Assembly code is converted into executable machine code by a utility program referred to as an assembler. The conversion process is referred to as assembly, as in assembling the source code. Assembly language usually has one statement per machine instruction (1:1), but comments and statements that are assembler directives, macros, and symbolic labels of program and memory locations are often also supported.
The term "assembler" is generally attributed to Wilkes, Wheeler and Gill in their 1951 book The preparation of programs for an electronic digital computer, who, however, used the term to mean "a program that assembles another program consisting of several sections into a single program".
Each assembly language is specific to a particular computer architecture and sometimes to an operating system. However, some assembly languages do not provide specific syntax for operating system calls, and most assembly languages can be used universally with any operating system, as the language provides access to all the real capabilities of the processor, upon which all system call mechanisms ultimately rest. In contrast to assembly languages, most high-level programming languages are generally portable across multiple architectures but require interpreting or compiling, a much more complicated task than assembling.

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