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Path: utzoo!watmath!clyde!rutgers!cbmvax!vu-vlsi!colin
From: colin@vu-vlsi.UUCP
Newsgroups: comp.terminals
Subject: Re: A Question About the Infamous VT100 Line Wrap Bug
Message-ID: <581@vu-vlsi.UUCP>
Date: Fri, 23-Jan-87 13:21:37 EST
Article-I.D.: vu-vlsi.581
Posted: Fri Jan 23 13:21:37 1987
Date-Received: Sat, 24-Jan-87 06:45:18 EST
References: <1504@blia.BLI.COM>
Reply-To: colin@vu-vlsi.UUCP (Colin Kelley)
Organization: Villanova Univ. EE Dept.
Lines: 28

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's not a bug, it's a feature!  Honest.  First off, this only applies when
line wrap is set on in SETUP.  Here goes:

The behavior is that when a character is printed in column 80, most terminals
advance the cursor to column 1 of the next line.  The vt100 and its
descendants don't do this--they just make an internal note somewhere that the
cursor _should_ be on column 1 of the next line; the blinking cursor is left
where it was, in column 80.  Now if a printable character arrives next, it
is put in column 1 of the next line, and the cursor is advanced to column 2,
just the way line wrap behaves on most "normal" terminals.  However, if a
newline arrives next (carriage return followed by line feed) the cursor winds
up in column 1 of the next line, not two lines down as you'd expect on "normal"
terminals...

Why this behavior?  Well, for one reason, it allows the last character position
on the screen (line 24 column 80) to be used.  Also, it works pretty well when
just dumping lines of text to the screen separated by newlines (e.g. the VMS
TYPE command):  lines of exactly 80 characters will look fine, rather than
leaving an extra blank line following...

I hope this helps!

	-Colin Kelley  ..{cbmvax,pyrnj,psuvax1,bpa}!vu-vlsi!colin