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In computing, a compiler is a computer program that transforms source code written in a programming language or computer language (the source language ), into another computer language (the target language, often having a binary form known as object code or machine code ).

The Production Quality Compiler-Compiler, in the late 1970s, introduced the principles of compiler organization that are still widely used today (e.g., a front-end handling syntax and semantics and a back-end generating machine code).
Software for early computers was primarily written in assembly language, and before that directly in machine code.

It was the world's first self-compiling compiler – the compiler was first coded in simplified form in assembly language (the bootstrap ), then re-written in its own language and compiled by the bootstrap, and finally re-compiled by itself, making the bootstrap obsolete.

Compiler construction is normally considered as an advanced rather than a novice programming task, mainly due to the quantity of code needed (and the difficulties of grokking this amount of code) rather than the difficulty of any particular coding constructs.
To this most books about compilers have some blame.