08/22/83  New features in the Multics Communications System

06/15/81 Multiple canonicalization algorithms:
Multics now supports two different algorithms for converting a typed
string into its canonical representation.  The algorithm in effect is
controlled by a new mode named "can_type".

The command
     set_tty -modes can_type=replace
causes only the last character typed in any column to appear in the
input line.  This is "replacement canonicalization" as described below
and is intended for use with most video display terminals (CRTs).

The command
     set_tty -modes can_type=overstrike
causes all characters typed in any column to appear in the input line.
This is "overstriking canonicalization" as described below and was the
only form of canonicalization present in previous versions of Multics.


Description:
The description of this mode as given in the write-up of tty_ in MPM
Communications is:

can_type=overstrike, can_type=replace
    specifies the method to be used to convert an input string to
    canonical form.  Canonicalization is only performed when the I/O
    switch is in "can" mode.  (Default is can_type=overstrike.)


overstriking canonicalization:
When can_type=overstrike is specified, the canonicalization algorithm
in use is equivalent to the only algorithm available in previous
Multics releases.  This algorithm assumes that the user is typing input
on a terminal which is capable of displaying several characters in a
single column.


replacement canonicalization:
When can_type=replace is specified, the canonicalization algorithm
assumes that the user is typing input on a terminal which is not
capable of displaying multiple characters in a single column.
(Examples of these terminals include most modern video (CRT)
terminals.)  Only the last character entered into a column is visible;
replacement canonicalization causes the canonical form of typed input
to contain only the last character entered in any column.

For example with can_type=replace, typing:

    This is a tsetBBBest of tpying text.BBBBBBBBBBByp<LF>

where B is a backspace character and <LF> is the line-feed character
will appear on the screen and be input as:

    This is a test of typing text.



When using can_type=replace, it is not possible to overstrike a
character with the erase character.  In other words, it is not possible
to delete a character in the middle of a typed line without
repositioning to the character in question and retyping the rest of the
line.

Therefore, the user may wish to disable the erase character when using
replacement canonicalization.  This may be accomplished by the command
line:
    set_tty -edit \400


When using can_type=replace, the following assumptions are made about
the nature of the terminal being used:

o Overstriking a character with any other printing character or a space
  causes the first character to be erased.

o Entering a tab character simply moves the cursor position to the next
  tab stop (column 11, 21, etc.) without erasing any intervening
  characters.


For more detail on this feature, refer to the "Typing Conventions"
section of the Multics Programmer's Manual -- Communications I/O (Order
No. CC92).


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Historical Background

This edition of the Multics software materials and documentation is provided and donated
to Massachusetts Institute of Technology by Group BULL including BULL HN Information Systems Inc. 
as a contribution to computer science knowledge.  
This donation is made also to give evidence of the common contributions of Massachusetts Institute of Technology,
Bell Laboratories, General Electric, Honeywell Information Systems Inc., Honeywell BULL Inc., Groupe BULL
and BULL HN Information Systems Inc. to the development of this operating system. 
Multics development was initiated by Massachusetts Institute of Technology Project MAC (1963-1970),
renamed the MIT Laboratory for Computer Science and Artificial Intelligence in the mid 1970s, under the leadership
of Professor Fernando Jose Corbato. Users consider that Multics provided the best software architecture 
for managing computer hardware properly and for executing programs. Many subsequent operating systems 
incorporated Multics principles.
Multics was distributed in 1975 to 2000 by Group Bull in Europe , and in the U.S. by Bull HN Information Systems Inc., 
as successor in interest by change in name only to Honeywell Bull Inc. and Honeywell Information Systems Inc. .

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Permission to use, copy, modify, and distribute these programs and their documentation for any purpose and without
fee is hereby granted,provided that the below copyright notice and historical background appear in all copies
and that both the copyright notice and historical background and this permission notice appear in supporting
documentation, and that the names of MIT, HIS, BULL or BULL HN not be used in advertising or publicity pertaining
to distribution of the programs without specific prior written permission.
    Copyright 1972 by Massachusetts Institute of Technology and Honeywell Information Systems Inc.
    Copyright 2006 by BULL HN Information Systems Inc.
    Copyright 2006 by Bull SAS
    All Rights Reserved