Assembly language

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

An assembly language is a low-level language for programming computers. It implements 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 portable).

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.

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. A compiler, analogous to an assembler, is used to translate high-level language statements into machine code; or an interpreter executes statements directly.)

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.

Key concepts

Assembler

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. 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, optimize instruction scheduling to exploit the CPU pipeline efficiently.

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 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."

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:

• Binary: 10110000 01100001 (Hexadecimal: 0xb061)

The equivalent assembly language representation is easier to remember (more mnemonic):

• mov al, #061h

This instruction means:

• Move the hexadecimal value 61 (97 decimal) 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.

Every computer 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 than 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.

Language design

Basic elements

Instructions (statements) in assembly language are generally very simple, unlike in a high-level language. Each instruction typically consists of an operation or opcode plus zero or more operands. Most instructions refer to a single value, or pair of values. Generally, an opcode is a symbolic name for a single executable machine language instruction. Operands can be either immediate (typically one byte values, coded in the instruction itself) or the addresses of data elsewhere in storage.

Most assemblers also support pseudo-operations, which are directives obeyed by the assembler at assembly time instead of the CPU at run time. (For example, pseudo-ops would be used to reserve storage areas and optionally set their initial contents.) Often the names of pseudo-ops 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 constants and variable is given a name so that instructions can reference them by name, making the code more self-explanatory. In executable code, the name of each subroutine is associated with its entry point, so that calls to that subroutine can use its name. Inside subroutines, GOTO destinations are given labels. Some assemblers support local symbols which are lexically distinct from normal symbols (eg., "10$") for use as GOTO destinations.

Most assemblers 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 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.

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 — i.e. without comments, meaningful symbols, or data definitions — is quite difficult to read.

Macros

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

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

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

Macro assemblers often allow macros to take parameters. 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 emit a large number of assembly language instructions or data definitions, based on the macro arguments. This could be used to generate record-style data structures or "unrolled" loops, for example, or could generate entire algorithms based on complex parameters. An organization using assembly language that has been heavily extended using such a macro suite can be considered to be working in a (slightly) higher-level language such programmers are not working with a computer's lowest-level conceptual elements.

Macros were used to customize large scale software systems for specific customers in the mainframe era and were also used by customer personnel to satisfy their employers' needs by making specific versions of manufacturer operating systems; this was done, for example, by systems programmers working with IBM's Conversational Monitor System/Virtual Machine (CMS/VM) and with its "real time transaction processing" add-on, Customer Information Control System, CICS.

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

This was because, as was realized in the 1970s, the concept of "macro processing" is independent of the concept of "assembly", the former being in modern terms more word processing, text processing, than generating object code. The concept of macro processing in fact appeared in and appears in the C programming language, which supports "preprocessor instructions" to set variables, and make conditional tests on their values. Note that unlike certain previous macro processors inside assemblers, the C preprocessor was not "Turing complete" because it lacked the ability to either loop or "go to", the latter allowing the programmer to loop.

Despite the power of macro processing, it fell into desuetude 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 substititution 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.

Therefore the two high level languages to import macro processing, PL/I and C, featured macros, but this facility was underused or dangerous when used. Today, macro processing's ability to generate the specific from the general is handled much better by abstract classes and interfaces in object-oriented programming. This is because OO programming separates concerns about text from concerns about meaning and semantics.

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 developed by Marvin Zloof at IBM's Thomas Watson Research Center, which extended the S/370 macro assembler with IF/ELSE/ENDIF and similar control flow blocks. 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.

Use of assembly language

Historical perspective

Historically, a large number of programs have been written entirely in assembly language. Operating systems were almost exclusively written in assembly language until the widespread acceptance of C in the 1970s and early 1980s. 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 size (and hence speed), and reliability: the writers of Cardbox-Plus said simply "we use assembler because then all the bugs are ours". This held true for 8-bit versions of the program, which had no bugs at all, but ironically it turned out to be false with 16 bits: Cardbox-Plus 2.0 had to be upgraded to Cardbox-Plus 2.1 because a bug in Microsoft's macro assembler caused Cardbox-Plus to index the number "-0" differently from the number "0".

Typical examples of large assembly language programs from this time are the MS-DOS operating system, the early IBM PC spreadsheet program Lotus 1-2-3, and almost all popular games for the Commodore 64. Even into the 1990s, most console video games were written in assembly, including most games for the Mega Drive/Genesis and the Super Nintendo Entertainment System . The popular arcade game NBA Jam (1993) is another example.

The smallest symbolic assembler ever written was The Assembler for the unexpanded VIC-20, written by Don French and published by French Silk. It was only 1639 bytes in length but was packed with features for its size. The Assembler supported the usual symbolic addressing and the definition of character strings and hex strings, but it also allowed address expressions of any number of terms combined with +-*/, logical AND, logical OR, and exponentiation operators.

