What is macro assembler?

An assembler that can perform macro substitution and expansion. The programmer can define a macro that consists of several statements and then use the macro name later in the program, thus avoiding having to rewrite the statements. For example, a macro called swap exchanges the values of two variables: After defining swap, the programmer can then insert an instruction such as “swap a, b” in the assembly language program. While assembling, the assembler replaces the instruction with the statements within the macro that swap the values of the variables a and b.