• A character constant stands for one character in one byte and its value may be used in numeric expressions.
• String constants (properly called string literals) are potentially many bytes and their values may not be used in arithmetic expressions.

Characters

A single character may be written as a single quote immediately followed by that character. The same escapes apply to characters as to strings. So if you want to write the character backslash, you must write ’\\ where the first \ escapes the second \. As you can see, the quote is an ‘acute’ accent, not a ‘grave’ accent (`). A newline immediately following an acute accent is taken as a literal character and does not count as the end of a statement. The value of a character constant in a numeric expression is the machine’s byte-wide code for that character. as assumes your character code is ASCII: ’A means 65, ’B means 66, and so on.

Strings

A string is written between double-quotes. It may contain double-quotes or null characters. The way to get special characters into a string is to escape these characters: precede them with a backslash (\) character. For example \\ represents one backslash: the first \ is an escape which tells as to interpret the second character literally as a backslash (which prevents as from recognizing the second \ as an escape character). The complete list of escapes follows.
 

\b
Mnemonic for backspace; for ASCII this is octal code 010.

\f

Mnemonic for FormFeed; for ASCII this is octal code 014.

\n

Mnemonic for newline; for ASCII this is octal code 012.

\r

Mnemonic for carriage-Return; for ASCII this is octal code 015.

\t

Mnemonic for horizontal Tab; for ASCII this is octal code 011.

\ digit digit digit

An octal character code. The numeric code is 3 octal digits. For compatibility with other Unix systems, 8 and 9 are accepted as digits: for example, \008 has the value 010, and \009 the value 011.

\x hex-digit hex-digit

A hex character code. The numeric code is 2 hexadecimal digits. Either upper or lower case x works.

\\

Represents one \ character.

\"

Represents one " character. Needed in strings to represent this character, because an unescaped " would end the string.

\ anything-else

Any other character when escaped by \ gives a warning, but assembles as if the \ was not present. The idea is that if you used an escape sequence you clearly didn’t want the literal interpretation of the following character. However as has no other interpretation, so as knows it is giving you the wrong code and warns you of the fact.