Re: Q: Videx Videoterm

[mressl@macgui.com (Marc S Ressl)]

Michael J. Mahon wrote:
> Marc S Ressl wrote:
>> Michael J. Mahon wrote:
>> 
>>>Marc S Ressl wrote:
>>>
>>>>Hello there,
>>>>
>>>>I am emulating the Videx Videoterm for OpenEmulator (and it is mostly
>>>>done!), but I have a problem.
>>>>
>>>>When scrolling past the last line, there often appear ghosts from other
>>>>lines at the last line. This also happens (more rarely) on some other
>>>>parts
>>>>of the screen.
>>>>
>>>>I can understand this as the Videx firmware takes quite some time for
>>>>the
>>>>scrolling, but something does not feel right. Is this ghosting visible
>>>>on
>>>>real boards?
>>>
>>>Marc, what, exactly, does the "ghosting" look like?  Is it a mixture
>>>of the previous line with the new (scrolled) one?  Is it other random
>>>lines showing up?  If so, do the ghosts show up preferentially at one
>>>place, or are they pretty uniformly distributed?
>>>
>>>It's been years since I looked at the Videoterm code, but if it updates
>>>the SRAM asynchronously with 6845 refresh, then line "mixing" can easily
>>>occur.  (I don't mean electrically asynchronously, just unsynchronized
>>>with the video frame generation.)  This is the result of 6845 video
>>>generation proceeding as the buffer contents are moved.
>>>
>>>If the code makes an attempt to synchronize with the 6845 horizontal or
>>>vertical blanking interval (which would account for some slowness), then
>>>the emulation of the status reads that permit this synchronization may
>>>not be working properly, resulting in unsynchronized text buffer updates
>>>and consequent visible line mixing.
>>>
>>>Some simple test cases with scrolling lines of different contents
>>>and/or lengths should case some light on the cause.
>>>
>>>-michael
>>>
>>>NadaNet 3.1 for Apple II parallel computing!
>>>Home page:  http://home.comcast.net/~mjmahon/
>>>
>>>"The wastebasket is our most important design
>>>tool--and it's seriously underused."
>>>
>> 
>> 
>> Hi Michael!
>> 
>> Thanks a lot for your blessings :-)!
>> 
>> I guess an image is worth a thousand words, so here go two images:
>> 
>> http://openemulator.googlecode.com/files/Videx%201.png
>> (here you see the ghosting at the lower right, this happens fairly often,
>> several times per second)
>> 
>> http://openemulator.googlecode.com/files/Videx%202.png
>> (here you see the ghosting+another effect in the middle of the screen,
>> this
>> happens not very often, maybe one time per 2-3 seconds)
>> 
>> With the best wishes,
>> 
>> Marc.-
> 
> So the frequently occurring "ghost" line at the lower right is
> the line just scrolled off the top of the screen, while the
> less frequently occurring ghost looks like lines horizontally
> displaced from where they were prior to a few scrolls.
> 
> Do these "ghosts" only last one video frame, or do they persist
> longer?  (I'm betting only one frame, hence "ghost".)
> 
> I'm still guessing that this is a problem with unsynchronized
> updated to the card's text buffer.
> 
> I've forgotten how the Videoterm scrolls.  Does it actually move
> the buffer contents, leaving the 6845 address pointers alone, or
> does it treat the text buffer like a circular buffer, updating
> the 6845 address registers?
> 
> Clearly, something is not happening at the right time in this
> process.
> 
> -michael
> 
> NadaNet 3.1 for Apple II parallel computing!
> Home page:  http://home.comcast.net/~mjmahon/
> 
> "The wastebasket is our most important design
> tool--and it's seriously underused."
>

Hi Michael:

yes, by ghosts I mean single frames that are incorrectly rendered.

The Videoterm uses the MC6845's start address register for scrolling (as it
is supposed to be used). So when you scroll, the video RAM start address is
advanced 80 characters, and therefore some characters of older lines could
be visible at the line that just "appeared".

I just did some math, and 80*24 = 1920. The Videx has 2048 bytes video
memory, so there is extra space after the last line for 2048-1920 = 128
characters.

So right now I think I must be doing something wrong, as there must be some
provision in the Videoterm code for clearing the extra characters after the
last visible line. Or maybe there wasn't and the ghosting did in fact occur.

I just thought it would be easier to ask on CSA2 instead of spending time
trying to understand 1980's code :-).

With the best wishes,

Marc.-