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Old 05-21-09, 12:53 PM   #1
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Join Date: May 2008
Posts: 2
Default the truth about MTRR's and PAT?

I have a basic understanding of what MTRR's are. In my experience due to a buggy BIOS, linux has a lot of trouble setting these up correctly on boot. My goal is to set them all up by hand, but even after extensive research, I'm still unclear about a few things.

1. How does the PAT relate to MTRR's? Do they work together or does having PAT enabled eliminate the need for MTRR entries?

2. I understand the video memory should be marked as type "write-combining". Are the write-back and uncachable entries necessary? Is it OK to have memory space that's not assigned to any MTRR?

3. This is an example of one I set up by hand on my LanParty NF4 with 8800 GTS 512MB card. The card has 512MB memory but (for whatever reason) linux only lets me use 256MB @ d0000000. (according to Xorg.log and lspci)

reg00: base=0x000000000 ( 0MB), size= 2048MB, count=1: write-back <--- (kernel created)
reg01: base=0x07ff00000 ( 2047MB), size= 1MB, count=1: uncachable <--- is this necessary? (kernel created it)
reg02: base=0x080000000 ( 2048MB), size= 1024MB, count=1: uncachable <--- is this necessary? (i created as placeholder)
reg03: base=0x0c0000000 ( 3072MB), size= 256MB, count=1: uncachable <--- is this necessary? (i created as placeholder)
reg04: base=0x0d0000000 ( 3328MB), size= 256MB, count=1: write-combining <--- my video memory (this one is correct)

4. If I do a "lspci -v" I see that my card is actually reserving 3 sections in memory... one 256MB, one 16MB, one 32MB. What are the 32MB and 16MB entries for?

5. How do you manipulate the PAT in linux? (ex: the MTRR's have a /proc/mtrr interface)

Please help me understand all this.

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