Date   : Mon, 02 Jul 2018 21:49:57 +0100
From   : soruk@... (Michael McConnell)
Subject: Announce: Brandy Basic - with teletext?!

I'm thinking about double buffering too, but that is a project for 
another day!

On 02/07/18 20:59, Rob wrote:
> If you can emulate an entire BBC Micro in *javascript* within a browser 
> window, then I wouldn't expect a compiled native application to have any 
> problems at all.
> 
> FWIW, teletext graphics, and the way the various control codes interact, 
> can be quite tricky to get right. Hold Graphics is the one that trips a 
> lot of people up, as it can show up mistakes in the coding of other 
> control codes (some take effect on the cell with the code, some only on 
> the cell after!) And then you have to decide if you are implementing a 
> "proper" viewdata/teletext display, where a double height code maps the 
> line onto the line below, and ignores (but still remembers!) whatever is 
> stored in memory for the second line, or a BBC Micro mode 7 where it 
> uses that second line and can therefore produce funky effects. It is 
> very satisfying when you get it all right!
> 
> Even on a Beeb it's not that easy: Back in the 80s, I was contacted to 
> write a viewdata terminal emulator for the Beeb. It was actually quite 
> hard to get double height working "properly" using the Beeb's hardware. 
> I.e. What happens if you overwrite the double height control code on the 
> first line? All the terminal emulators I had tested got it wrong, as 
> they had already discarded the second line (in order to cater for the 
> Beeb's hardware, which needs both to be the same) and which should now 
> be visible, and is allowed to contain something completely different to 
> the first!? My approach was to write the data into a separate buffer, 
> and copy into screen memory on the vertical sync timer. That way I could 
> deal with double height, and Reveal (Conceal being another code that the 
> Beeb obeys, but then has no way of revealing the hidden characters.)
> 
> Rob.
> -- 
> Sent from a mobile device. Please excuse brevity or spoiling mistooks.
> 
> 
> On Mon, 2 Jul 2018, 19:38 Pete Turnbull, <pete@... 
> <mailto:pete@...>> wrote:
> 
>     On 02/07/2018 15:59, Michael McConnell wrote:
>      > I guess, with the way the screen is currently done (including the
>     other
>      > modes) that it would be computationally far to expensive to scan the
>      > screen for what should be flashing pixels. For Mode 7 at the very
>     least
>      > to get a 100% true emulation it would need to maintain a 1000 byte
>      > screen memory buffer (somewhat akin to the BBC Micro screen memory at
>      > &7C00) and re-render the entire screen periodically
> 
>     Hmm.? Sometime in the late 1990s I wrote a Viewdata client for Unix (or
>     more specifically, for IRIX on an SGI Indy running at 180MHz).? I
>     needed
>     it for HOBS, the Bank of Scotland online banking system, which I had
>     previously used on a Beeb and then an A440.? I had graphics, flash,
>     double height and separate/contiguous etc working properly then, so I
>     find it hard to believe it can't be done two decades later on 20x
>     faster
>     processors, though I guess it might depend on what it's written on top
>     of.? I've forgotten exactly how I did it but AFAIR I did use a double
>     buffer.
> 
>     -- 
>     Pete
>     Pete Turnbull
> 
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