I believe the limitation is actually in the CPU -- a 32-bit address bus 
can only span a total of 4 "billion" (nearest binary power) addresses.  
Since the video buffers, and perhaps some other peripherals, must map into 
some of this space, the OS cannot simultaneously make use of the full 4GB of 
installed RAM.  [Motherboards that are capable of installing more than 3 GB 
have emerged only within the last year or so.]

  It would be possible to use all 4GB, and more, by re-inventing the tricks 
like EMS and XMS that were used to expand past the 1MB (20-bit address bus) 
of the original 8086/8088, but it will be cleaner, simpler, and faster to 
jump to the 64-bit version OSes to make use of this additional RAM (and much 
more).  [Presumably, the 32-bit version of the OS must assume a 32-bit CPU 
even if the installed chip includes 64-bit capability.]

David Gillett


On 2 Oct 2006 at 21:06, Thomas Mayer wrote:

> Trey
> 
> I've done some research on your situation and, other than getting a 
> headache, I found that you are not alone. I started on the premise that 
> XP, assuming that is what you are using, may have a 3GB limitation on 
> the amount of physical RAM that it will use. However, as best as I can 
> tell, it has a 4GB limit for the 32 system. If you use a 64 system the 
> limitation is much higher. There is some thought that the 4GB limitation 
> includes the page file and the size of the page file limits the amount 
> of physical memory XP can use. This seems to be disproved by some people 
> who have disabled the page file without any affect on the amount of 
> physical RAM used by XP. There are a number of mentions of system files 
> (space) being reserved in the 3 to 4GB range by XP, the BIOS, chip 
> set(s), video memory, etc. or some combination thereof depending upon 
> what "expert" is discussing the situation. In any eventuality, a number 
> of persons share your problem with reports of XP only using somewhere 
> between 3 and about 3.6GB of physical RAM. Just seems weird to me that 
> this "reservation system" only kicks in when the physical RAM is more 
> than 3GB.
> 
> I did not see any real answer to this situation other than going to a 64 
> system.
> 
> Tom Mayer
> 
> Trey Holm wrote:
> > I am not sure.  It came with four RAM slots, all filled with 512's.
> > After about a year I replaced two of the sticks with 1G sticks.  Windows
> > picked these up without a problem. Six months later, the present day, I
> > replaced the second set.  However, this time the new sticks were not
> > picked up.  The configuration of the sticks before this last upgrade
> > was:  512-1G-512-1G.  Now, the BIOS shows: 1G-1G-1G-1G, but Windows says
> > 3024 KB.
> >
> > Does this help you help me?
> >
> > -----Original Message-----
> > From: Personal Computer Hardware discussion List
> > [mailto:[log in to unmask]] On Behalf Of James Dicomes
> > Sent: Monday, October 02, 2006 8:16 PM
> > To: [log in to unmask]
> > Subject: Re: [PCBUILD] How to get new memory to work?
> >
> > I had the same idea about my Dell but further investigation into the 
> > computer came up with a maximum of 512 for my Dell model. Could that be
> > your 
> > problem?
> > ----- Original Message ----- 
> > From: "Trey Holm" <[log in to unmask]>
> > To: <[log in to unmask]>
> > Sent: Sunday, October 01, 2006 10:46 AM
> > Subject: [PCBUILD] How to get new memory to work?
> >
> >
> > Is there anyway someone could tell me how to determine why Windows is
> > not picking up new RAM I just installed?  I have a Dell Dimension 4700
> > that came with  2 dual 512 sticks.  Later I put 2 1G sticks that Windows
> > picked up.  I have now replaced the old 512's with 2 1G sticks.  The
> > BIOS setup picks the new sticks up, but when in the properties menu
> > under My Computer, it shows the total from the old configuration.  How
> > do I determine whether the sticks or bad or Windows is the problem?
> >
> >
> >
> > Thanks,
> >
> > Trey Holm
> >
> > [log in to unmask]
> >
> >
> >
> >
> >        
> >
> >   
> 
>                          PCBUILD's List Owners:
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>                        Drew Dunn<[log in to unmask]>

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