Macro Instructions in Assembly Programming

Macro Instructions

• Macro instructions are single-line abbreviations for groups of instructions.
• For every occurrence of a macro instruction within a program, the instruction must be replaced by the entire block.

Advantages of Macros

• Simplify and reduce the amount of repetitive coding.
• Reduce the possibility of errors caused by repetitive coding.
• Make an assembly program more readable.

Macro Processors

• A preprocessor can be any program that processes its input data to produce output, which is used as an input to another program.
• The outputs of the macro processors are assembly programs that become inputs to the assembler.
• The macro processor may exist independently and be called during the assembling process or be a part of the assembler implementation itself.

Difference between Macro and Subroutine

• Macro name in the mnemonic field leads to expansion only.
• Macro is completely handled by the assembler during assembly time.
• Subroutine name in a call statement in the program leads to execution.
• Subroutines are completely handled by the hardware at runtime.
• Macro definition and macro expansion are executed by the assembler.
• Hardware executes the subroutine call instruction.

Macro Definition and Call

• A macro consists of a name, a set of formal parameters, and a body of codes.
• A macro can be defined by enclosing a set of statements between a macro header and a macro end statement.
• The formal structure of a macro includes:
  • Macro prototype statement: specifies the name of the macro and name and type of formal parameters.
  • Model statements: specify the statements in the body of the macro from which assembly language statements are to be generated during expansion.
  • Macro preprocessor statement: specifies the statement used for performing auxiliary function during macro expansion.
• A macro can be called by writing the name of the macro in the mnemonic field of the assembly language.
• The syntax of a typical macro call can be of the following form: [ [,…]].
• The MACRO directive in the mnemonic field specifies the start of the macro definition.
• The MEND directive specifies the end of the macro definition.