DEALING WITH CHARACTERS The input to an assembler consists of a stream of characters which represent assembly information in several different ways integers, real numbers, labels, quoted characters and strings, register names, and various kinds of punctuation. If character processing is mixed in with algorithms for building symbol tables or algorithms for code generation, the result is a complex mess that would challenge even the best programmers. The problem is that this organization (or disorganization) forces programmers to deal with two distinct kinds of abstraction simultaneously. The result is difficult algorithm development, a large number of errors, and difficulty in debugging. These problems are magnified if the assembler requires maintenance at a later time. A good general design principle is to assign responsibility for different kinds of abstractions to different program modules. This principle is crucial for large designs, but is a good practice for smaller designs as well. Dividing responsibilities for different kinds of abstractions into different modules allows programmers to focus their attention on one aspect of the problem at a time. For an assembler, the implication is that there should be a module dedicated to handling text at the level of characters. This module is called a lexical analyser or scanner.
