ASCII is an acronym for the American Standard Code for Information
Interchange. Pronounced ask-ee, ASCII is a character and control code
encoding standard used by computers, telecommunications equipment, and other
devices. There are 128 valid codes in ASCII, including 95 printable
characters (including the "space" character) and 33 control codes.
The 8-bit Atari computers use a modified version of ASCII called Atari ASCII,
or ATASCII. The ATASCII character set, contained in Operating System (OS)
ROM, includes 256 printable characters.
The XL OS contains both the standard ATASCII character set (as found in the
400/800 OS) and an alternate International Character Set, where 29 ATASCII
standard "graphics characters" are replaced primarily with characters with
European accent marks.
ATASCII printable characters are a functional superset of ASCII, with a few
exceptions: ATASCII lacks the left curly bracket, right curly bracket, tilde,
and grave accent printable ASCII characters. The similarity between ATASCII
and ASCII character coding facilitates exchange of text between the Atari and
other computers or devices also using character encodings based on ASCII.
16 of the ATASCII character codes also have control code functions when
processed by the E: Screen Editor device of the Atari OS. ATASCII control
codes are quite different from ASCII control codes. The most important
functional difference between ATASCII and other variations of ASCII is how a
"newline" (line ending, end of line (EOL), line break) is coded. The Atari's
ATASCII uniquely uses the single decimal character code 155 to indicate a
newline. Thus, when exchanging text files between the Atari and other
computing devices a "text conversion" is necessary in order for newlines to be
recognized as expected both before and after transmission.
The Wikipedia article on ATASCII provides links to several ATASCII/ASCII
character tables, and a list of the ATASCII control codes.