An identifier consists of a case-sensitive sequence of alphanumeric characters (A-Z, a-z, 0-9), and the following special characters:
. (period)
_ (underscore)
$ (dollar sign)
@ (at sign)
? (question mark)
Identifiers can be up to 31 characters long, and the first character cannot be numeric (0-9).
If an identifier is not defined to the assembler (only referenced), the assembler assumes that the identifier is an external symbol. The assembler then treats the identifier like a name specified by an .extern directive.
If the identifier is defined to the assembler and the identifier has not been specified as global, the assembler assumes that the identifier is a local symbol.