On Thu, Apr 06, 2000 at 09:38:35AM +0200, Helge Hafting wrote:
> > The video RAM reported on video boards as "2Mb, 4Mb,... NMb.", is
> > not RAM that can be directly accessed via ISA or PCI et all, buses.
> > It's SRAM mappable to the Video device chip, and assessible only
> > through a port from the "outside". It isn't even the "Screen regen buffer"
> > you see at 0xB8000 which is typically only 16 kb. To access that RAM, you
> > have to enable "graphics mode" and then it's available as 64k pages
> > from 0xA0000 by writing to page registers through a port. This is only
> > 64k per page and the kernel has no way of "knowing" when to change the
>
> Most new cards have all the video memory directly acessible via PCI (or AGP)
> busses. They have the ugly 0xA0000 64k-block stuff too, but that's only for
> backward compatibility with VGA. Most cards support a directly accessible
> linear framebuffer.
This reminds of a hack I saw when I was in college (1975 - 1979, which as my
daughter likes to inform me, was when dinosaurs roamed the earth :-). The
unveristy had a couple of Terak (spelling?) computers that had a P-code Pascal
compiler on them. The compiler would use the black and white video memory as a
scratch pad (and naturally get 'random' patterns on the screen). After some
amount of practice, you could usually tell what phase the compiler was in by
the patterns on the screen, just like I can identify when I'm linking GCC by
the disk noise.
-- Michael Meissner, Cygnus Solutions, a Red Hat company. PMB 198, 174 Littleton Road #3, Westford, Massachusetts 01886, USA Work: meissner@redhat.com phone: +1 978-486-9304 Non-work: meissner@spectacle-pond.org fax: +1 978-692-4482- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.tux.org/lkml/
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