A macro assembler is a specialized type of assembler that facilitates the use of macros, which serve as shorthand representations for groups of instructions. When a macro is invoked in an assembly language program, the macro assembler performs macro expansion by replacing the macro call with the corresponding sequence of machine language instructions that it represents.

The macro expansion process initiates when the assembler encounters a macro definition in the source code. This definition is then stored in a designated area of memory known as the macro library. Each macro definition comprises the macro’s name, the parameters it can accept, and the sequence of instructions it represents.

When the assembler detects a macro call in the source code, it searches for the corresponding macro definition in the macro library. It then substitutes the macro call with the defined sequence of instructions. If the macro accepts parameters, the assembler also integrates the actual parameter values from the macro call into the instruction sequence.

This macro expansion technique enables programmers to create more concise and readable code. Instead of repeatedly writing lengthy sequences of instructions for common operations, programmers can define a macro for that operation and invoke it when needed. This not only streamlines the coding process but also enhances code readability and maintainability.

However, it is essential to be cautious when using macros. While they can contribute to more compact and understandable code, they may also introduce complexity and make debugging more challenging if not managed properly. Since macros are expanded by the assembler rather than the compiler, any errors within macros can be more difficult to identify. Additionally, because each macro call is replaced with an actual sequence of instructions, excessive macro usage may lead to larger and slower programs. Consequently, while macros are a powerful programming tool, they should be employed judiciously.