Re: character sssssset

[dempson@actrix.gen.nz (David Empson)]

Adalbert Goertz <gj380@cleveland.Freenet.Edu> wrote:

> character set:
> 
> Can someone explaint to me what is meant by
> 
> 7-bit character set and 8-bit character set.

The ASCII character set that is used on most computers only defines 128
characters, therefore requiring 7 bits to hold a character code.

Since most computers can easily deal with 8 bits per character, various
computers have different ideas about what to do with the extra 128
characters that can be represented in a single byte.

On the Apple II text screen, the extra 128 characters are used by the
screen hardware to display inverse, flashing and/or mousetext, depending
on the computer model, firmware version and video ROM.  (The character
set is backwards on the Apple II screen - "normal" ASCII is characters
128-255, inverse/flash/mousetext are in characters 0-127.  This is
hidden from the user most of the time.)

Foreign languages are handled on the text screen by replacing rarely
used symbols with the more common foreign letters.

On the PC, there are multiple standards for what to put into the extra
characters.  MS-DOS has a mechanism called "code pages" that allow the
extra characters to be redefined.  This is mostly used for extra
characters required in languages other than English.  The standard code
page used on a US-based PC has mathematical symbols, some foreign
letters, and box drawing characters.

On the Mac, the extra characters are a reasonable selection of foreign
letters.  The precise characters that are available depend on the font
used, and some fonts redefine a variety of characters to make room for
special symbols.

IIgs fonts work like the Mac.

As you can see, this is all rather confusing.  If you have an arbitrary
chunk of 8-bit text, you have no way of knowing what the characters from
128 to 255 are supposed to be, short of trying various standards until
you get something that makes sense.

One of the features of the MIME standard is to provide a mechanism to
define the symbol set used for characters 128-255.  The most commonly
used standard on the Internet is called the "ISO Latin 1" alphabet,
which is sometimes referred to as ISO-8859-1, if I remember right.

Another alphabet could be used if special characters are required that
are not represented in ISO Latin 1.

MIME also deals with one of the major limitations of Internet News and
E-Mail: you cannot reliably include 8-bit data in a news article or mail
message (particularly control characters).  MIME defines a mechanism
called "quotable printable", in which characters that cannot be
transmitted are encoded as an equals sign followed by two hexadecimal
digits.  (An equals sign must itself be encoded, by using two equals
signs.)

The Unicode standard is also worth mentioning at this point. It uses 16
bits per character instead of 8 bits.  The first 256 characters in
Unicode are the same as ISO Latin 1, including ASCII in the first 128
characters.  Everything numbered higher than 256 is well defined, and
allocated to various symbol sets and/or languages.  I haven't seen a
detailed breakdown, but I have read that a limited subset of at least
one variant of Chinese is included, which uses up something like half
the available characters.

Unicode is gradually developing as a usable standard.  It is already
supported at the file system level on the Mac and PC (HFS Plus and NTFS
respectively).

> Is there a way to handle 8-bit characters in the
> Apple//GS world and how?

Anything generated on a Mac should display fine in a GS/OS desktop
program, assuming standard fonts were used.

For anything else, including display of Mac files on the text screen,
you are probably out of luck.  Many ISO-Latin-1 characters can be
translated to Mac characters in the range 128 to 255, so you could
display most text by going through a character remapping utility.

Another option is to use a special font which has the appropriate
characters in the positions from 128 to 255.  For example, you could
display an IBM PC text file which uses box drawing characters by using a
font which has the box drawing characters in the appropriate positions.

> I am told the Netscape allows  8-bit characters.

Sure - no problem.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand