VT100 under SIMH

[Rob O'Donnell]

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 
time.

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.

Rob.