Assembly languages are a type 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 family is thus specific to a certain physical (or virtual) computer architecture. This is in contrast to most high-level languages, which are (ideally) 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.
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. 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 as inline, instead of called subroutines, or even generate entire programs or program suites.
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.
More sophisticated high-level assemblers provide language abstractions such as:
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."
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.
Hexadecimal: B0 61 (Binary: 10110000 01100001)
The equivalent assembly language representation is easier to remember (example in Intel syntax, more mnemonic):
MOV AL, 61h
This instruction means:
The mnemonic "mov" represents the opcode 1011 which actually copies the value in the second operand into the register indicated by the first operand. The mnemonic was chosen by the designer of the instruction set to abbreviate "move", making it easier for the programmer to remember. Typical of an assembly language statement, a comma-separated list of arguments or parameters follows the opcode.
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 (partial) reverse by a disassembler. Unlike 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 (essentially macros) which expand into several machine language instructions to provide commonly needed functionality. For example, for a machine that lacks a "branch if greater or equal" instruction, an assembler may provide a pseudoinstruction that expands to the machine's "set if less than" and "branch if zero (on the result of the set instruction)". Most full-featured assemblers also provide a rich macro language (discussed below) which is used by vendors and programmers to generate more complex code and data sequences.
Each computer architecture and processor architecture usually 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.
Any Assembly language consists of 3 types of instruction statements which are used to define the program operations:
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.
There are instructions used to define data elements to hold data and variables. They define 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.
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.
Many assemblers support predefined macros, and others support programmer-defined (and repeatedly redefinable) macros involving sequences of text lines that variables and constants are embedded in. 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 (or predefined) macros for system calls and other special code sequences, such as the generation and storage of data realized through advanced bitwise and boolean operations used in gaming, software security, data management, and cryptography.
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 macro, 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.
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. 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 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. 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.
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, although assembly language is almost always handled and generated by compilers, it is still used for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues. Typical uses are device drivers, low-level embedded 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. 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 . 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.
There have always been debates over the usefulness and performance of assembly language relative to high-level languages. 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. This has made raw code execution speed a non-issue for many programmers.
There are some situations in which practitioners might choose to use assembly language, such as when:
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. 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.
Hard-coded assembly language is typically used in a system's boot ROM (BIOS on IBM-compatible PC systems). 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.
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. Programs using such facilities, such as the Linux kernel, can then construct abstractions utilizing different assembly language on each hardware platform. 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. Tools such as the Interactive Disassembler make extensive use of disassembly for such a purpose.
A particular niche that makes use of assembly language is the demoscene. Certain competitions require the contestants to restrict their creations to a very small size (e.g. 256B, 1KB, 4KB or 64 KB), and assembly language is the language of choice to achieve this goal. When resources, particularly CPU-processing constrained systems, like the earlier Amiga models, 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 coders in an attempt to minimize the number of CPU cycles used; the CPU constraints are so great that every CPU cycle counts. However, using such techniques has enabled systems like the Commodore 64 to produce real-time 3D graphics with advanced effects, a feat which might be considered unlikely or even impossible for a system with a 0.99MHz processor.
The following page has a list of different assemblers for the different computer architectures, along with any associated information for that specific assembler:
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.
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. FASM and NASM have similar syntax, but each support different macros that could make them difficult to translate to each other. The basics are all the same, but the advanced features will differ.
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. An instruction set simulator (which would ideally be written in an assembler language) can, in theory, process the object code/ binary of any assembler to achieve portability even across platforms (with an overhead no greater than a typical bytecode interpreter). This is essentially what microcode achieves when a hardware platform changes internally.
For example, many things in libc depend on the preprocessor to do OS-specific, C-specific things to the program before compiling. 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. To address this issue, FASMLIB project provides a portable assembly library for Win32 and Linux platforms, but it is yet very incomplete.
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. The Forth programming language commonly contains an assembler used in CODE words.
Many people use an emulator to debug assembly-language programs.
|Address||Label||Instruction (AT&T syntax)||Object code|
|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|
Example of a selection of instructions (for a virtual computer) with the corresponding address in memory where each instruction will be placed. These addresses are not static, see memory management. Accompanying each instruction is the generated (by the assembler) object code that coincides with the virtual computer's architecture (or ISA).
Assembly Language is a set of mnemonic languages with a 1 to 1 logical mapping of instructions to the machine code of various architectures. Assembly is usually used when the programming task is small and local, as it has very little modularity and is platform-dependent, unlike higher-level languages. Other uses of assembly are in program debugging (in which machine instructions can be executed one at a time), and in reverse-engineering compiled programs by disassembly (in which there is no higher-level code to associate with).
