(Or "character encoding scheme") A mapping of
binary values to code positions and back; generally a 1:1
(bijective) mapping.
In the case of
ASCII, this is generally a f(x)=x mapping:
code point 65 maps to the byte value 65, and vice versa. This
is possible because ASCII uses only code positions
representable as single bytes, i.e., values between 0 and 255,
at most. (
US-ASCII only uses values 0 to 127, in fact.)
Unicode and many
CJK coded character sets use many more
than 255 positions, requiring more complex mappings: sometimes
the characters are mapped onto pairs of bytes (see
DBCS).
In many cases, this breaks programs that assume a one-to-one
mapping of bytes to characters, and so, for example, treat any
occurrance of the byte value 13 as a
carriage return. To
avoid this problem, character encodings such as
UTF-8 were
devised.