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From: gwyn@brl-smoke.UUCP
Newsgroups: comp.terminals
Subject: Re: A Question About the Infamous VT100 Line Wrap Bug
Message-ID: <5604@brl-smoke.ARPA>
Date: Sun, 8-Feb-87 00:42:29 EST
Article-I.D.: brl-smok.5604
Posted: Sun Feb  8 00:42:29 1987
Date-Received: Mon, 9-Feb-87 02:39:28 EST
References: <1504@blia.BLI.COM> <121@lmi-angel.UUCP> <581@vu-vlsi.UUCP> <5559@brl-smoke.ARPA> <103@Mannix.iros1.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Distribution: na
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 30

In article <103@Mannix.iros1.UUCP> burj@iros1.UUCP (Justin Bur) writes:
>... the "xn" capability was modified to work with vt100s.

No, it wasn't.  Don't believe everything you read.  "xn" means that
the terminal ignores a line-feed immediately after an auto-margin wrap,
no more and no less.  This does not correctly characterize the DEC VT100.

>...  In particular, with wrap turned
>on, vi works fine whether the termcap has "am" and "xn" set or not.
>(Just don't set "am" without also setting "xn".)

No, no, no!  I don't know whether "vi" happens to work, but the VT100
in general violates the simple model for auto-wrap assumed by termcap,
as several people have discussed already.  Its actual behavior is very
state-dependent, and attempting to specify "am" exacerbates the problem.
The VT100 does NOT behave in a way that could reasonably permit one to
specify "am"; in particular, it has not wrapped after displaying the
character in the last column, so its cursor is in the wrong place for
an "am" terminal.  (Sometimes the VT100 will compensate for this and
sometimes it won't, but in any case the visual result is wrong.)

The only complete solution for the VT100 is to NOT specify "am", and if
you leave auto-wrap enabled (as most people do) then set "co#79", so
that the VT100 firmware bug is not triggered.

Don't get me wrong; I've been rather fond of VT100s, but they really
do have a bug in their auto-wrap logic.

	- Douglas A. Gwyn
	  4.3BSD TERMCAP(5) editor (with assistance of others)