Problems with AMD64 and 8 GB RAM?

Peter Wemm peter at
Wed Mar 30 15:22:28 PST 2005

On Wednesday 30 March 2005 03:15 pm, Greg 'groggy' Lehey wrote:
> On Wednesday, 30 March 2005 at 16:01:14 -0700, Scott Long wrote:
> > Greg 'groggy' Lehey wrote:
> >> On Wednesday, 30 March 2005 at 14:35:46 -0800, Steve Kargl wrote:
> >>> On Thu, Mar 31, 2005 at 07:54:39AM +0930, Greg 'groggy' Lehey 
> >>>> None of these problems occur when I use 4 GB memory.  About the
> >>>> only strangeness, which seems to come from the BIOS, is that it
> >>>> recognizes only 3.5 GB.  If I put all DIMMS in, it recognizes
> >>>> the full 8 GB memory.
> >>>
> >>> I had 4 bad out of 12 tested where the DIMMs were Crucial PC2700
> >>> 2GB Reg. ECC DIMMs.
> >>
> >> OK, this makes sense.  It might also explain why the 4 GB
> >> configuration only recognizes 3.5 GB.
> >
> > No, and I'm going to make this an FAQ and post it in a very obvious
> > place, since 4+ GB is so easy to get and people don't seem to
> > understand the PC architecture very well.
> That's not easy to understand when it's barely documented.  Thanks
> for the info: it helps a lot.
> This may still be a hint, though: that memory hole doesn't show up
> during a boot with 8 GB RAM.  How come?  Is the system trying to map
> RAM over the PCI hole?

Nope, its still there.  When you boot -v, you'll see the hole in the 
"Physical memory chunk(s)" list.

However, I suspect that some of the bioses will set the 4GB hole 
partition in the physical ram lower so that there will be 4.5GB of ram 
above the 4GB mark.  I haven't looked too closely  to see for sure.

> It looks as if I should get a verbose boot listing with 8 GB.  It'll
> be a couple of hours before I find time to reboot this machine.  In
> the meantime, there's a verbose boot with 4 GB at
>  I'm told it
> shows a number of strange things, including incorrect reporting of
> on-chip cache sizes.

Nope, it is correct.  You have 1MB of L2 cache.
L1 data cache: 64 kbytes, 64 bytes/line, 1 lines/tag, 2-way associative
L1 instruction cache: 64 kbytes, 64 bytes/line, 1 lines/tag, 2-way 
L2 unified cache: 1024 kbytes, 64 bytes/line, 1 lines/tag, 16-way 

> Greg
> --
> See complete headers for address and phone numbers.

Peter Wemm - peter at; peter at; peter at
"All of this is for nothing if we don't go to the stars" - JMS/B5

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