Assembly language explained

See the terminology section below for information regarding inconsistent use of the terms assembly and assembler.An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices in which each statement corresponds to a single machine language instruction. An assembly language is specific to a certain computer architecture, in contrast to most high-level programming languages, which may be more portable.

Assembly language allows the programmer to use symbolic representation for machine operation codes (usually called mnemonics), memory locations, registers and other parts of an instruction.

A utility program called an assembler is used to translate assembly language statements into the target computer's machine code.

In normal professional usage, the term assembler is used to refer both to an assembly language, and to software which assembles an assembly-language program. Thus: "CP/CMS was written in S/360 assembler" as well as "ASM-H was a widely-used S/370 assembler."

Many assemblers offer additional mechanisms to facilitate program development, control the assembly process, and aid debugging. Assemblers often include a macro facility (described below), and are called macro assemblers.

Key concepts

Assembler

An 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 as inline, instead of called subroutines.

Assemblers are generally simpler to write than compilers for high-level languages, and have been available since the 1950s. Modern assemblers, especially for RISC architectures, such as SPARC or POWER, as well as x86 and x86-64, optimize Instruction scheduling to exploit the CPU pipeline efficiently.

Number of passes

There are two types of assemblers based on how many passes through the source are needed to produce the executable program.

The original reason for the use of one-pass assemblers was speed of assembly; however, modern computers perform multi-pass assembly without unacceptable delay. The advantage of the multi-pass assembler is that the absence of a need for errata makes the linker (or the loader if the assembler directly produces executable code) simpler and faster.

High-level assemblers

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

See Language design below for more details.

Assembly language

A program written in assembly language consists of a series of (mnemonic) processor instructions and meta-statements (known variously as directives, pseudo-instructions and pseudo-ops), comments and data. Assembly language instructions usually consist of an opcode mnemonic followed by a list of data, arguments or parameters.[3] These are translated by an assembler into machine language instructions that can be loaded into memory and executed.

For example, the instruction that tells an x86/IA-32 processor to move an immediate 8-bit value into a register. The binary code for this instruction is 10110 followed by a 3-bit identifier for which register to use. The identifier for the AL register is 000, so the following machine code loads the AL register with the data 01100001.[4] 10110000 01100001This binary computer code can be made more human-readable by expressing it in hexadecimal as follows B0 61Here, B0 means 'Move a copy of the following value into AL, and 61 is a hexadecimal representation of the value 01100001, which is 97 in decimal. Intel assembly language provides the mnemonic MOV (an abbreviation of move) for instructions such as this, so the machine code above can be written as follows in assembly language, complete with an explanatory comment if required, after the semicolon. This is much easier to read and to remember.

asm

MOV AL, 61h       ; Load AL with 97 decimal (61 hex)
In some assembly languages the same mnemonic such as MOV may be used for a family of related instructions for loading, copying and moving data, whether these are immediate values, values in registers, or memory locations pointed to by values in registers. Other assemblers may use separate opcodes such as L for "move memory to register", ST for "move register to memory", LR for "move register to register", MVI for "move immediate operand to memory", etc.

The Intel opcode 10110000 (B0) copies an 8-bit value into the AL register, while 10110001 (B1) moves it into CL and 10110010 (B2) does so into DL. Assembly language examples for these follow.[4]

asm

MOV AL, 1h        ; Load AL with immediate value 1
MOV CL, 2h        ; Load CL with immediate value 2
MOV DL, 3h        ; Load DL with immediate value 3
The syntax of MOV can also be more complex as the following examples show.[5]

asm

MOV EAX, [EBX]	  ; Move the 4 bytes in memory at the address contained in EBX into EAX
MOV [ESI+EAX], CL ; Move the contents of CL into the byte at address ESI+EAX
In each case, the MOV mnemonic is translated directly into an opcode in the ranges 88-8E, A0-A3, B0-B8, C6 or C7 by an assembler, and the programmer does not have to know or remember which.[4]

Transforming assembly language into machine code is the job of an assembler, and the reverse can at least partially be achieved by a disassembler. Unlike high-level languages, there 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 has its own machine language. Computers differ in the number and type of operations they support, in the different sizes and numbers of registers, and in the representations of data in storage. While most general-purpose computers are able to carry out essentially the same functionality, the ways they do so differ; 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

There is a large degree of diversity in the way the authors of assemblers categorize statements and in the nomenclature that they use. In particular, some describe anything other than a machine mnemonic or extended mnemonic as a pseudo-operation (pseudo-op). A typical assembly language consists of 3 types of instruction statements that are used to define program operations:

Opcode mnemonics and extended mnemonics

Instructions (statements) in assembly language are generally very simple, unlike those in high-level language. Generally, a mnemonic is a symbolic name for a single executable machine language instruction (an opcode), and there is at least one opcode mnemonic defined for each machine language instruction. Each instruction typically consists of an operation or opcode plus zero or more operands. Most instructions refer to a single value, or a pair of values. Operands can be immediate (value coded in the instruction itself), registers specified in the instruction or implied, or the addresses of data located elsewhere in storage. This is determined by the underlying processor architecture: the assembler merely reflects how this architecture works. Extended mnemonics are often used to specify a combination of an opcode with a specific operand, e.g., the System/360 assemblers use B as an extended mnemonic for BC with a mask of 15 and NOP for BC with a mask of 0.

Extended mnemonics are often used to support specialized uses of instructions, often for purposes not obvious from the instruction name. For example, many CPU's do not have an explicit NOP instruction, but do have instructions that can be used for the purpose. In 8086 CPUs the instruction xchg ax,ax is used for nop, with nop being a pseudo-opcode to encode the instruction xchg ax,ax. Some disassemblers recognize this and will decode the xchg ax,ax instruction as nop. Similarly, IBM assemblers for System/360 and System/370 use the extended mnemonics NOP and NOPR for BC and BCR with zero masks. For the SPARC architecture, these are known as synthetic instructions[6]

Some assemblers also support simple built-in macro-instructions that generate two or more machine instructions. For instance, with some Z80 assemblers the instruction ld hl,bc is recognized to generate ld l,c followed by ld h,b.[7] These are sometimes known as pseudo-opcodes.

Data sections

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. Some assemblers classify these as pseudo-ops.

Assembly directives

Assembly directives, also called pseudo opcodes, pseudo-operations or pseudo-ops, are instructions that are executed by an assembler at assembly time, not by a CPU at run time. They can make the assembly of the program dependent on parameters input by a programmer, so that one program can be assembled different ways, perhaps for different applications. They also can be used to manipulate presentation of a program to make it easier to read and maintain.

(For example, directives would be used to reserve storage areas and optionally their initial contents.) The names of directives often start with a dot to distinguish them from machine instructions.

Symbolic assemblers let programmers 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).

Some 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, 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.

Macros

Many assemblers support predefined macros, and others support programmer-defined (and repeatedly re-definable) macros involving sequences of text lines in which variables and constants are embedded. This sequence of text lines may include opcodes or directives. Once a macro has been defined its name may be used in place of a mnemonic. When the assembler processes such a statement, it replaces the statement with the text lines associated with that macro, then processes them as if they existed in the source code file (including, in some assemblers, expansion of any macros existing in the replacement text).

Note that this definition of "macro" is slightly different from the use of the term in other contexts, like the C programming language. C macro's created through the #define directive typically are just one line, or a few lines at most. Assembler macro instructions can be lengthy "programs" by themselves, executed by interpretation by the assembler during assembly.

Since macros can have 'short' names but expand to several or indeed many lines of code, they can be used to make assembly language programs appear to be far shorter, requiring fewer lines of source code, as with higher level languages. They can also be used to add higher levels of structure to assembly programs, optionally introduce embedded debugging code via parameters and other similar features.

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 computer reservations systems (CRS) and credit card systems today.

It was also possible to use solely the macro processing abilities 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 1960s, 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 appeared, 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 programs to loop.

Despite the power of macro processing, it fell into disuse in many high level languages (major exceptions being C/C++ and PL/I) while remaining a perennial for assemblers.

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 aload a*bthe 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, the macro expansion of load a-c*b occurs. To avoid any possible ambiguity, users of macro processors can parenthesize formal parameters inside macro definitions, or callers can parenthesize the input parameters.

PL/I and C/C++ feature macros, but this facility 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.

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. 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.[8]

Use of assembly language

Historical perspective

Assembly languages date to the introduction of the stored-program computer. The EDSAC computer (1949) had an assembler called initial orders featuring one-letter mnemonics.[9] Nathaniel Rochester wrote an assembler for an IBM 701 (1954). SOAP (Symbolic Optimal Assembly Program) (1955) was an assembly language for the IBM 650 computer written by Stan Poley.[10]

Assembly languages eliminated much of the error-prone and time-consuming first-generation programming needed with the earliest computers, freeing programmers 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 microcomputers), their use had largely been supplanted by high-level languages, in the search for improved programming productivity. Today assembly language 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 entirely written in assembly language until the introduction of the Burroughs MCP (1961), which was written in ESPOL, an Algol dialect. 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 IBM PC DOS operating systems and early applications such as the spreadsheet program Lotus 1-2-3. 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.[11] The popular arcade game NBA Jam (1993) is another example. Assembly language has long been the primary development language for many popular home computers of the 1980s and 1990s (such as the Sinclair ZX Spectrum, Commodore 64, Commodore Amiga, and Atari ST). This was in large part because 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 the 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.

