemonic is a symbolic name for a single executable machine language instruction (an opcode ), and there is at least one opcode mnemonic defined for each machine language instruction. Each instruction typically consists of an operation or opcode plus zero or more operands . Most instructions refer to a single value or a pair of values. Operands can be immediate (value coded in the instruction itself), registers specified in
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#17327823970591056-410: A move between a byte-sized register and either another register or memory, and the second byte, E0h, is encoded (with three bit-fields) to specify that both operands are registers, the source is AH , and the destination is AL . In a case like this where the same mnemonic can represent more than one binary instruction, the assembler determines which instruction to generate by examining the operands. In
1144-522: 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 ( labels or symbols ) with memory locations and various constants. Usually, every constant and variable
1232-400: 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. Since the information about pseudoinstructions and macros defined in the assembler environment
1320-470: A result, loses efficiency and the ability for low-level manipulation). Debugging is a very important task in the software development process since having defects in a program can have significant consequences for its users. Some languages are more prone to some kinds of faults because their specification does not require compilers to perform as much checking as other languages. Use of a static code analysis tool can help detect some possible problems. Normally
1408-400: A second pass would require storing the symbol table in memory (to handle forward references ), rewinding and rereading the program source on tape , or rereading a deck of cards or punched paper tape . Later computers with much larger memories (especially disc storage), had the space to perform all necessary processing without such re-reading. The advantage of the multi-pass assembler is that
1496-459: A visual environment. Different programming languages support different styles of programming (called programming paradigms ). The choice of language used is subject to many considerations, such as company policy, suitability to task, availability of third-party packages, or individual preference. Ideally, the programming language best suited for the task at hand will be selected. Trade-offs from this ideal involve finding enough programmers who know
1584-410: Is Entity-Relationship Modeling ( ER Modeling ). Implementation techniques include imperative languages ( object-oriented or procedural ), functional languages , and logic programming languages. It is very difficult to determine what are the most popular modern programming languages. Methods of measuring programming language popularity include: counting the number of job advertisements that mention
1672-403: Is a one-to-one correspondence between many simple assembly statements and machine language instructions. However, in some cases, an assembler may provide pseudoinstructions (essentially macros) which expand into several machine language instructions to provide commonly needed functionality. For example, for a machine that lacks a "branch if greater or equal" instruction, an assembler may provide
1760-447: Is a hexadecimal representation of the value 01100001, which is 97 in decimal . Assembly language for the 8086 family provides the mnemonic MOV (an abbreviation of move ) for instructions such as this, so the machine code above can be written as follows in assembly language, complete with an explanatory comment if required, after the semicolon. This is much easier to read and to remember. In some assembly languages (including this one)
1848-453: 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.g., to generate common short sequences of instructions as inline , instead of called subroutines . Some assemblers may also be able to perform some simple types of instruction set -specific optimizations . One concrete example of this may be
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#17327823970591936-404: Is always completely unable to recover source comments. Each computer architecture has its own machine language. Computers differ in the number and type of operations they support, in the different sizes and numbers of registers, and in the representations of data in storage. While most general-purpose computers are able to carry out essentially the same functionality, the ways they do so differ;
2024-607: Is directly executed by the central processing unit . Proficient programming usually requires expertise in several different subjects, including knowledge of the application domain , details of programming languages and generic code libraries , specialized algorithms, and formal logic . Auxiliary tasks accompanying and related to programming include analyzing requirements , testing , debugging (investigating and fixing problems), implementation of build systems , and management of derived artifacts , such as programs' machine code . While these are sometimes considered programming, often
2112-508: 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. Many assemblers support predefined macros , and others support programmer-defined (and repeatedly re-definable) macros involving sequences of text lines in which variables and constants are embedded. The macro definition
2200-401: Is given a name so instructions can reference those locations by name, thus promoting self-documenting code . In executable code, the name of each subroutine is associated with its entry point, so any calls to a subroutine can use its name. Inside subroutines, GOTO destinations are given labels. Some assemblers support local symbols which are often lexically distinct from normal symbols (e.g.,
2288-464: Is little more than a different notation for a machine language, two machines with different instruction sets also have different assembly languages. High-level languages made the process of developing a program simpler and more understandable, and less bound to the underlying hardware . The first compiler related tool, the A-0 System , was developed in 1952 by Grace Hopper , who also coined