The architecture is the most important thing to know when programming in Assembly Language. The architecture in question might be the specific hardware that the application is designed to run on, or a virtual machine. A virtual machine is an example of abstract hardware, and usually also has its own version of machine code. The architecture dictates the internal representations used by data types, instructions understood by the CPU, and available resources. Since Assembly Language is a 1 to 1 logical mapping to machine code, the steps taken to implement an algorithm are frequently much smaller and more numerous than those in higher level languages. A typical hardware architecture contains a CPU, registers, and memory.
The CPU (Central Processing Unit) acts as a kind of brain for the computer system. It usually contains a cache and registers. The cache is typically used to store segments of program code, while registers are used for immediate data access. The CPU successively executes instructions from the cache until it encounters a branch or interrupt.
Registers are a part of the CPU, and are the fastest memory available. The number, size, and use of registers available depends upon the architecture in question. A register is usually considered as an integer, but can be conceptualized as anything from a character to a pointer.
Memory access is a trade-off between speed and size. Accessing memory is much faster than disk I/O, but much slower than accessing a register. Memory contains both program code and variable storage, and has the advantage of being able to store large or complex objects.
An instruction in Assembly Language usually declares both where this data is located, and the format of the expected data. Instructions can be categorized as mathematical, logical, flow control, or memory operations. Common mathematical operations include adding, subtracting, multiplying, dividing, and shifting, while common logical operations are the bitwise operations AND, OR, XOR, and NOT. Flow control operations are most often branches or comparisons, and memory is often loaded, stored, or moved. Each architecture comes with an instruction set which is in machine code (1's and 0's) for the machine but presented to the human programmer in more readable forms. See MIPS architecture - a fine example.
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An assembly language is a programming language that can be used to directly tell the computer what to do. An assembly language is almost exactly like the machine language that a computer can understand, except that it uses words in place of numbers. A computer cannot really understand an assembly program directly. However, it can easily change the program into machine code by replacing the words of the program with the numbers that they stand for. A program that does that is called an assembler.
Programs written in assembly language are usually made of instructions, which are small tasks that the computer performs when it is running the program. They are called instructions because the programmer uses them to instruct the computer what to do. The part of the computer that follows the instructions is the processor.
The assembly language of a computer is a low-level language, which means that it can only be used to do the simple tasks that a computer can understand directly. In order to perform more complex tasks, one must tell the computer each of the simple tasks that are part of the complex task. For example, a computer does not understand how to print a sentence on its screen. Instead, a program written in assembly must tell it how to do all of the small steps that are involved in printing the sentence.
Such an assembly program would be composed of many, many instructions, that together do something that seems very simple and basic to a human. This makes it hard for humans to read an assembly program. In contrast, a high-level programming language may have a single instruction such as PRINT "This is a sentence" that will tell the computer to perform all of the small tasks for you.
When computer scientists first built programmable machines, they programmed them directly in machine code, which is a series of numbers that instructed the computer what to do. Writing machine language was very hard to do and took a long time, so eventually assembly language was made. Assembly language is easier for a human to read and can be written faster, but it is still much harder for a human to use than a high-level programming language which tries to mimic human language.
To program in machine code, the programmer needs to know what each instruction looks like in binary (or hexadecimal). Although it is easy for a computer to quickly figure out what machine code means, it is hard for a programmer. Each instruction can have several forms, all of which just look like a bunch of numbers to people. Any mistake that someone makes while writing machine code will only be noticed when the computer does the wrong thing. Figuring out the mistake is hard because most people cannot tell what machine code means by looking at it.
An example of what machine code looks like:
05 2A 00
With assembly language, each instruction can be written as a short word, called a mnemonic, followed by other things like numbers or other short words. The mnemonic is used so that the programmer does not have to remember the exact numbers in machine code needed to tell the computer to do something. Examples of mnemonics in assembly language include add, which adds data, and mov, which moves data from one place to another. Because 'mnemonic' is an uncommon word, the phrase instruction type or just instruction is sometimes used instead, often incorrectly. The words and numbers after the first word give more information about what to do. For instance, things following an add might be what two things to add together and the things following mov say what to move and where to put it.
For example, the machine code in the previous section (05 2A 00) can be written in assembly as:
Assembly language also allows programmers to write the actual data the program uses in easier ways. Most assembly languages have support for easily making numbers and text. In machine code, each different type of number like positive, negative or decimal, would have to be manually converted into binary and text would have to be defined one letter at a time, as numbers.
Assembly language provides what is called an abstraction of machine code. When using assembly, programmers do not need to know the details of what numbers mean to the computer, the assembler figures that out instead. Assembly language actually still lets the programmer use all the features of the processor that they could with machine code. In this sense, assembly language has a very good, rare trait: it has the same ability to express things as the thing it is abstracting (machine code) while being much easier to use. Because of this, machine code is almost never used as a programming language.
