accessing the contents of video memory

Prabhu nath gprabhunath at
Tue Jan 18 03:11:37 EST 2011

Oops. Surprised to see it crashed :). Never I have seen a crash on my


On Tue, Jan 18, 2011 at 1:29 PM, Alexandre Courbot <gnurou at> wrote:

> On Tue, Jan 18, 2011 at 3:40 PM, Prabhu nath <gprabhunath at>
> wrote:
> > If you are working on a desktop machine, then the following idea works.
> > Usually, the VGA compatible controller memory will be mapped to the
> physical
> > address space which can be viewed with the help of "lspci -vv" command.
> Then
> > open the device file "/dev/mem" and mmap with the offset equal to the
> > physical address of the Graphics card. Here is a small snippet which
> works
> > on my machine
> > Now open the device mem as
> >           fd = open = ("/dev/mem", O_RDWR)
> > and do an mmap as
> >           vgamem = mmap (NULL, 0x10000000, PROT_READ | PROT_WRITE,
> >                                      MAP_SHARED, fd, 0xd0000000)
> > 0x10000000 = 256MB of graphics card memory
> > 0xd0000000 = base of the physical address of graphics card memory
> >
> > vgamem will be the user virtual address which is mapped to the physical
> > address of Graphics card memory.
> >
> > To verify the mapping, write some bit patterns to 256MB of memory
> starting
> > from vgamem.
> > If you see distortion on your desktop, then you have written on to your
> > graphics card
> > memory.
> Just gave it a shot, for the fun of it (program attached - needs root
> rights). Indeed after running it my screen was stripped with little
> black dots, as one could expect. However it also screwed my fonts and
> after a few seconds crashed my system. ;) Indeed, the video memory
> does not only contain the screen framebuffer, it also hosts other data
> related to 3d management, compositing, and probably data structures
> that X rely on. But at least this validates your approach to easily
> access video memory.
> Now all that is needed for Elvis to get the screen contents is to know
> the boundaries of the screen's buffer, as well as its format.
> Following your idea, he could probably access the memory-mapped
> registers of his video card, but this is highly vendor-dependant.
> Cannot think of a more generic way though.
> Alex.
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