Re: LINE/PAGE SEPARATOR semantics in terminal emulators

From: Paul Keinanen (keinanen@sci.fi)
Date: Tue Nov 03 1998 - 01:15:49 EST


At 15:27 2.11.1998 -0800, Markus Kuhn wrote:

>I think, we should get a few brains together and write a Unicode
>technical report about VT100/ISO 6429 terminal emulators and what has to
>be considered when they are extended to support Unicode/UTF-8.

As long as I have been using VT-100 (and later VT-200 series) terminals, I
have found it unfortunate that these devices have implemented FF as a single
line feed. Fortunately, the operating systems that I have been using, have
all included features to expand the FF to at least a few line feeds
(originally intended for Teletype style devices with continuous roll of paper).

On some other terminals, FF moved the cursor to the top left without
clearing the screen (not very usable in my opinnion).

Others types cleared the screen and started to write text from the left top
corner until the page was full before starting scrolling. In my opinnion,
this helped readability, when the text was not constantly scrolling. This
would also be better in line with hard copy terminals.

>I suggest that such a technical report should roughly specify the
>following cursor control actions to be associated for these control
>characters (talking about left-to-right mode here only at the moment):
>
>CR: column=1
>LF: column=1, row++
>FF: column=1, row++
>LS: column=1, row++
>PS: column=1, row+=2

While on a hardcopy device the FF and PS should have the same semantics
(move past the paper perforation on a tractor feed system or load a new
sheet of paper), I hope that this is not taken as an example to justify
using on display devices (nearly) the same semantics for PS as for FF on
VT-100 series terminals.

Implementing the PS on a display terminal requires some additions to the
terminal design anyway, so I hope this would be done correctly this time
i.e. clear the page and start over from the top left. This avoids scrolling,
if only a dozen or two lines are to be displayed.

In order to improve readability for text just before the PS, some means may
be required, but this should be implementation defined.

The terminal could for instance stop the output (by using software e.g.
Xon/Xoff or hardware e.g. RTS/CTS hanshaking) until the user presses a
designated key (send/scroll/clear-page etc.) or if the operating system
supports it, request resumption of the output in somewhat similar way as
block mode terminals do.

This would bring the operation closer to the operation of hard copy devices
and also with IBM style block mode devices or old display system using
storage tubes that had to be manually cleared after each page.

On terminal emulators and unix/ms-dos utilities such as "more" this, should
also be quite easy to implement.

Paul Keinšnen



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