When programs are finished, they have already been transformed into machine code so that the processor can actually run them. Sometimes, however, if the program has a bug (mistake) in it, programmers will want to be able to tell what each part of the machine code is doing. Disassemblers are programs that help programmers do that by transforming the machine code of the program back into assembly language, which is much easier to understand. Disassemblers, which turn machine code into assembly language, do the opposite of assemblers, which turn assembly language into machine code.
An understanding of how computers are organized, how they seem to work at a very low level, is needed to understand how an assembly language program works. At the most simplistic level, computers have three main parts:
In most computers, memory is divided up into bytes. Each byte contains 8 bits. Each byte in memory also has an address which is a number that says where the byte is in memory. The first byte in memory has an address of 0, the next one has an address of 1, and so on. Dividing memory into bytes makes it byte addressable because each byte gets a unique address. Addresses of byte addressable memories cannot be used to refer to a single bit of a byte. A byte is the smallest piece of memory that can be addressed.
Even though an address refers to a particular byte in memory, processors allow for using several bytes of memory in a row. The most common use of this feature is to use either 2 or 4 bytes in a row to represent a number, usually an integer. Single bytes are sometimes also used to represent integers, but because they are only 8 bits long, they can only hold 28 or 256 different possible values. Using 2 or 4 bytes in a row raises the number of different possible values to be 216, 65536 or 232, 4294967296, respectively.
When a program uses a byte or a number of bytes in a row to represent something like a letter, number, or anything else, those bytes are called an object because they are all part of the same thing. Even though objects are all stored in identical bytes of memory, they are treated as though they have a 'type', which says how the bytes should be understood: either as an integer or a character or some other type (like a non-integer number). Machine code can also be thought of as a type that is interpreted as instructions. The notion of a type is very, very important because it defines what things can and can’t be done to the object and how to interpret the bytes of the object. For instance, it is not valid to store a negative number in a positive number object and it is not valid to store a fraction in an integer.
An address that points to (is the address of) a multi-byte object is the address to the first byte of that object – the byte that has the lowest address. As an aside, one important thing to note is that you can’t tell what the type of an object is - or even its size - by its address. In fact, you can’t even tell what type an object is by looking at it. An assembly language program needs to keep track of which memory addresses hold which objects, and how big those objects are. A program that does so is type safe because it only does things to objects that are safe to do on their type. A program that doesn’t will probably not work properly. Note that most programs do not actually explicitly store what the type of an object is, they just access objects consistently - the same object is always treated as the same type.
The processor runs (executes) instructions, which are stored as machine code in main memory. As well as being able to access memory for storage, most processors have a few small, fast, fixed-size spaces for holding objects that are currently being worked with. These spaces are called registers. Processors usually execute three types of instructions, although some instructions can be a combination of these types. Below are some examples of each type in x86 assembly language.
The following x86 assembly language instruction reads (loads) a 2-byte object from the byte at address 4096 (0x1000 in hexadecimal) into a 16-bit register called 'ax':
In this assembly language, square brackets around a number (or a register name) mean that the number should be used as an address to the data that should be used. The use of an address to point to data is called indirection. In this next example, without the square brackets, another register, bx, actually gets the value 20 loaded into it.
Because no indirection was used, the actual value itself was put into the register.
If the operands (the things that come after the mnemonic), appear in the reverse order, an instruction that loads something from memory instead writes it to memory:
Here, the memory at address 1000h gets the value of ax. If this example is executed right after the previous one, the 2 bytes at 1000h and 1001h will be a 2 byte integer with the value of 20.
Some instructions do things like subtraction or logical operations like not:
The machine code example earlier in this article would be this in assembly language:
Here, 42 and ax are added together and the result is stored back in ax. In x86 assembly it is also possible to combine a memory access and mathematical operation like this:
This instruction adds the value of the 2 byte integer stored at 1000h to ax and stores the answer in ax.
This instruction computes the or of the contents of the registers ax and bx and stores the result back into ax.
Usually, instructions are executed in the order they appear in memory, which is the order they are typed in the assembly code. The processor just executes them one after another. However, in order for processors to do complicated things, they need to execute different instructions based on what the data they were given is. The ability of processors to execute different instructions depending on something's outcome is called branching. Instructions that decide what the next instruction should be are called branch instructions.
In this example, suppose someone wants to calculate the amount of paint they will need to paint a square with a certain side length. However, due to economy of scale the paint store will not sell them any less than amount of paint needed to paint a 100 x 100 square.
