The main use of macros is to insert assembler language statements into a source program.

You call a named sequence of statements (the macro definition) by using a macro instruction, or macro call. The assembler replaces the macro call by the statements from the macro definition and inserts them into the source module at the point of call. The process of inserting the text of the macro definition is called macro generation or macro expansion. Macro generation occurs during conditional assembly.

The expanded stream of code then becomes the input for processing at assembly time; that is, the time at which the assembler translates the machine instructions into object code.