An assembly program has a series of instructions--mnemonics that relate to a stream of executable instructions, when translated by an assembler that may be loaded into memory and then executed.
For an example, an x86/IA-32 processor may execute the following binary instruction as expressed in machine language (see x86 assembly language):
- Binary: 10110000 01100001 (Hexadecimal: B0 61)
The equivalent assembly language representation is simpler to remember (example in the Intel synta x, more mnemonic):
Means of this instruction:
- Move the value 61h (or 97 decimal; the h-suffix means hexadecimal; into the processor register named "AL".
The mnemonic "mov" represents the opcode 1011 which moves the value in the second operand into the register specified by the first operand. The mnemonic was selected by the instruction set designer to abbreviate "move", by making it easier for the programmer to remember. A comma-separated list of parameters or arguments follows the opcode; it is a typical assembly language statement.
In practice many programmers drop the word mnemonic and which is technically incorrect, call "mov" an op code. When they do this they are referring to the underlying binary code which it represents. To put it as a way, a mnemonic such as "mov" is not an op code, but it symbolizes an op code, one can refer to "the op code mov" for instance when one intends to refer to the binary op code it symbolizes rather than to the symbol -- the mnemonic -- itself. As few modern programmers have required being attentive of actually what binary patterns are the op codes for specific instructions, the distinction has in practice become a bit blurred among programmers but not among processor designers.
An assembler transforms assembly language into machine language, and the reverse by disassemble. There is typically a one-to-one correspondence between assembly statements unlike in high-level languages, and the machine language instructions. But, in some cases, an assembler can provide pseudo instructions which expand into various machine language instructions to provide commonly required functionality. For instance, for a machine that lacks a "branch if greater or equal" instruction, an assembler can be used a pseudo instruction that expands to the machine's "branch if zero (on the result of the set instruction)"and "set if less than" and. All the full-featured assemblers also provide a rich macro language mostly (discussed below) which is used by programmers and vendors to generate more difficult code and data sequences.
Each processor architecture and computer architecture has its own machine language. On this level, each instruction is enough simple to be executed by using a relatively small number of electronic circuits. Computers differ by the type and number of operations they support. For instance, a new 64-bit machine would have different circuitry from a 32-bit machine. They can also have different sizes and numbers of registers and different representations of data types in storage. Whereas most general-purpose computers are able to carry out really the same functionality, the ways they do so differ; the equivalent assembly languages reflect these differences.
Multiple sets of assembly-language syntax or mnemonics may exist for a single instruction set, typically instantiated in different assembler programs. In these types of cases, the most popular one is usually that supplied by the manufacturer and used in its documentation.