To figure out the amount of paint they will need to get based on the length of the square they want to paint, they come up with this set of steps:
That algorithm can be expressed in the following code where ax is the side length.
This example introduces several new things, but the first two instructions are familiar. They copy the value of ax into bx and then subtract 100 from bx.
One of the new things in this example is called a label, a concept found in assembly languages in general. Labels can be anything the programmer wants (unless it is the name of an instruction, which would confuse the assembler). In this example, the label is 'continue'. It is interpreted by the assembler as the address of an instruction. In this case, it is the address of mult ax.
Another new concept is that of flags. On x86 processors, many instructions set 'flags' in the processor that can be used by the next instruction to decide what to do. In this case, if bx was less than 100, sub will set a flag that says the result was less than zero.
The next instruction is jge which is short for 'Jump if Greater than or Equal to'. It is a branch instruction. If the flags in the processor specify that the result was greater than or equal to zero, instead of just going to the next instruction the processor will jump to the instruction at the continue label, which is mul ax.
This example works fine, but it is not what most programmers would write. The subtract instruction set the flag correctly, but it also changes the value it operates on, which required the ax to be copied into bx. Most assembly languages allow for comparison instruction that do not change any of the arguments they are passed, but still set the flags properly and x86 assembly is no exception.
Now, instead of subtracting 100 from ax, seeing if that number is less than zero, and assigning it back to ax, ax is left unchanged. The flags are still set the same way, and the jump is still taken in the same situations.
While input and output are a fundamental part of computing, there is no one way they are done in assembly language. This is because the way I/O works depends on the set up of the computer and the operating system its running, not just what kind of processor it has. In the example section the Hello World example uses MS-DOS operating system calls and the example after it uses BIOS calls.
It is possible to do I/O in assembly language. Indeed, assembly language can generally express anything that a computer is capable of doing. However, even though there are instructions to add and branch in assembly language that will always do the same thing there are no instructions in assembly language that always do I/O.
The important thing to note is that the way that I/O works is not part of any assembly language because it is not part of how the processor works.
Even though assembly language is not directly run by the processor - machine code is, it still has a lot to do with it. Each processor family supports different features, instructions, rules for what the instructions can do, and rules for what combination of instructions are allowed where. Because of this, different types of processors still need different assembly languages.
Because each version of assembly language is tied to a processor family, it lacks something called portability. Something that has portability or is portable can be easily transferred from one type of computer to another. While other types of programming languages are portable, assembly language, in general, is not.
Although assembly language allows for an easy way to use all the processor's features, it is not used for modern software projects for several reasons:
As a result of these drawbacks, high-level languages like Pascal, C, and C++ are used for most projects instead. They allow programmers to express their ideas more directly instead of having to worry about telling the processor what to do every step of the way. They're called high-level because the ideas the programmer can express in the same amount code are more complicated.
Programmers writing code in compiled high level languages use a program called a compiler to transform their code into assembly language. Compilers are much harder to write than assemblers are. Also, high-level languages do not always allow programmers to use all the features of the processor. This is because high-level languages are designed to support all processor families. Unlike assembly languages, that only support one type of processor, high-level languages are portable.
Even though compilers are more complicated than assemblers, decades of making and researching compilers has made them very good. Now, there is not much reason to use assembly language anymore for most projects, because compilers can usually figure out how to express programs in assembly language as well or better than programmers.
A Hello World Program written in x86 Assembly:
.data hello_message db 'Hello, World!',0dh,0ah,'$'
.code main proc
mov ax,@data mov ds,ax
mov ah,9 mov dx,offset hello_message int 21h
mov ax,4C00h int 21h
main endp end main
A function that prints a number to the screen using BIOS interrupts written in NASM x86 assembly. Modular code is possible to write in assembly, but it takes extra effort. Note that anything that comes after a semicolon on a line is a comment and is ignored by the assembler. Putting comments in assembly language code is very important because large assembly language programs are so hard to understand.
printn: push bp mov bp, sp push ax push bx push cx push dx push si
mov si, 0 mov ax, [bp + 4] ; number mov cx, [bp + 6] ; base
gloop: inc si ; length of string mov dx, 0 ; zero dx div cx ; divide by base cmp dx, 10 ; is it ge 10? jge num add dx, '0' ; add zero to dx jmp anum num: add dx, ('A'- 10) ; hex value, add 'A' to dx - 10. anum: push dx ; put dx onto stack. cmp ax, 0 ; should we continue? jne gloop
mov bx, 7h ; for interrupt tloop: pop ax ; get its value mov ah, 0eh ; for interrupt int 10h ; write character dec si ; get rid of character jnz tloop
pop si pop dx pop cx pop bx pop ax pop bp ret 4