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Old 03-06-2005, 10:11 AM   #1
Registered: Oct 2003
Location: AU
Distribution: Ubuntu
Posts: 57

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XFree ghost image video memory

Hello... This one has puzzled me for a while, and I cant find much information on it.

Recently I installed debian on a pentium laptop with 24mb ram via floppy it took me a long time but i was pleased with the result.

When I finally managed to install XFree, and booted X, a fuzzy screenshoot looking windows 95 booting image was displayed on the screen, for a good 15 seconds, before the X background and mouse were initialized.

I have noticed this before on my other computers, when X starts i get a very quick fuzzy dump of what yesterdays desktop looked like before X was killed...

so im thinking this crap im seeing as X is starting comes from a portion or ram that is re-used as a sort of frame buffer, and becuase the machine boots into pretty much the same portion of ram every time, X would use the same bit of ram every boot right ??

the windows 95 one on this laptop troubled me though, becuase it was a good 2 days since windows was booted ... the 24mb of ram has been used and re-used many a time trying all the different 'linux on a floppy' distros out there.

so this garbage im seeing cant be coming from ram. and why do i see only X garbage (ie a mangled version of my previous kde desktop) and not a fuzzy version of ncurses or terminal screen or the last thing the graphics card showed on the screen before power off?

is there another form of video memory in computers that stores this data? does anybody know anything about this kinda stuf?

or has anybody experienced the same thing? lol



this is an image of the X session before reboot:

this is the garbage displayed for about 15 seconds as X is starting up after a reboot (which took 7 minutes :P )


Last edited by d_t_baker; 03-06-2005 at 10:27 AM.
Old 03-06-2005, 01:09 PM   #2
Senior Member
Registered: Jun 2004
Posts: 2,553

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wow that's wild

X would use the same bit of ram every boot right ??
not necesarily

i think it depends on the way whatever driver you are using is written

it looks like x here on my machine is using mapped memory
like you said it's essentially a buffer that acts as a virtual memory page so writes to the buffer go to
the video cards ram. (i think )
like for me the buffer treats /dev/nvidia0 like it's a file mapped into memory and writes to that region on memory go straight to the device
now when you create a memory map like that the first argument can either be a start address so the map always takes up the same spot or NULL which allows the kernel to pick it's own starting spot.
but i don't think the information is ever actually in the system RAM but rather just pointers ???
i think i'm loosing myself here.
now since your laptop is very old it must be using "shared" or system ram for video
but with only 24 mb of ram the video ram must be geting swapped out to a swap partition constantly
that is the pointers in the paging tables come to point to hard drive rather than ram.
i'm guessing that on startup x memory is swapped out and what you are seeing is the results of whats in the swap file or swap partition or whatever you are using. but this is just a wild guess.

this still in no way accounts for the win 95 image does it ?


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