2376-678: Is more than one assembler for the same architecture, and sometimes an assembler is specific to an operating system or to particular operating systems. Most assembly languages do not provide specific syntax for operating system calls, and most assembly languages can be used universally with any operating system, as the language provides access to all the real capabilities of the processor , upon which all system call mechanisms ultimately rest. In contrast to assembly languages, most high-level programming languages are generally portable across multiple architectures but require interpreting or compiling , much more complicated tasks than assembling. In
2464-441: Is most commonly a mixture of assembler statements, e.g., directives, symbolic machine instructions, and templates for assembler statements. This sequence of text lines may include opcodes or directives. Once a macro has been defined its name may be used in place of a mnemonic. When the assembler processes such a statement, it replaces the statement with the text lines associated with that macro, then processes them as if they existed in
2552-407: 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
2640-511: Is similar to learning a foreign language . Opcode In computing , an opcode (abbreviated from operation code ) is an enumerated value that specifies the operation to be performed. Opcodes are employed in hardware devices such as arithmetic logic units (ALUs) and central processing units (CPUs) as well as in some software instruction sets. In ALUs the opcode is directly applied to circuitry via an input signal bus, whereas in CPUs,
2728-542: Is still strong in corporate data centers often on large mainframe computers , Fortran in engineering applications, scripting languages in Web development, and C in embedded software . Many applications use a mix of several languages in their construction and use. New languages are generally designed around the syntax of a prior language with new functionality added, (for example C++ adds object-orientation to C, and Java adds memory management and bytecode to C++, but as
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2816-401: Is the composition of sequences of instructions, called programs , that computers can follow to perform tasks. It involves designing and implementing algorithms , step-by-step specifications of procedures, by writing code in one or more programming languages . Programmers typically use high-level programming languages that are more easily intelligible to humans than machine code , which
2904-524: Is universally enforced by their syntax. For example, in the Intel x86 assembly language, a hexadecimal constant must start with a numeral digit, so that the hexadecimal number 'A' (equal to decimal ten) would be written as 0Ah or 0AH , not AH , specifically so that it cannot appear to be the name of register AH . (The same rule also prevents ambiguity with the names of registers BH , CH , and DH , as well as with any user-defined symbol that ends with
2992-434: Is usually easier to code in "high-level" languages than in "low-level" ones. Programming languages are essential for software development. They are the building blocks for all software, from the simplest applications to the most sophisticated ones. Allen Downey , in his book How To Think Like A Computer Scientist , writes: Many computer languages provide a mechanism to call functions provided by shared libraries . Provided
3080-475: The xchg ax , ax instruction as nop . Similarly, IBM assemblers for System/360 and System/370 use the extended mnemonics NOP and NOPR for BC and BCR with zero masks. For the SPARC architecture, these are known as synthetic instructions . Some assemblers also support simple built-in macro-instructions that generate two or more machine instructions. For instance, with some Z80 assemblers
3168-470: The CPU pipeline as efficiently as possible. Assemblers have been available since the 1950s, as the first step above machine language and before high-level programming languages such as Fortran , Algol , COBOL and Lisp . There have also been several classes of translators and semi-automatic code generators with properties similar to both assembly and high-level languages, with Speedcode as perhaps one of
3256-504: The Jacquard loom could produce entirely different weaves by changing the "program" – a series of pasteboard cards with holes punched in them. Code-breaking algorithms have also existed for centuries. In the 9th century, the Arab mathematician Al-Kindi described a cryptographic algorithm for deciphering encrypted code, in A Manuscript on Deciphering Cryptographic Messages . He gave
3344-439: The source code editor , but the content aspects reflect the programmer's talent and skills. Various visual programming languages have also been developed with the intent to resolve readability concerns by adopting non-traditional approaches to code structure and display. Integrated development environments (IDEs) aim to integrate all such help. Techniques like Code refactoring can enhance readability. The academic field and
3432-755: 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. An assembler program creates object code by translating combinations of mnemonics and syntax for operations and addressing modes into their numerical equivalents. This representation typically includes an operation code (" opcode ") as well as other control bits and data. The assembler also calculates constant expressions and resolves symbolic names for memory locations and other entities. The use of symbolic references
3520-568: The 9th century, a programmable music sequencer was invented by the Persian Banu Musa brothers, who described an automated mechanical flute player in the Book of Ingenious Devices . In 1206, the Arab engineer Al-Jazari invented a programmable drum machine where a musical mechanical automaton could be made to play different rhythms and drum patterns, via pegs and cams . In 1801,
3608-526: The AE in 1837. In the 1880s, Herman Hollerith invented the concept of storing data in machine-readable form. Later a control panel (plug board) added to his 1906 Type I Tabulator allowed it to be programmed for different jobs, and by the late 1940s, unit record equipment such as the IBM 602 and IBM 604 , were programmed by control panels in a similar way, as were the first electronic computers . However, with
