On Wed, Mar 11, 2020 at 07:28:06AM +1100, Chris Angelico wrote:
> That's exactly what "ASCII compatible" means. Since ASCII is a
> seven-bit encoding, an encoding is ASCII-compatible if (a) every ASCII
> character is represented by the corresponding byte value, and (b)
> every seven-bit value represents that ASCII character.
Sorry Chris, that explanation left me more confused than I started :-(
Let me have a go...
The ASCII encoding is a mapping between *seven-bit numeric values* and
128 distinct characters, some of which are human-readable:
A = 1000001
B = 1000010
a = 1100001
and some of which are considered to be "binary" characters:
NUL = 0000000
SOH = 0000001
DEL = 1111111
In practice today, seven bits are inconvenient, so these are always
padded with a leading 0 bit.
An encoding is compatible with ASCII if, and only if, the following is
true:
* all 128 of the ASCII characters are handled by the encoding;
* each of those characters are mapped to the same eight-bit value as
the ASCII encoding would use (including the leading 0 bit);
* no non-ASCII character is mapped to one of those eight-bit values;
* or to something which could be confused with one of those eight-bit
values by a naive application that processed them a byte at a time.
E.g. if an encoding mapped some character ∇ to the 16-bit value:
01000001 11110000
that would not be considered ASCII-compatible, because the first byte
would be interpreted as "A" by a naive application.
Most (all?) of the "extended ASCII" eight-bit encodings are ASCII
compatible, because they use only bytes with a leading 1 for the
non-ASCII characters.
UTF-8 is also ASCII compatible.
UTF-16 and UTF-32 are *not* ASCII compatible.
How did I go?
--
Steven
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