Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!olivea!uunet!cbmvax!cbmehq!cbmger!peterk
From: peterk@cbmger.UUCP (Peter Kittel GERMANY)
Newsgroups: comp.sys.amiga.emulations
Subject: Re: Bridgeboard Graphics Displays - are they as slow as I think?
Message-ID: <769@cbmger.UUCP>
Date: 18 Jan 91 16:07:56 GMT
References: <6474@munnari.oz.au>
Reply-To: peterk@cbmger.UUCP (Peter Kittel GERMANY)
Organization: Commodore Bueromaschinen GmbH, West Germany
Lines: 29

In article <6474@munnari.oz.au> ianr@mullian.ee.mu.OZ.AU (Ian ROWLANDS) writes:
>
>	Since I got my bridgeboard, I've noticed that the display is rather
>slow when displaying graphics. This occurs even with the window having a 
>priority of 10, and no other processes running.

Yes, that can't be avoided since there is really some overhead.
When the PC (or AT) side writes something into its video RAM area,
that occurs with full speed and appears immediately on the Amiga
side in the dual-ported RAM. But this is NOT the Amiga display RAM!
So there is a circuitry that detects every change in this RAM area
and causes an interrupt for the Amiga CPU. In that interrupt routine
the Amiga copies the changed data to the appropriate bitmaps in its
Chip memory, so that it becomes visible. So there is a delay until
a change appears visible on the Amiga screen, but the overall
performance of the PC side is NOT affected. (This can turn into a
disadvantage when an A2286 scrolls a DIR so fast that you lose most
of it when viewing it in an Amiga window...)

>Also, how does the display perform with an accelerator in the system (the Amiga
>side, I mean)? Does the speed pick up or is the display still slow?

Yes, this helps, as the CPU can do the above mentioned things faster.
But you can never achieve the speed of a simple PC-side pure-text
display.

-- 
Best regards, Dr. Peter Kittel  // E-Mail to  \\  Only my personal opinions... 
Commodore Frankfurt, Germany  \X/ {uunet|pyramid|rutgers}!cbmvax!cbmger!peterk