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لغات البرمجة


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لغة التجميع
في علوم الحاسوب، لغة التجميع (بالإنكليزية: Assembly language) هي الصيغة السهلة القراءة للبشر المقابلة للغة الآلة التي تشكل الأوامر التي ينفذها حاسوب بتصميم ما. فلغة الآلة عبارة عن تتابع من البتات (bits) تمثل عملية حاسوبية أو أمر للحاسوب، تصبح أسهل للقراءة عندما تستبدل برموز تعبر عنها.

ولكل معالج لغة تجميع خاصة به كما أن له "لغة آلة Machine Language" خاصة به. وتحتاج لغة التجميع ما يسمى "المجمـِّع" (بالإنكليزية: Assembler) وهو الذي يقوم بتحويل لغة التجميع التي يستطيع البشر قراءتها والتعديل فيها إلى لغة الآلة التي يستطيع المعالج تنفيذها. وتستخدم هذه اللغة الآن من قبل البشر وذلك لبرمجة أجزاء من نظم التشغيل أو للتاكد من سرعة وكفاءة تنفيذ بعض البرامج التي يحتاج فيها الكفاءة كبرامج المحاكاة والألعاب.

و تتكون اسطر برامج التجميع من ثلاثة أجزاء:

العلامة (Label) و هو ما يتم به الاشارة لسطر ما في سطور أخرى.
الأمر (Instruction) و هو يكون مناظر في الغالب لأمر في المعالج و هو ما سيقوم المعالج بتنفيذه عند الوصول لهذا السطر أثناء تنفيذ البرنامج.
المعامل (Operand) و هو المتغير الذي سيتم تطبيق الأمر عليه.



Assembly languages are a family of low-level languages for programming computers, microprocessors, microcontrollers, and other (usually) integrated circuits. They implement a symbolic representation of the numeric machine codes and other constants needed to program a particular CPU architecture. 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. An assembly language is thus specific to a certain physical or virtual computer architecture (as opposed to most high-level languages, which are usually portable).

A utility program called an assembler is used to translate assembly language statements into the target computer's machine code. The assembler performs a more or less isomorphic translation (a one-to-one mapping) from mnemonic statements into machine instructions and data. This is in contrast with high-level languages, in which a single statement generally results in many machine instructions.

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.

Contents [hide]
1 Key concepts
1.1 Assembler
1.2 Assembly language
2 Language design
2.1 Basic elements
2.1.1 Opcode mnemonics
2.1.2 Data sections
2.1.3 Assembly directives / pseudo-ops
2.2 Macros
2.3 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 List of assemblers for different computer architectures
6 Further details
7 Example listing of assembly language source code
8 See also
9 References
10 Further reading
11 External links
11.1 Software

Compare with: Microassembler.
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.[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.g., to generate common short sequences of instructions to run inline, instead of in a subroutine.

Assemblers are generally simpler to write than compilers for high-level languages, and have been available since the 1950s. Modern assemblers, especially for RISC based architectures, such as MIPS, Sun SPARC, and HP PA-RISC, as well as x86(-64), optimize instruction scheduling to exploit the CPU pipeline efficiently.

There are two types of assemblers based on how many passes through the source are needed to produce the executable program. One-pass assemblers go through the source code once and assumes that all symbols will be defined before any instruction that references them. Two-pass assemblers (and multi-pass assemblers) create a table with all unresolved symbols in the first pass, then use the 2nd pass to resolve these addresses. The advantage in one-pass assemblers is speed - which is not as important as it once was with advances in computer speed and capabilities. The advantage of the two-pass assembler is that symbols can be defined anywhere in the program source. As a result, the program can be defined in a more logical and meaningful way. This makes two-pass assembler programs easier to read and maintain.[2]

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.

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."[citation needed]

[edit] 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.

For example, an x86/IA-32 processor can execute the following binary instruction as expressed in machine language (see x86 assembly 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; the pound sign means move the immediate value, not location) into the processor register named "AL".
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. Unlike in high-level languages, there is usually a one-to-one correspondence between simple assembly statements and machine language instructions. 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.

Each computer architecture and processor architecture has its own machine language. 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. In these cases, the most popular one is usually that supplied by the manufacturer and used in its documentation.

