Assembly language




While macro instructions can be defined by a programmer for any set of native assembler program instructions, typically macros are associated with macro libraries delivered with the operating system allowing access to operating system functions such as

  • peripheral access by access methods (including macros such as OPEN, CLOSE, READ and WRITE)
  • operating system functions such as ATTACH, WAIT and POST for subtask creation and synchronization. Typically such macros expand into executable code, e.g., for the EXIT macroinstruction,
  • a list of define constant instructions, e.g., for the DCB macro—DTF (Define The File) for DOS—or a combination of code and constants, with the details of the expansion depending on the parameters of the macro instruction (such as a reference to a file and a data area for a READ instruction);
  • the executable code often terminated in either a branch and link register instruction to call a routine, or a supervisor call instruction to call an operating system function directly.

In older operating systems such as those used on IBM mainframes, full operating system functionality was only available to assembler language programs, not to high level language programs (unless assembly language subroutines were used, of course), as the standard macro instructions did not always have counterparts in routines available to high-level languages.

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