VT100 under SIMH

From: Rob O'Donnell <classiccmp.org_at_irrelevant.fsnet.co.uk>
Date: Thu Sep 2 04:27:19 2004

At 01:53 02/09/2004, der Mouse wrote:

>Making double-width and double-height work was one of the things I put
>some trouble into when writing mterm. Its decansi mode is a pretty
>good VT-100 emulator for output, and in many respects for input too.

This reminds me of my experiments with double height..

Double height characters were also a feature of the teletext character set
used by Prestel (and Micronet) in the early 80's. I was commissioned to
write a terminal program for use on this for the BBC Micro at the time, [to
be embedded into another software package] and also took some care over this.

One of the "features" of the specification was that a double height
character "overflowed" onto the line below, hiding whatever text was sent
for that line. Because of the way the BBC display circuitry worked, you
actually had to replicate the data on the double-height line on this line
too, to make it display the bottom half of the characters. Most terminal
emulators took this into account, and then ignored anything that was sent
for this second line.

I however had access to a "real" hardware terminal, and noted an
interesting thing - if you removed the double height sequence from the
first line, the second line would then show up with whatever text has been
sent for it. If this was itself defined as double height, it would
overflow on to the third line, hiding it's original text - if that line had
had double height, it would of course be removed, revealing the text on
line 4, etc. etc.

It was possible therefore to send two sets of 12 lines of double height
text or graphics, interleaved, and swap between them simply by sending a
single space character to overwrite first double height intro sequence at
the top of the screen.

By doing this, then putting it back, it was possible to flick backwards and
forwards between two /completely different/ images "instantly". (ok, in the
time taken to send 2 or 3 characters, but compare to the time taken to send
a maximum 768 characters at 1200 baud!) Absolutely amazing to see at the

Of course, not a single one of the software emulated terminals actually
coped with this, (except the one I wrote). Pity, as there were very few
users using real ones.

Received on Thu Sep 02 2004 - 04:27:19 BST

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