I mentioned a week or so back the problems that I've been having with
the display under RiscBSD since I upgraded to 2Mbytes of VRAM.

Basically a 640x480x256 graphics mode seems to work okay. Whenever I
try anything more ambitious I get strange disruption of the display.
The most common effect is one where alternate display lines appear to
be horizontally shifted with respect to each other by about one
character position. If "*" stands for a pixel then a vertical line
would look something like this:

        *
                *
        *
                *
        *
                *
        *

instead of:

        *
        *
        *
        *
        *
        *
        *

I've also seen effects which look similar to the left and right halfs
of a character being swapped over.

Has anyone seen anything like this before? Anybody got any suggestions
for possible fixes?

It's a bit frustrating, now that the X11 sets are out for 1.2-beta. X
in a 640x480 window is only marginally more useful than the proverbial
chocolate tea pot. :-)

My current configuration is:

	bsd-4626 kernel
	1.2beta sets
	RiscPC 600 with 32Mbytes DRAM and 2Mbytes VRAM.

The problem did not appear when I had 0 or 1Mbyte of VRAM, and it went
away when I tried removing the VRAM SIMM and configuring to use DRAM
for video. I'm assuming (for the moment) that VRAM hardware is okay -
it works fine in RISC OS anyway.

Also - is it possible to use DRAM for video even if there is VRAM
fitted to the machine? This might be a useful diagnostic, or a
possible work around until I can get the VRAM working properly. I've
tried just configuring the DRAM size and booting but I seem to get
black text on a black background - I can see the cursor, and it moves
when I type, but otherwise the display stays blank. Perhaps the kernel
is writing stuff in the DRAM but the VIDC is still displaying the
contents of the VRAM (or vice-versa)?

Regards,

+-------------------------------------------------------------------------+
Neil Hoggarth                                 Departmental Computer Officer
<neil.hoggarth@physiol.ox.ac.uk>                   Laboratory of Physiology
http://www.physiol.ox.ac.uk/~njh/                     Oxford University, UK
+-------------------------------------------------------------------------+