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3696-544: The Intel 8080 family and the Intel 8086/8088. Because Intel claimed copyright on its assembly language mnemonics (on each page of their documentation published in the 1970s and early 1980s, at least), some companies that independently produced CPUs compatible with Intel instruction sets invented their own mnemonics. The Zilog Z80 CPU, an enhancement of the Intel 8080A , supports all the 8080A instructions plus many more; Zilog invented an entirely new assembly language, not only for
3784-527: The V20 and V30 actually wrote in NEC's assembly language rather than Intel's; since any two assembly languages for the same instruction set architecture are isomorphic (somewhat like English and Pig Latin ), there is no requirement to use a manufacturer's own published assembly language with that manufacturer's products. There is a large degree of diversity in the way the authors of assemblers categorize statements and in
3872-462: The Z80, NEC invented new mnemonics for all of the 8086 and 8088 instructions, to avoid accusations of infringement of Intel's copyright. (It is questionable whether such copyrights can be valid, and later CPU companies such as AMD and Cyrix republished Intel's x86/IA-32 instruction mnemonics exactly with neither permission nor legal penalty.) It is doubtful whether in practice many people who programmed
3960-401: The absence of errata makes the linking process (or the program load if the assembler directly produces executable code) faster. Example: in the following code snippet, a one-pass assembler would be able to determine the address of the backward reference BKWD when assembling statement S2 , but would not be able to determine the address of the forward reference FWD when assembling
4048-424: The architecture, these elements may also be combined for specific instructions or addressing modes using offsets or other data as well as fixed addresses. Many assemblers offer additional mechanisms to facilitate program development, to control the assembly process, and to aid debugging . Some are column oriented, with specific fields in specific columns; this was very common for machines using punched cards in
4136-501: 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. Computer programming Computer programming or coding
4224-442: The assembler operates and "may affect the object code, the symbol table, the listing file, and the values of internal assembler parameters". Sometimes the term pseudo-opcode is reserved for directives that generate object code, such as those that generate data. The names of pseudo-ops often start with a dot to distinguish them from machine instructions. Pseudo-ops can make the assembly of the program dependent on parameters input by
4312-586: The better-known examples. There may be several assemblers with different syntax for a particular CPU or instruction set architecture . For instance, an instruction to add memory data to a register in a x86 -family processor might be add eax,[ebx] , in original Intel syntax , whereas this would be written addl (%ebx),%eax in the AT&;T syntax used by the GNU Assembler . Despite different appearances, different syntactic forms generally generate
4400-674: The branch statement S1 ; indeed, FWD may be undefined. A two-pass assembler would determine both addresses in pass 1, so they would be known when generating code in pass 2. More sophisticated high-level assemblers provide language abstractions such as: See Language design below for more details. A program written in assembly language consists of a series of mnemonic processor 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 an opcode mnemonic followed by an operand , which might be
4488-414: The circumstances. The first step in most formal software development processes is requirements analysis , followed by testing to determine value modeling, implementation, and failure elimination (debugging). There exist a lot of different approaches for each of those tasks. One approach popular for requirements analysis is Use Case analysis. Many programmers use forms of Agile software development where
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#17327823970594576-495: The code, making it easy to target varying machine instruction sets via compilation declarations and heuristics . Compilers harnessed the power of computers to make programming easier by allowing programmers to specify calculations by entering a formula using infix notation . Programs were mostly entered using punched cards or paper tape . By the late 1960s, data storage devices and computer terminals became inexpensive enough that programs could be created by typing directly into
4664-467: The computers. Text editors were also developed that allowed changes and corrections to be made much more easily than with punched cards . Whatever the approach to development may be, the final program must satisfy some fundamental properties. The following properties are among the most important: Using automated tests and fitness functions can help to maintain some of the aforementioned attributes. In computer programming, readability refers to
4752-548: The concept of the stored-program computer introduced in 1949, both programs and data were stored and manipulated in the same way in computer memory . Machine code was the language of early programs, written in the instruction set of the particular machine, often in binary notation. Assembly languages were soon developed that let the programmer specify instructions in a text format (e.g., ADD X, TOTAL), with abbreviations for each operation code and meaningful names for specifying addresses. However, because an assembly language
4840-453: 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. 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
4928-565: The data (known as operands ) the operation will act upon, although some instructions may have implicit operands or none at all. Some instruction sets have nearly uniform fields for opcode and operand specifiers, whereas others (e.g., x86 architecture) have a less uniform, variable-length structure. Instruction sets can be extended through the use of opcode prefixes which add a subset of new instructions made up of existing opcodes following reserved byte sequences. Opcodes can be found in so-called byte codes and other representations intended for
