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A macro instruction is a line of computer program coding that results in one or more lines of program coding in the target programming language, sets variables for use by other statements, etc. In the mid-1950s, when assembly language programming was commonly used to write programs for digital computers, the use of macro instructions was initiated for two main purposes: to reduce the amount of program coding that had to be written by generating several assembly language statements from one macro instruction and to enforce program writing standards, e.g. specifying input/output commands in standard ways. Macro instructions were effectively a middle step between assembly language programming and the high-level programming languages that followed, such as FORTRAN and COBOL. Two of the earliest programming installations to develop "macro languages" for the IBM 705 computer were at Dow Chemical Corp. in Delaware and the Air Material Command, Ballistics Missile Logistics Office in California. A macro instruction written in the format of the target assembly language would be processed by a macro compiler, which was a pre-processor to the assembler, to generate one or more assembly language instructions to be processed next by the assembler program that would translate the assembly language instructions into machine language instructions.
By the late 1950s the macro language was followed by the Macro Assemblers. This was a combination of both where one program served both functions, that of a macro pre-processor and an assembler in the same package. This allowed assembly language programmers to implement their own macro-language and allowed limited portability of code between two machines running the same CPU but different operating systems, for example, early versions of MSDOS and CPM-86. The macro library would need to be written for each target machine but not the overall assembly language program. Note that more powerful macro assemblers allowed use of conditional assembly constructs in macro instructions that could generate different code on different machines or different operating systems, reducing the need for multiple libraries.
In the 1980s and early 1990s, desktop PCs were only running at a few MHz and assembly language routines were commonly used to speed up programs written in C, Fortran, Pascal and others. These languages, at the time, used different calling conventions. Macros could be used to interface routines written in assembly language to the front end of applications written in almost any language. Again, the basic assembly language code remained the same, only the macro libraries needed to be written for each target 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) and other 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, 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); he 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.
In modern operating systems such as Unix and its derivatives, operating system access is provided through subroutines, usually provided by dynamic libraries. High-level languages such as C offer comprehensive access to operating system functions, obviating the need for assembler language programs for such functionality.