A macro instruction is a request to the assembler program to process a predefined sequence of instructions called a macro definition. From this definition, the assembler generates machine and assembler instructions, which it then processes as if they were part of the original input in the source module.

IBM supplies macro definitions for input/output, data management, and supervisor operations that you can call for processing by coding the required macro instruction. (These IBM-supplied macro instructions are described in the applicable Macro instructions manual.)

You can also prepare your own macro definitions, and call them by coding the corresponding macro instructions. Rather than code all this sequence each time it is needed, you can create a macro instruction to represent the sequence and then, each time the sequence is needed, code the macro instruction statement. During assembly, the sequence of instructions represented by the macro instruction is inserted into the source program.

A complete description of the macro facility, including the macro definition, the macro instruction, and the conditional assembly language, is given in Introduction to macro language through MHELP instruction.