Assembly language, commonly called assembly, asm or symbolic machine code, is a human-readable notation for the machine language that a specific computer architecture uses. Machine language, a pattern of bits encoding machine operations, is made readable by replacing the raw values with symbols called mnemonics.
For example, a computer with the appropriate processor will understand this x86/IA-32 machine language:
10110000 01100001
For programmers, however, it is easier to remember the equivalent assembly language representation:
mov al, 061h
which means to move the hexadecimal value 61 (97 decimal) into the processor register with the name "al". The mnemonic "mov" is short for "move", and a comma-separated list of arguments or parameters follows it; this is a typical assembly language statement.
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 1-to-1 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)".
Every computer architecture has its own machine language, and therefore its own assembly language. Computers differ by the number and type of operations that they support. They may also have different sizes and numbers of registers, and different representations of data types in storage. While all general-purpose computers are able to carry out essentially the same functionality, the way they do it differs, and the corresponding assembly language must reflect these differences.
In addition, multiple sets of mnemonics or assembly-language syntax may exist for a single instruction set. In these cases, the most popular one is usually that used by the manufacturer in their documentation.
Contents [hide]
1 Machine instructions
2 Assembly language directives
3 Usage of assembly language
4 Example listing of assembly language source code
5 See also
6 References
7 Books
8 External links
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Machine instructions
Instructions in assembly language are generally very simple, unlike in a high-level language. Any instruction that references memory (for data or as a jump target) will also have an addressing mode to determine how to calculate the required memory address. More complex operations must be built up out of these simple operations. Some operations available in most instruction sets include:
moving
set a register (a temporary "scratchpad" location in the CPU itself) to a fixed constant value
move data from a memory location to a register, or vice versa. This is done to obtain the data to perform a computation on it later, or to store the result of a computation.
read and write data from hardware devices
computing
add, subtract, multiply, or divide the values of two registers, placing the result in a register
perform bitwise operations, taking the conjunction/disjunction (and/or) of corresponding bits in a pair of registers, or the negation (not) of each bit in a register
compare two values in registers (for example, to see if one is less, or if they are equal)
affecting program flow
jump to another location in the program and execute instructions there
jump to another location if a certain condition holds
jump to another location, but save the location of the next instruction as a point to return to (a call)
Some computers include one or more "complex" instructions in their instruction set. A single "complex" instruction does something that may take many instructions on other computers. Such instructions are typified by instructions that take multiple steps, may issue to multiple functional units, or otherwise appear to be a design exception to the simplest instructions which are implemented for the given processor. Some examples of such instructions include:
saving many registers on the stack at once
moving large blocks of memory
complex and/or floating-point arithmetic (sine, cosine, square root, etc.)
performing an atomic test-and-set instruction
instructions that combine ALU with an operand from memory rather than a register
A form of complex instructions that has become particularly popular recently are SIMD operations that perform the same arithmetic operation to multiple pieces of data at the same time, which have appeared under various trade names beginning with MMX and AltiVec.
The design of instruction sets is a complex issue, with a simpler instruction set (generally grouped under the concept RISC) perhaps offering the potential for higher speeds, while a more complex one (traditionally called CISC) may offer particularly fast implementations of common performance-demanding tasks, may use memory (and thus cache) more efficiently, and be somewhat easier to program directly in assembly. See instruction set for a fuller discussion of this point.
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Assembly language directives
In addition to codes for machine instructions, assembly languages have extra directives for assembling blocks of data, and assigning address locations for instructions or code.
They usually have a simple symbolic capability for defining values as symbolic expressions which are evaluated at assembly time, making it possible to write code that is easier to read and understand.
Like most computer languages, comments can be added to the source code; these often provide useful additional information to human readers of the code but are ignored by the assembler and so may be used freely.
They also usually have an embedded macro language to make it easier to generate complex pieces of code or data.
In practice, the absence of comments and the replacement of symbols with actual numbers makes the human interpretation of disassembled code considerably more difficult than the original (high level) source would be.
