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From: diamant@hpfclp.UUCP
Newsgroups: comp.os.minix
Subject: Re: Minix runs on Faraday motherboard
Message-ID: <9490003@hpfclp.HP.COM>
Date: Tue, 14-Apr-87 00:18:50 EST
Article-I.D.: hpfclp.9490003
Posted: Tue Apr 14 00:18:50 1987
Date-Received: Sat, 18-Apr-87 04:47:02 EST
References: <406@netxcom.UUCP>
Organization: HP, Fort Collins, CO
Lines: 30

> >What happens is that after 50 or 75 lines have been displayed, the next
> >15 or so aren't painted -- I see the cursor move, but no characters
> >are displayed.  Then after about 15 of these "ghost" lines, the
> >display begins to paint again, and "as if by magic" the ghost lines
> >appear on the display.
> >
> >Has anyone else seen this, or is it just me?
> 
> Yes.  The Toshiba T1100+, supposedly a real CGA clone, does exactly the
> same thing.  I haven't figured it out yet...

We have  seen this on a Leading  Edge  (don't  know the  model,  but the
problem appeared on a Mono.  display) as well.  However, in that case, I
think it was 25 lines on and 25 off.  The problem is presumed to be that
the  display  buffer  has  more  memory  than a true  IBM and  that  the
scrolling  algorithm  in MINIX  assumes that when you get to the bottom,
the next line will be the top.  If you look in the scroll_screen routine
in the tty driver  (kernel/tty.c)  there are  references to a video mask
(vid_mask)  which is set to 16K for  color and 4K for  monochrome  (line
4000 in the book  sources).  If the sizes are  wrong for your  hardware,
then the behavior you describe would be likely.  We haven't tried fixing
the  constant  yet  (haven't  found an easy way to find and  patch it as
opposed to  rebuilding  the entire system just to test it).  Post a note
if you try it and it works  (also let me know how you patched it if that
is what you do).

John Diamant
SCO				UUCP:  {hplabs,hpfcla}!hpfclp!diamant
Hewlett Packard Co.		ARPA Internet: diamant%hpfclp@hplabs.HP.COM
Fort Collins, CO