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SubjectRE: Using video memory as system memory
On Wed, 10 Apr 2002, Holzrichter, Bruce wrote:

> I thought that this was interesting as well, and had a couple of questions,
> as I am no expert in this stuff.
>
> You don't have the frame buffer enabled for display when trying to use this
> as system memory, correct?

Correct. :) In fact, text mode can only take a maximum of the first 256KB of
memory of the card (extended text paging). So as long as you only target the
rest of the memory (and don't use X or svgalib) you should be fine.

> Are there implications of the BIOS shadowing video memory to system memory,
> or is that not an issue once Linux takes over memory control?

Not that I'm aware of. This is PCI-mapped prefetchable memory.

> That is a neat idea, though. The PCI/AGP bus may be a limiting factor for
> this as well, correct? As far as speed, I believe most video cards have
> fast memory, vram, or sram, but it's only useful transferring between the
> Video GPU, and Video cards memory, as the bus to the video card is the
> bottleneck.

Yeah. In fact in some responses the 'slow speed' consideration was so much that
they all say I'd be better off writing a block driver and making use of the
memory more as a swap device rather than as system RAM.

Has anyone out there done this yet? I figure I'd ask before reinventing
anything.. :)

-Byron

--
Byron Stanoszek Ph: (330) 644-3059
Systems Programmer Fax: (330) 644-8110
Commercial Timesharing Inc. Email: byron@comtime.com

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