Conditional assembly

Conditional assembly (originally in assembly language, later called conditional compilation in other programming languages)) is a mechanism used to produce multiple versions of a program's object code from a single set of source code.

It consists of conditional statements, inserted into the source, which conditionally direct the assembler/compiler to ignore source until a matching statement further down the source directs it to resume normal processing. The operation/non-operation of the conditional control statements are controlled by expressions in the statements, using variables which are assigned values in a number of ways (e.g. in source files, on the command line, etc.)