[gentoo-amd64] Re: 965 chipset and 2 DIMM slots will never give 4GB available? Is that correct?

[Evert]

Duncan wrote:
> Volker Armin Hemmann <volker.armin.hemmann@××××××××××××.de> posted
> 200710181500.24885.volker.armin.hemmann@××××××××××××.de, excerpted below,
> on  Thu, 18 Oct 2007 15:00:24 +0200:
>
>> On Donnerstag, 18. Oktober 2007, Evert wrote:
>>
>>> I read their text again and noticed: 'OS will allocate 3~4GB memory
>>> address for onboard device'.
>>> Wouldn't that mean it's simply a (kernel)configuration issue...?
>> no, it is a 'stupid bios' issue. The bios could put the pci/other device
>> memory space somewhere in the high terrabyte area, but instead it
>> chooses to put it at the end of the 4gb range, because of historical
>> reasons.
>
> This is correct.  Here's a bit more detail.
>
> Many legacy PCI devices (and possibly some PCI-E devices, I don't know as
> my board is PCI-X, not yet PCI-E) and/or their drivers (mainly a problem
> with closed source drivers if the device and board handles 64-bit, AFAIK)
> are designed to run in a 32-bit address space only.  As such, PCI device
> address space is normally located at the top of the 32-bit addressable
> memory area -- 3.5 to 4 GB.  Naturally, if devices are using that address
> space, it's not available to be used by memory, so there's a "memory
> hole" at that location -- about a half GB just below the 4 GB boundary,
> thus under /normal/ circumstances, limiting actual usable/addressable
> memory to ~3.5 GB if one has 4 GB memory, or ~half a GB less than total
> memory if one has > 4 GB.
>
> There is, however, a BIOS workaround, depending on whether the mobo and
> BIOS installed support it or not.  Basically what it does is move the
> memory otherwise covered by the "hole" up beyond 4 GB, so with 4 GB
> memory, you'll actually have usable memory addresses running to ~4.5 GB.
> (Or, some implementations move the entire GB, or even two gigs, so you
> may see addresses to 5 or even 6 gigs, but only have 4 gigs memory.  Mine
> seems to move 2 gigs, so my 8 gigs memory appears as 10 gigs at POST.)
>
> If the BIOS supports this memory address move, you'll see one or two
> different options therein.  Since I'm booted ATM I don't have immediate
> access to my BIOS to check and I don't remember exactly what they are,
> but I have two separate options here.  IIRC one of them is something like
> "contiguous memory" vs. something else.  The other one I don't remember
> at all ATM, only that I had it.  I never was entirely clear at what the
> specific interaction between them was, but one definitely has to be set
> correctly to get the addressing working right, while the other... I'm not
> sure about.  I just played with it and a kernel option or two until I got
> it working.
>
> However, the first thing is that your BIOS supports it at all.  If it
> doesn't, you're simply SOL, unless you are lucky and they have a BIOS
> update adding the feature.  Unfortunately, given the reply from the
> board's support people as you quoted, specifically that they didn't
> mention any such feature at all, it would appear their BIOS doesn't
> support it, and you are stuck with 3.5 GB.  Of course, if they don't
> support Linux, they may not have known that it can take advantage of such
> an option given the chance, and may have given you the standard MSWormOS
> (or whatever) support answer, and their BIOS actually /does/ have the
> option.
>
> Note that I've read of advanced users flashing a BIOS with the features
> they want but designed for another board using the same base chipset.  Of
> course, that's going to void your warrantee and could well end up
> bricking your board, but it's an option if you want to risk it.  It's
> also possible to replace the BIOS chip itself, and to buy BIOS chips pre-
> flashed with various BIOS images, so if you decided to go that route, you
> could buy one flashed with some BIOS with that feature and see if it
> worked, without risking overwriting the supported BIOS on your current
> chip.  Of course, that's well beyond anything I've tried, and in the
> general case, if you were advanced enough to work with that sort of
> thing, you'd probably know all this including the above about the PCI
> device memory hole already, so it's well beyond anything I'd recommend.
> However, the option is there if you are suitably determined, and have
> access to the appropriate resources (a friend that has the tools and
> knows what they are doing, or funds to purchase the service commercially,
> or such that the risk of bricking isn't a major worry).
>
> If you'd like the specific BIOS settings, I can reboot and look them up,
> but I'm not going to bother unless given the above, you think you might
> have the options, need to know more, and specifically request that info.
>

Thank you for putting the whole problem in cleartext. I have forwarded your text to AOpen,
hoping that even they understand it... ;-)
(I understood the problem already, but it's not easy to get AOpen to see this/my point of
view on the issue...)


Thanks! :-)

   Evert

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