NWG/RFC#373 14 July 1972
NIC 11058 SU-AI
ARBITRARY CHARACTER SETS
by John McCarthy
It would be nice to be able to have documents stored in computers that
could include arbitrary characters and to be able to display them on
any CRT screen, edit them using any keyboard, and print them on any
printer. The object of this memorandum is to suggest how to get there
from here with special reference to the ARPA network.
Where are we now?
(1) At present, there is 96 character ASCII, and everyone agrees that
it should be included in any larger set.
(2) Many installations are dependent on 64 character sets which do not
even include the lower case latin alphabet.
(3) At the Stanford Artificial Intelligence Laboratory, we have a 114
character set that includes 96 character ASCII and which is
implemented in our keyboards, displays, and line printer
(4) Printers are becoming available that get their character designs
out of memory, for example, the Xerox XGP printer, one of which we are
getting.
(5) The IMLAC type display has the character designs in main memory so
that changing the displayed set is just a matter of reloading the
memory.
(6) Many display systems share the character generator among many
display units. In some of these, e.g. the Datadisc, arbitrary sets
are probably feasible (using kludgery to be described later), but in
other systems, e.g. our III's arbitrary sets are not feasible.
One possible approach to communication in eXPanded character sets is
to prodUCe an expanded standard set of characters, perhaps using 8 or
9 bits and expect new equipment to implement this set. This approach
has the disadvantage that it will be very hard to get agreement on
what the next step should be, and even if formal agreement is
realized, many groups will find it in their interest to ignore the
standard.
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ARBITRARY CHARACTER SETS by John McCarthy
Therefore, I would like to suggest that the next step be to arbitrary
character sets. I suggest implementing this in the following way:
(1) There be established a registry of characters. Anyone can
register a new character. Each character has a unique number, 17 bits
should be enough even to include Chinese. Besides this, each
character has a name in ASCII usually mnemonic. Finally, the
character has a design which is a picture on a 50 by 50 dot matrix.
(2) Besides the registry of characters, there is a registry of
characters sets, which different groups are using for different
classes of documents. A registered character set has a registry
number and a table giving the correspondence between the character
codes as bit sequences and the registered character numbers.
(3) Associated with a document is a statement of the character code
used therein. This may be one of the registered codes or it may
contain in addition modifications described by an auxiliary table
giving the code correspondence with registered character numbers. A
character code may have an escape character that says that the next
character is described by its registry number. The statement of the
character code may be a header on the document or the receiver may
have to learn it by some other means, e.g. because its library
catalog entry contains this information.
(4) Devices such as printers and displays draw characters in different
ways and standardization doesn't seem feasible at present. Therefore,
it is necessary to provide a way of going from the standard
description of a character using a 50 by 50 dot matrix to whatever
method the device uses. This is up to the programmers who are
supporting the device. Some may choose to manually create files
describing how registered characters are implemented. They may find
it too much work to provide for all the characters and to update their
files when new characters are registered. Others will provide
programs for going from the registered descriptions to descriptions
compatible with their implementations. Perhaps most will hand tailor
the characters most used and provide a program for the others.
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NWG/RFC# 373 JMC 14-JUL-72 12:41 11058
ARBITRARY CHARACTER SETS by John McCarthy
(5) The easiest device to handle is the line printer because it is
slow. At the beginning of the print job, the SPOOL program will look
up the character set and load the printer's memory with the character
designs used in the particular document. Sometimes, it may have to go
through the network to one of the computers that stores the registry
in order to find out what to do.
(6) Display systems that have a character memory for each display unit
can be handled in about the same way. Users will occasionally
experience delays when the display programs are surprised by
unfamiliar characters.
(7) Display systems that share character memories require more
complicated treatment. The object is to keep the memory large enough
to keep all the characters that the current set of users is using and
to handle the required table lookups from the different character
codes in a nice way. There will be limitations on the diversity of
character sets that can be in use simultaneously. Systems like the
Datadisc that only look up the character when it is first written can
be extended to work with large sets. Systems that have to look up
each character code 30 times per second in order to maintain the
display won't work so well.
I have no special ideas about how to make keyboards adaptable to
arbitrary sets. Each user may have to fend for himself.
In this memorandum so far, I have ignored typography, i.e. the fact
that in printed documents the same letter may be printed in many
fonts. Perhaps, each character in each font will require a separate
registered description, but with a constant difference between the
numbers of the same character in different fonts. Installations will
again have to decide what font distinctions they will implement.
Some other issues that might be considered are whether means can be
provided to adapt texts automatically to the line and page lengths of
the different devices.
It seems to me most likely that the typographical problems cannot be
solved at this time, and it would be best to adopt conventions for
registering character designs at this time, and leave typography for
later.
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NWG/RFC# 373 JMC 14-JUL-72 12:41 11058
ARBITRARY CHARACTER SETS by John McCarthy
In my opinion, there is no real obstacle to establishing the registry
in the ARPA network now, getting the standards organization to work,
and being able to exchange documents in extended character sets as
soon as the various installations can acquire the printers and display
devices.
It is the present policy of the Stanford Artificial Intelligence
Laboratory to acquire no more devices that are wedded to fixed
character sets.
[ This RFCwas put into machine readable form for entry ]
[ into the online RFCarchives by BBN Corp. under the ]
[ direction of Alex McKenzie. 1/97 ]
