## 17 Machine Descriptions

A machine description has two parts: a file of instruction patterns (`.md` file) and a C header file of macro definitions.

The `.md` file for a target machine contains a pattern for each instruction that the target machine supports (or at least each instruction that is worth telling the compiler about). It may also contain comments. A semicolon causes the rest of the line to be a comment, unless the semicolon is inside a quoted string.

See the next chapter for information on the C header file.