ASSEMBLER MODULES Thus an assembler has four main modules, A scanner, A pass 1 module, A pass 2 module, and • A main program module. The responsibilities of these modules are described in more detail in the following sections. LEXICAL ANALYSIS THE SCANNER The primary purpose of the scanner module is processing characters into higher level units that are more meaningful for the pass 1 and pass 2 modules. These units are called tokens. The process of forming these groups is called lexical analysis. The need fora scanner arises inmost programs that deal with complex input. Part of the design of a scanner involves deciding precisely what a token is that is, deciding the level of the units that the scanner delivers to the pass modules. Some possibilities are discussed in Options for Scanner Interfaces.
