Macro instructions are single instructions in computer programming that are replaced by a block of code. They were first used in assembly language programming to reduce repetitive code and provide commonly used instruction sets. Macro instruction libraries were developed to simplify programming and ensure consistent operations. This led to higher-level languages with their own libraries for extensive functionality.
A macro instruction in computer programming is a single instruction that, when a program is compiled, is replaced by a predefined block of code. The term was originally applied to instructions used in assembly language programming, where long, repetitive code was sometimes needed for very simple operations. The development of macro instructions was one of the first steps towards creating a language based on a library of representative functions so that minimal actions could be collected together in a single command call in a separate program. Early libraries of macro instructions were more frequently provided by hardware and software manufacturers to help programmers use and access functionality correctly.
Programmers used macro instructions for a variety of reasons, but the most common were to reduce the amount of repetitive code in the program, eliminate the possibility of error within the macro code, and provide commonly used instruction sets so that different programmers could use the same macro instruction. In assembly language, every single step necessary to obtain a certain result must be written explicitly. An example would be a program that added three numbers together.
The addition operator in higher-level languages is often taken for granted, but the operation of adding three numbers in assembly language requires at least six separate lines of code. The first two lines load numbers into fields called registers which actually correspond to physical circuitry inside the computer. The third line adds the two registers and a fourth line stores the result in another register. The fifth line loads the last number to be added into a register and the final statement adds the previous result to the third number.
This sequence of operations can easily be turned into a macro instruction, so only one line of code is needed, along with the three numbers to add as parameters. When the program is finally compiled by an assembler and turned into machine code, actions called preprocessing are performed before the assembly is executed. The preprocessor takes the macro statement and parameters and expands them into the required lines of code, substituting the parameters for representative placeholders within the actual macro code.
After the use of macro instructions became widespread, many commonly used blocks of code would be packaged into macro instruction libraries. These libraries contained several macros to simplify assembly language programming and to ensure that certain operations were performed consistently between different programs. The extensive use of macro instruction libraries eventually led to higher level languages that relied on their own libraries to provide extensive functionality with far less repetitive programming requirements.
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