What is M- notation?

The M- notation refers to a way of representing characters that are produced by pressing the Meta key (often the Alt key on many keyboards) in combination with another key. This notation is commonly used in text editors and command-line interfaces to denote special characters or commands.

Key Points about M- Notation:

1. Meta Key: The Meta key is a modifier key that can be used to access additional commands or characters. On many systems, this is typically the Alt key.

2. Character Representation: When you see a character prefixed with M-, it indicates that the character is generated by holding down the Meta key while pressing another key. For example:

   • M-x means "press the Meta (or Alt) key and then press x."
   • M-a means "press the Meta (or Alt) key and then press a."

3. ASCII Values: In terms of ASCII, characters represented with M- are often in the range of 128-255. For example, M-a corresponds to the ASCII value of 97 (which is a) plus 128, resulting in 225.

4. Usage in Editors: This notation is frequently used in text editors like Emacs, where M- commands are common for executing various functions or commands.

Example:

If you see a command like M-x compile, it means you should press Alt (or Meta) and then x, which will typically prompt you to enter a command to compile your code.

In summary, M- notation is a way to indicate that a character or command is accessed through the Meta key, allowing for a broader range of inputs and commands in text-based interfaces.