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From: wsr@lmi-angel.UUCP
Newsgroups: comp.terminals
Subject: Re: A Question About the Infamous VT100 Line Wrap Bug
Message-ID: <121@lmi-angel.UUCP>
Date: Mon, 26-Jan-87 12:08:56 EST
Article-I.D.: lmi-ange.121
Posted: Mon Jan 26 12:08:56 1987
Date-Received: Fri, 30-Jan-87 05:39:10 EST
References: <1504@blia.BLI.COM>
Reply-To: wsr@lmi-angel.UUCP (Wolfgang Rupprecht)
Organization: LISP Machine, Inc (Cambridge Engineering HQ)
Lines: 27

In article <1504@blia.BLI.COM> forrest@blia.BLI.COM (Jon Forrest) writes:
>Could someone summarize the bug with vt100 terminals having
>to do with line wrapping. I recall it has something to do with
>what happens when a certain character appears in column 79 (80?).

It sounds like you are refering to the infamous 'xn' bug (the bug
described by the 'xn' entry in termcap.)  This is only a problem in
line wrap mode (ie. if you are using the 'am' flag in your termcap,
and have line wrap set on your terminal).

The bug is that the cursor 'sticks' in the 80th column after you enter
the 80th character. For forward motion the cursor is in the 81's
column, eg. the next character will end up in the 1st column of the
next line. For reverse motion (deleting and/or arrow keys) the cursor
is in the 80th column, and the first deletion or other backwards
motion takes you to the 79th column. Where you are depends on where
you are going.  (It's not a bug it's an undocumented feature !!) :-)

What is sad, is that this bug was actually introduced (by some
misguided fool) in an attempt to solve another problem. With line wrap
on and the normal behavior on column 80, it would have been impossible
to write the last character on the last line of the screen without the
terminal scrolling. (Now it's impossible to write the last character
on any line..... without lots of extra work.) Why I wonder, didn't
they just leave it alone, and tell people to turn off line wrap?
-- 
Wolfgang Rupprecht	{harvard|decvax!cca|mit-eddie}!lmi-angel!wsr