What is an Assembler?

The first idea a new computer programmer has of how a computer works is learned from a programming language. Invariably, the language is a textual or symbolic method of encoding programs to be executed by the computer. In fact, this language is far removed from what the computer hardware actually "understands". At the hardware level, after all, computers only understand bits and bit patterns. Somewhere between the programmer and the hardware the symbolic programming language must be translated to a pattern of bits. The language processing software which accomplishes this translation is usually centered around either an assembler, a compiler, or an interpreter. The difference between these lies in how much of the meaning of the language is "understood" by the language processor.
An interpreter is a language processor which actually executes programs written in its source language. As such, it can be considered to fully understand that language. At the lowest level of any computer system, there must always be some kind of interpreter, since something must ultimately execute programs. Thus, the hardware may be considered to be the interpreter for the machine language itself. Languages such as BASIC, LISP, and SNOBOL are typically implemented by interpreter programs which are themselves interpreted by this lower level hardware interpreter.
Interpreters running as machine language programs introduce inefficiency because each instruction of the higher level language requires many machine instructions to execute. This motivates the translation of high level language programs to machine language. This translation is accomplished by either assemblers or compilers. If the translation can be accomplished with no attention to the meaning of the source language, then the language is called an assembly or low level language, and the translator is called an assembler. If the meaning must be considered, the translator is called a compiler and the source language is called a high level language. The distinction between high and low level languages is somewhat artificial since there is a continuous spectrum of possible levels of complexity in language design. In fact, many assembly languages contain some high level features, and some high level languages contain low level features.
Since assemblers are the simplest of symbolic programming languages, and since high level languages are complex enough to be the subject of entire texts, only assembly languages will be discussed here. Although this simplifies the discussion of language processing, it does not limit its applicability; most of the problems faced by an implementor of an assembly language are also faced in high level language implementations. Furthermore, most of these problems are present in even the simplest of assembly languages. For this reason, little reference will be made to the comparatively complex assembly languages of real machines in the following sections.