Current usage

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[12] to render high-level languages into code that can run as fast as hand-written assembly, despite the counter-examples that can be found. The complexity of modern processors and memory sub-system makes effective optimization increasingly difficult for compilers, as well as assembly programmers. 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 developers might choose to use assembly language:

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. This is analogous to children needing to learn the basic arithmetic operations (e.g., long division), although calculators are widely used for all except the most trivial calculations.

Typical applications

Related terminology

List of assemblers for different computer architectures

The following page has a list of different assemblers for the different computer architectures, along with any associated information for that specific assembler:

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. An instruction set simulator can 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 similar to use of microcode to achieve compatibility across a processor family.

Some higher level computer languages, such as C and Borland Pascal, support inline assembly where sections of assembly code, in practice usually brief, can be embedded into the high level language code. The Forth language commonly contains an assembler used in CODE words.

An emulator can be used to debug assembly-language programs.

Example listing of assembly language source code

AddressLabelInstruction (AT&T syntax)Object code
.begin
.org 2048
a_start.equ 3000
2048ld length,%
2064be done00000010 10000000 00000000 00000110
2068addcc %r1,-4,%r110000010 10000000 01111111 11111100
2072addcc %r1,%r2,%r410001000 10000000 01000000 00000010
2076ld %r4,%r511001010 00000001 00000000 00000000
2080ba loop00010000 10111111 11111111 11111011
2084addcc %r3,%r5,%r310000110 10000000 11000000 00000101
2088done:jmpl %r15+4,%r010000001 11000011 11100000 00000100
2092length:2000000000 00000000 00000000 00010100
2096address:a_start00000000 00000000 00001011 10111000
.org a_start
3000a:

Example of a selection of instructions (for a virtual computer[20]) with thecorresponding 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

Further reading

External links

Notes and References

  1. David Salomon (1993). Assemblers and Loaders
  2. Hyde, Randall. "Chapter 12 – Classes and Objects". The Art of Assembly Language, 2nd Edition. No Starch Press. © 2010.
  3. Book: Intel Architecture Software Developer’s Manual, Volume 2: Instruction Set Reference. 1999. INTEL CORPORATION. 18 November 2010.
  4. Book: Intel Architecture Software Developer’s Manual, Volume 2: Instruction Set Reference. 1999. INTEL CORPORATION. 442 and 35. 18 November 2010.
  5. Web site: Evans. David. x86 Assembly Guide. University of Virginia. 18 November 2010. 2006.
  6. Web site: The SPARC Architecture Manual, Version 8. SPARC, International. 1992.
  7. http://www.z80.de/z80/z80code.htm
  8. http://neshla.sourceforge.net/ NESHLA: The High Level, Open Source, 6502 Assembler for the Nintendo Entertainment System
  9. Book: Salomon. Assemblers and Loaders. 2012-01-17. 7.
  10. Web site: The IBM 650 Magnetic Drum Calculator. 2012-01-17.
  11. http://www.eidolons-inn.net/tiki-index.php?page=SegaBase+Saturn Eidolon's Inn : SegaBase Saturn
  12. Web site: Rusling. David A.. The Linux Kernel. Mar 11, 2012.
  13. Web site: BLAS Benchmark-August2008. eigen.tuxfamily.org. 2008-08-01. 2010-03-04.
  14. Web site: x264.git/common/x86/dct-32.asm. git.videolan.org. 2010-09-29. 2010-09-29.
  15. Techically BAL was only the assembler for BPS; the others were macro assemblers.
  16. Stroustrup, Bjarne, The C++ Programming Language, Addison-Wesley, 1986, ISBN 0-201-12078-X: "C++ was primarily designed so that the author and his friends would not have to program in assembler, C, or various modern high-level languages. [use of the term ''assembler'' to mean ''assembly language'']"
  17. Saxon, James, and Plette, William, Programming the IBM 1401, Prentice-Hall, 1962, LoC 62-20615. [use of the term ''assembly program'']
  18. Web site: Microsoft Corporation. MASM: Directives & Pseudo-Opcodes. March 19, 2011.
  19. http://www.encyclopedia.com/doc/1O11-metaassembler.html (John Daintith, ed.) A Dictionary of Computing: "meta-assembler"
  20. http://iiusatech.com/~murdocca/POCA Principles of Computer Architecture