[edit] Language design
[edit] Basic elements
Any Assembly language consists of 3 types of instruction statements which are used to define the program operations:

opcode mnemonics
data sections
assembly directives
[edit] Opcode mnemonics
Instructions (statements) in assembly language are generally very simple, unlike those in high-level languages. Generally, an opcode is a symbolic name for a single executable machine language instruction, 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 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.

[edit] Data sections
There are instructions used to define data elements to hold data and variables. They define what type of data, length and 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.

[edit] Assembly directives / pseudo-ops
Assembly Directives are instructions that are executed by the Assembler at assembly time, not by the CPU at run time. They can make the assembly of the program dependent on parameters input by the programmer, so that one program can be assembled different ways, perhaps for different applications. They also can be used to manipulate presentation of the program to make it easier for the programmer to read and maintain.

(For example, pseudo-ops would be used to reserve storage areas and optionally 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 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 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. 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. Good use of comments is even more important with assembly code than with higher-level languages, as the meaning and purpose 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.

[edit] 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 a 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 or indeed many lines of code, they can be used to make assembly language programs appear to be much shorter (require less lines of source code from the application programmer - as with a higher level language). They can also be used to add higher levels of structure to assembly programs, optionally introduce embedded de-bugging code via parameters and other similar features.

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

Macro assemblers often allow macros to take parameters. Some assemblers include quite sophisticated macro languages, incorporating such high-level language elements as optional parameters, symbolic variables, conditionals, string manipulation, and arithmetic operations, all usable during the execution of a given macros, and allowing macros to save context or exchange information. Thus a macro might generate 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 higher-level language, since 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 IBM's "real time transaction processing" add-ons, CICS, Customer Information Control System, and ACP/TPF, the airline/financial system that began in the 1970s and still runs many large Global Distribution Systems (GDS) and credit card systems today.

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.

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, Prolog, and Forth, retain the power of assembly language macros because they are able to manipulate their own code as data.

[edit] Support for structured programming
Some assemblers have incorporated structured programming elements to encode execution flow. The earliest example of this approach was in the Concept-14 macro set, originally proposed by Dr. H.D. Mills (March, 1970), and implemented by Marvin Kessler at IBM's Federal Systems Division, which extended the S/360 macro assembler with IF/ELSE/ENDIF and similar control flow blocks.[3] This was a way to reduce or eliminate the use of GOTO operations in assembly code, one of the main factors causing spaghetti code in assembly language. This approach was widely accepted in the early 80s (the latter days of large-scale assembly language use).

A curious design was A-natural, a "stream-oriented" assembler for 8080/Z80 processors[citation needed] from Whitesmiths Ltd. (developers of the Unix-like Idris operating system, and what was reported to be the first commercial C compiler). 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.[4] 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.[5]

[edit] Use of assembly language
[edit] Historical perspective
Assembly languages were first developed in the 1950s, when they were referred to as second generation programming languages. 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. 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. 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.

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. 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.

Most early microcomputers relied on hand-coded assembly language, including most operating systems and large applications. This was because these systems had severe resource constraints, imposed idiosyncratic memory and display architectures, and provided limited, buggy system services. Perhaps more important was the lack of first-class high-level language compilers suitable for microcomputer use. A psychological factor may have also played a role: the first generation of microcomputer programmers retained a hobbyist, "wires and pliers" attitude.

In a more commercial context, the biggest reasons for using assembly language were minimal bloat (size), minimal overhead, greater speed, and reliability.

Typical examples of large assembly language programs from this time are 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. 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[citation needed]. According to some industry insiders, the assembly language was the best computer language to use to get the best performance out of the Sega Saturn, a console that was notoriously challenging to develop and program games for [6]. The popular arcade game NBA Jam (1993) is another example. 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 Assembler for the VIC-20 was written by Don French and published by French Silk. At 1639 bytes in length, its author believes it is the smallest symbolic assembler ever written. The assembler supported the usual symbolic addressing and the definition of character strings or hex strings. It also allowed address expressions which could be combined with addition, subtraction, multiplication, division, logical AND, logical OR, and exponentiation operators