5016-510: The ease with which a human reader can comprehend the purpose, control flow , and operation of source code . It affects the aspects of quality above, including portability, usability and most importantly maintainability. Readability is important because programmers spend the majority of their time reading, trying to understand, reusing, and modifying existing source code, rather than writing new source code. Unreadable code often leads to bugs, inefficiencies, and duplicated code . A study found that
5104-527: The engineering practice of computer programming are concerned with discovering and implementing the most efficient algorithms for a given class of problems. For this purpose, algorithms are classified into orders using Big O notation , which expresses resource use—such as execution time or memory consumption—in terms of the size of an input. Expert programmers are familiar with a variety of well-established algorithms and their respective complexities and use this knowledge to choose algorithms that are best suited to
5192-425: The first decades of computing, it was commonplace for both systems programming and application programming to take place entirely in assembly language. While still irreplaceable for some purposes, the majority of programming is now conducted in higher-level interpreted and compiled languages. In " No Silver Bullet ", Fred Brooks summarised the effects of the switch away from assembly language programming: "Surely
5280-400: The first description of cryptanalysis by frequency analysis , the earliest code-breaking algorithm. The first computer program is generally dated to 1843 when mathematician Ada Lovelace published an algorithm to calculate a sequence of Bernoulli numbers , intended to be carried out by Charles Babbage 's Analytical Engine . However, Charles Babbage himself had written a program for
5368-480: The first example, the operand 61h is a valid hexadecimal numeric constant and is not a valid register name, so only the B0 instruction can be applicable. In the second example, the operand AH is a valid register name and not a valid numeric constant (hexadecimal, decimal, octal, or binary), so only the 88 instruction can be applicable. Assembly languages are always designed so that this sort of lack of ambiguity
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#17327823970595456-404: The first step in debugging is to attempt to reproduce the problem. This can be a non-trivial task, for example as with parallel processes or some unusual software bugs. Also, specific user environment and usage history can make it difficult to reproduce the problem. After the bug is reproduced, the input of the program may need to be simplified to make it easier to debug. For example, when a bug in
5544-465: The following examples show. In each case, the MOV mnemonic is translated directly into one of the opcodes 88-8C, 8E, A0-A3, B0-BF, C6 or C7 by an assembler, and the programmer normally does not have to know or remember which. Transforming assembly language into machine code is the job of an assembler, and the reverse can at least partially be achieved by a disassembler . Unlike high-level languages , there
5632-464: The functions in a library follow the appropriate run-time conventions (e.g., method of passing arguments ), then these functions may be written in any other language. Computer programmers are those who write computer software. Their jobs usually involve: Although programming has been presented in the media as a somewhat mathematical subject, some research shows that good programmers have strong skills in natural human languages, and that learning to code
5720-458: The instruction ld hl,bc is recognized to generate ld l,c followed by ld h,b . These are sometimes known as pseudo-opcodes . Mnemonics are arbitrary symbols; in 1985 the IEEE published Standard 694 for a uniform set of mnemonics to be used by all assemblers. The standard has since been withdrawn. There are instructions used to define data elements to hold data and variables. They define
5808-526: The instruction or implied, or the addresses of data located elsewhere in storage. This is determined by the underlying processor architecture: the assembler merely reflects how this architecture works. Extended mnemonics are often used to specify a combination of an opcode with a specific operand, e.g., the System/360 assemblers use B as an extended mnemonic for BC with a mask of 15 and NOP ("NO OPeration" – do nothing for one step) for BC with
5896-404: The language to build a team, the availability of compilers for that language, and the efficiency with which programs written in a given language execute. Languages form an approximate spectrum from "low-level" to "high-level"; "low-level" languages are typically more machine-oriented and faster to execute, whereas "high-level" languages are more abstract and easier to use but execute less quickly. It
5984-465: The language, the number of books sold and courses teaching the language (this overestimates the importance of newer languages), and estimates of the number of existing lines of code written in the language (this underestimates the number of users of business languages such as COBOL). Some languages are very popular for particular kinds of applications, while some languages are regularly used to write many different kinds of applications. For example, COBOL
6072-402: The letter H and otherwise contains only characters that are hexadecimal digits, such as the word "BEACH".) Returning to the original example, while the x86 opcode 10110000 ( B0 ) copies an 8-bit value into the AL register, 10110001 ( B1 ) moves it into CL and 10110010 ( B2 ) does so into DL . Assembly language examples for these follow. The syntax of MOV can also be more complex as
6160-427: The mnemonics may be built-in and some user-defined. Many operations require one or more operands in order to form a complete instruction. Most assemblers permit named constants, registers, and labels for program and memory locations, and can calculate expressions for operands. Thus, programmers are freed from tedious repetitive calculations and assembler programs are much more readable than machine code. Depending on