Current usage

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

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

• When a stand-alone binary executable is required, i.e. one that must execute without recourse to the run-time components or libraries associated with a high-level language; this is perhaps the most common situation. These are embedded programs that store only a small amount of memory and the device is intended to do single purpose tasks. Such examples consist of telephones, automobile fuel and ignition systems, air-conditioning control systems, security systems, and sensors.
• When interacting directly with the hardware.
• When using processor-specific instructions not exploited by or available to the compiler. A common example is the bitwise rotation instruction at the core of many encryption algorithms.
• When extreme optimization is required, e.g., in an inner loop in a processor-intensive algorithm. Some game programmers are experts at writing code that takes advantage of the capabilities of hardware features in systems enabling the games to run faster.
• When a system with severe resource constraints (e.g., an embedded system) must be hand-coded to maximize the use of limited resources; but this is becoming less common as processor price/performance improves
• When no high-level language exists, e.g., on a new or specialized processor
• Real-time programs that need precise timing and responses, such as simulations, flight navigation systems, and medical equipment. (For example, in a fly-by-wire system, telemetry must be interpreted and acted upon within strict time constraints. Such systems must eliminate sources of unpredictable delays – such as may be created by interpreted languages, automatic garbage collection, paging operations, or preemptive multitasking. Some higher-level languages incorporate run-time components and operating system interfaces that can introduce such delays. Choosing assembly or lower-level languages for such systems gives the programmer greater visibility and control over processing details.)
• When complete control over the environment is required (for example in extremely high security situations, where nothing can be taken for granted).
• When writing computer viruses, bootloaders, certain device drivers, or other items very close to the hardware or low-level operating system.
• When reverse-engineering existing binaries, which may or may not have originally been written in a high-level language.
• Reverse engineering and modification of video games (known as ROM Hacking), commonly done to games for Nintendo hardware such as the SNES and NES, is possible with a range of techniques, of which the most widely employed is altering the program code at the assembly language level.
• ASM is also still used for writing games and other software for graphics calculators for example see http://tifreakware.net/tutorials/89/a/calc/fargoii.htm
• Finally, compiler writers usually write code to generate assembly code

Few programmers today need to use assembly language on a daily basis. Languages such as C are more appropriate for almost all low-level programming.

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.

Typical applications

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 the assembly language is the demoscene. Certain competitions require the contestants to restrict their creations to a very small size(e.g. 1kb, 4kbs, 64kbs), and assembly language becomes the language of choice to achieve this aim.

Related terminology

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

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

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

• The use of the word assembly dates from the early years of computers (cf. short code, speed code/"speedcoding").

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

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

Further details

For any given personal computer, mainframe, embedded system, and game console, both past and present, at least one--possibly dozens--of assemblers have been written. For some examples, see the list of assemblers.

On Unix systems, the assembler is traditionally called as, although it is not a single body of code, being typically written anew for each port. A number of Unix variants use GAS.

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. However, it is not possible to link portably with C libraries that require the caller to use preprocessor macros that may change between operating systems.

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, 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 portable assembly library for Win32 and Linux platforms, but it is yet very incomplete.

Some higher level computer languages, such as C, support inline assembly where relatively brief sections of assembly code can be embedded into the high level language code. Borland Pascal also had an assembler compiler, which was initialized with a keyword "asm". It was mainly used to create mouse and COM port drivers. The Forth programming language commonly contains an assembler used in CODE words.

Many people use an emulator to debug assembly-language programs.

Example listing of assembly language source code

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

Example of a selection of instructions (for a virtual computer) 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).

See also

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

Books

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

External links

• Howto, Tips and Tricks in Assembly Language and some hardware help about programming its work
• WinAsm Studio, The Assembly IDE - Free Downloads, Source Code, a free Assembly IDE, a lot of open source programs to download and a popular Board
• The ASM Community, a great ASM programming resource including a Messageboard and an ASM Wiki Book
• Intel Assembly 80x86 CodeTable (a cheat sheet reference)
• MenuetOS - hobby Operating System for the PC written entirely in 64bit assembly language
• Unix Assembly Language Programming
• PPR: Learning Assembly Language
• CodeTeacher
• Assembly Language Programming Examples
• Typed Assembly Language (TAL)
• Authoring Windows Applications In Assembly Language
• The Program Transformation Wiki
• GoAsm - a component of the free "Go" tools: 32-bit and 64-bit Windows programming for x86 and x86-64
• GNU lightning is a library that generates assembly language code at run-time which is useful for Just-In-Time compilers
• "information on assembly programming under different platforms: IA32 (x86), IA64 (Itanium), x86-64, SPARC, Alpha, or whatever platform we find contributors for."
• "Terse: Algebraic Assembly Language for x86"
• Iczelion's Win32 Assembly Tutorial
• SB-Assembler for most 8-bit processors/controllers
• Assembly Tutorials BeginnersCode.com
• IBM z/Architecture Principles of Operation IBM manuals on mainframe machine language and internals.
• IBM High Level Assembler IBM manuals on mainframe assembler language.
• Tools and tutorials for x86 programmers
• Assembly Optimization Tips by Mark Larson
• Mainframe Assembler Forum