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

[Duncan]

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.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

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