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Usage of assembly language
Historically, a large number of programs have been written entirely in assembly language. A classic example was the early IBM PC spreadsheet program Lotus 123. Even into the 1990s, the majority of console video games were written in assembly language, including most games written for the Sega Genesis and the Super Nintendo Entertainment System. The popular arcade game NBA Jam (1993) was also coded entirely using assembly language.
There is some debate over the continued usefulness of assembly language. It is often said that modern compilers can render higher-level languages into codes that run as fast as hand-written assembly, but counter-examples can be made, and there is no clear consensus on this topic. It is reasonably certain that, given the increase in complexity of modern processors, effective hand-optimization is increasingly difficult and requires a great deal of knowledge.
However, some discrete calculations can still be rendered into faster running code with assembly, and some low-level programming is actually easier to do with assembly. Some system-dependent tasks performed by operating systems simply cannot be expressed in high-level languages. In particular, assembly is often used in writing the low level interaction between the operating system and the hardware, for instance in device drivers. Many compilers also render high-level languages into assembly first before fully compiling, allowing the assembly code to be viewed for debugging and optimization purposes.
It's also common, especially in relatively low-level languages such as C, to be able to embed assembly language into the source code with special syntax. Programs using such facilities, such as the Linux kernel, often construct abstractions where different assembly language is used on each platform the program supports, but it is called by portable code through a uniform interface.
Many embedded systems are also programmed in assembly to obtain the absolute maximum functionality out of what is often very limited computational resources, though this is gradually changing in some areas as more powerful chips become available for the same minimal cost.
Another common area of assembly language use is in the system BIOS of a computer. This low-level code is used 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, code written in higher level languages can be used, but almost always the code running immediately after power is applied is written in assembly language.
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.
MenuetOS, a floppy-based operating system with a fully functional GUI is written entirely in assembly. A 64bit version is also available. The author claims that only through assembly language could he produce his system in less than 1.4 megabytes.
While the modern usage of assembly differs greatly from its prior level in that most software developers do not use assembly coding for entire projects anymore, it is still a very valuable tool to use when writing frequently accessed components of an application or an operating system; a commonly cited example is an operating system's boot loader, which is almost always written entirely in assembly language.
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Example listing of assembly language source code
• Addr • Label • Instruction • • Object code • [1]
.begin
.org 2048
a_start .equ 3000
2048 ld [length],&r1 11000010 00000000 00101000 00101100
2052 ld [address],%r2 11000100 00000000 00101000 00110000
2056 addcc %r3,%r0,%r3 10000110 10001000 11000000 00000000
2060 loop: addcc %r1,%r1,%r0 10000000 10001000 01000000 00000001
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: 25 00000000 00000000 00000000 00011001
3004 -10 11111111 11111111 11111111 11110110
3008 33 00000000 00000000 00000000 00100001
3012 -5 11111111 11111111 11111111 11111011
3016 7 00000000 00000000 00000000 00000111
.end
Example of a selection of instructions (for a virtual computer[2]) 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).
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See also
Assembler
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References
^ Murdocca, Miles J.; Vincent P. Heuring (2000). Principles of Computer Architecture, Prentice-Hall. ISBN 0-201-43664-7.
^ Principles of Computer Architecture (POCA) – ARCTools virtual computer available for download to execute referenced code, accessed August 24, 2005
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Books
The Art of Assembly Language Programming, by Randall Hyde
Computer-Books.us, Online Assembly Language Books
PC Assembly Language by Dr Paul Carter; *PC Assembly Tutorial using NASM and GCC by Paul Carter
Programming from the Ground Up by Jonathan Bartlett
The x86 ASM Book by the ASM Community
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External links
Wikibooks has more about this subject:
Programming:AssemblyThe ASM Community Messageboard
MenuetOS - hobby Operating System for the PC written entirely in 64bit assembly language
List of resources; books, websites, newsgroups, and IRC channels
Linux Assembly
Unix Assembly Language Programming
PPR: Learning Assembly Language
CodeTeacher
Assembly Language Programming Examples
Typed Assembly Language (TAL)
Authoring Windows Applications In Assembly Language
RosAsm assembler/ RosAsm assembly Forum
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