6248-422: 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
6336-463: The new instructions but also for all of the 8080A instructions. For example, where Intel uses the mnemonics MOV , MVI , LDA , STA , LXI , LDAX , STAX , LHLD , and SHLD for various data transfer instructions, the Z80 assembly language uses the mnemonic LD for all of them. A similar case is the NEC V20 and V30 CPUs, enhanced copies of the Intel 8086 and 8088, respectively. Like Zilog with
6424-400: 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: Instructions (statements) in assembly language are generally very simple, unlike those in high-level languages . Generally,
6512-424: The opcode is the portion of a machine language instruction that specifies the operation to be performed. Opcodes are found in the machine language instructions of CPUs as well as in some abstract computing machines . In CPUs, an opcode may be referred to as instruction machine code , instruction code , instruction syllable , instruction parcel or opstring . For any particular processor (which may be
6600-405: The operation, and if necessary, pad it with one or more " no-operation " instructions in a later pass or the errata. In an assembler with peephole optimization , 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
6688-476: The programmer can try to skip some user interaction from the original problem description and check if the remaining actions are sufficient for bugs to appear. Scripting and breakpointing are also part of this process. Debugging is often done with IDEs . Standalone debuggers like GDB are also used, and these often provide less of a visual environment, usually using a command line . Some text editors such as Emacs allow GDB to be invoked through them, to provide
6776-498: The same mnemonic is used for different instructions, that means that the mnemonic corresponds to several different binary instruction codes, excluding data (e.g. the 61h in this example), depending on the operands that follow the mnemonic. For example, for the x86/IA-32 CPUs, the Intel assembly language syntax MOV AL, AH represents an instruction that moves the contents of register AH into register AL . The hexadecimal form of this instruction is: The first byte, 88h, identifies
6864-489: 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
6952-415: The same numeric machine 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 as FASM -syntax, TASM -syntax, ideal mode, etc., in the special case of x86 assembly programming). There are two types of assemblers based on how many passes through the source are needed (how many times the assembler reads
7040-432: The source code file (including, in some assemblers, expansion of any macros existing in the replacement text). Macros in this sense date to IBM autocoders of the 1950s. Macro assemblers typically 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
7128-407: The source) to produce the object file. 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
7216-411: The term software development is used for this larger overall process – with the terms programming , implementation , and coding reserved for the writing and editing of code per se. Sometimes software development is known as software engineering , especially when it employs formal methods or follows an engineering design process . Programmable devices have existed for centuries. As early as
7304-409: The term 'compiler'. FORTRAN , the first widely used high-level language to have a functional implementation, came out in 1957, and many other languages were soon developed—in particular, COBOL aimed at commercial data processing, and Lisp for computer research. These compiled languages allow the programmer to write programs in terms that are syntactically richer, and more capable of abstracting
7392-451: The term to mean "a program that assembles another program consisting of several sections into a single program". The conversion process is referred to as assembly , as in assembling the source code . The computational step when an assembler is processing a program is called assembly time . Because assembly depends on the machine code instructions, each assembly language is specific to a particular computer architecture . Sometimes there
7480-495: The type of data, the length and the alignment of data. These instructions can also define whether the data is available to outside programs (programs assembled separately) or only to the program in which the data section is defined. Some assemblers classify these as pseudo-ops. Assembly directives, 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
7568-413: The ubiquitous x86 assemblers from various vendors. Called jump-sizing , most of them are able to perform jump-instruction replacements (long jumps replaced by short or relative jumps) in any number of passes, on request. Others may even do simple rearrangement or insertion of instructions, such as some assemblers for RISC architectures that can help optimize a sensible instruction scheduling to exploit
7656-591: The use of "10$ " as a GOTO destination). Some assemblers, such as NASM , provide flexible symbol management, letting programmers manage different namespaces , automatically calculate offsets within data structures , and assign labels that refer to literal values or the result of simple computations performed by the assembler. Labels can also be used to initialize constants and variables with relocatable addresses. Assembly languages, like most other computer languages, allow comments to be added to program source code that will be ignored during assembly. Judicious commenting
7744-560: The various stages of formal software development are more integrated together into short cycles that take a few weeks rather than years. There are many approaches to the Software development process. Popular modeling techniques include Object-Oriented Analysis and Design ( OOAD ) and Model-Driven Architecture ( MDA ). The Unified Modeling Language ( UML ) is a notation used for both the OOAD and MDA. A similar technique used for database design
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