Problems with AMD64 and 8 GB RAM?

Peter Wemm peter at
Thu Mar 31 03:17:37 PST 2005

On Wednesday 30 March 2005 09:49 pm, Greg 'groggy' Lehey wrote:
> On Wednesday, 30 March 2005 at 22:27:43 -0700, Scott Long wrote:
> > Jon Noack wrote:
> >> On 03/30/05 23:14, Greg 'groggy' Lehey wrote:
> >>> On Wednesday, 30 March 2005 at 21:28:36 -0700, Scott Long wrote:
> >>>> Greg 'groggy' Lehey wrote:
> >>>>> On Wednesday, 30 March 2005 at 23:01:03 -0500, John Baldwin wrote:
> >>>>>> It would be interesting to see the contents of your MADT to see if
> >>>>>> it's trying to use a 64-bit PA for your APIC.
> >>>>>
> >>>>> Any suggestions about how to do so?
> >>>>
> >>>> man acpidump
> >>>
> >>> How do you run that on a system that won't boot?
> >>
> >> You said the system worked with 4 GB (albeit detecting only 3.5
> >> GB).
> Yes, this is correct.  A number of people have explained why it only
> detected 3.5 GB in this configuration.

You're also being confused by the implementation of the 'real memory' report.  
If you take a 30 second glance at the code, you'll see that it is reporting 
the same units that the hw.maxmem tunable uses.  ie: it is the LIMIT or 
Highest Address that the system has, not the sum total of all the parts.

eg: see the machdep.c comment next to the printf
         * Maxmem isn't the "maximum memory", it's one larger than the
         * highest page of the physical address space.  It should be
         * called something like "Maxphyspage".  We may adjust this
         * based on ``hw.physmem'' and the results of the memory test.

The SMAP lines are what you need to pay attention to.  In the output you 
posted with 8G, you can see the 4GB going from the 4->8GB range, exactly.  
SMAP type 1 is "usable memory".


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