Matching Memory with CPU

XP Pro can only access 2GB memory per application. If running 2 and 3 apps at same time, would having 4GB memory be useful?  If using 4GB memory, is it better to buy 2-2GB sticks or 4-1GB sticks?

How do I match Memory to CPU, by the Bandwidth or FSB or other spec? What spec is most important? Case in point is matching or optimizing RAM with a E6850 and a Q6600 processor, P5K  motherboard and Win XP Pro 32.
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Peter HartConnect With a Mentor Commented:
if you have an up and running system then have a scanner that can tell you what memory is required and what speed to buy.

what type of memory is dependant on the motherboard. the uerguide or website of the motherboard manufactorer will tell you what memory to buy. - get it wrong and it won't work.

XP (32bit)  only supports 4 Gig max. again depending on your motherboard it maybe best to buy the memory in pairs instead of a single stick (also cost)  - see for costs.
nobusConnect With a Mentor Commented:
Xp has a 4 Gb limit : - but it will use about 2.7 to 3.5 Gb depending on the system,
here a good article that explains it quite well :
here a list for different OS'es :
Windows NT 4.0: 4 GB
Windows 2000 Professional: 4 GB
Windows 2000 Standard Server: 4 GB
Windows 2000 Advanced Server: 8GB
Windows 2000 Datacenter Server: 32GB
Windows XP Professional: 4 GB
Windows Server 2003 Web Edition: 2 GB
Windows Server 2003 Standard Edition: 4 GB
Windows Server 2003 Enterprise Edition: 32 GB
Windows Server 2003 Datacenter Edition: 64 GB
Gary CaseConnect With a Mentor RetiredCommented:
You can alter the virtual space XP allows per process from 2GB to 3GB with the /3GB switch ... but that's rarely needed; and unless a specific application suggests it, I wouldn't use it.

As has already been noted, XP won't "see" all of the 4GB if you install that much memory.   A 32-bit OS can address 4GB, but the system has to assign addresses for a variety of reasons (interrupt vectors, BIOS shadowing, AGP aperture, etc.) => it assigns these at the "top" of the address space ... so they normally don't impact the addresses needed for system memory => UNLESS you install a full 4GB.   Just how much you lose depends primarily on how much memory is needed for video mapping, but typically you'll "see" 3.25 to 3.5GB unless you have dual video cards, in which case it will be a bit less.

If you do choose to install 4GB, it's better to install 2 x 2GB modules.   With unbuffered RAM, you'll get more stable performance of the memory subsystem with only 2 installed modules.  [More specifically, I don't recommend more than 4 "sides" of memory ==> 2 double-sided modules; or 4 single-sided modules would present the same load to the bus; but it's rare to find single-sided 1GB modules]

The best memory speed depends on the FSB of the CPU you're using -> which is different for the two CPU's you mentioned (E6850 and Q6600].   Ideally you want the memory speed to be synchronous with the FSB base frequency.
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MarcoVestAuthor Commented:
Thanks for feedback, it is very helpful.  E6850 with 333 base clock FSB and DDR2 800(PC 6400) with 400 base clock FSB a fairly close match but the Bandwidth is not, ie, E6850 at 10.6 GB/sec and DDR800 at 6.4GB/sec.  Am I on the right track? Is the bandwidth an issue? Crucial specs 4GB(2x2GB) DDR2667 for P5K motherboard but they did not reference the CPU.

So what I understand is, without the 3GB switch, no matter how many apps running simultaneously, XP 32 only will address up to 2GB RAM?  I was under impression that the 2GB limit was for each application, meaning that 4GB RAM may be useful with multiple simultaneous apps.

Looking to do either 4GB(2x2GB) DDR2-800 or 2GB(2x1GB) DDR2-1066 with E6850 on a P5K ASUS motherboard.  
Gary CaseConnect With a Mentor RetiredCommented:
Yes, you've got the right idea.   DDR2-667 is twice the base 333MHz clock ==> so it's the optimal memory (which is why Crucial specs it I'm sure).   Faster memory will result in memory wait states; slower memory will result in CPU wait states (except for cache hits -- which don't cause any access to main memory).   But the higher bandwidth memory is still faster even with wait states on the asynchronous cycles.

XP always address 4GB.   With 4GB of memory you'll "see" about 3-3.5GB because of the other functions that are mapped to the upper end of the 4GB address space (BIOS shadow, Interrupt vectors, AGP aperture, etc.) ... but XP will use ALL of the memory that it "sees."   The 2GB limitation is for the virtual address space assigned to each process -- which you can change to 3GB with the /3GB switch.   XP may use memory above that for itself; or it may be mapped to other processes as part of their 2GB.   Remember, the 2GB is what XP allows each process to use ==> you could, for example, have ten processes running, each with 2GB of virtual space.   The actual mapping of memory at any given instant depends on which part of each application's virtual space is currently resident in "real" memory.
MarcoVestAuthor Commented:
Asus P5K Mother board manual recommends installing less than 3GB RAM when using XP 32, suggesting to me XP will not recognize 3 or 4GB regardless of simultaneous applications running.

So your last statement, garycase, means that virtual memory resides elsewhere and real memory is RAM.  As multiple applications are processed simultaneously, XP will only recognize 2GB RAM, or thereabouts, therefore, installing more than 2GB RAM is wasted?  Just want to make sure I understand how RAM and 32bit OS function with multiple simultaneous applications.  If my understanding is true, then I should look at 2GB(2x1GB) set modules.....highest bandwidth I can afford, even though the latency is also higher?
Peter HartConnect With a Mentor Commented:
32bit XP the operating system can address 4Gig .
if the PC has only 512Meg  installed then it uses the disk as virutal memory and the system behaves like a dog with no legs.
at the time of writing 1Gig is suffienct for XP 32bit but 2 Gig will be good.
Gary CaseConnect With a Mentor RetiredCommented:
"... virtual memory resides elsewhere and real memory is RAM ..." => Virtual memory refers to the total address space an application can use.   This is limited to 2GB per process ... no matter how much "real" memory a system has.   The system manages the total virtual address space by mapping the real memory (the actual RAM) to memory pages retained on the hard disk (the "paging file") ... and swaps data to/from disk as needed by the running processes.

I agree that 2GB is generally plenty for XP ... if you decide to install more, I'd use 2 x 1GB modules and 2 x 512MB modules for a total of 3GB.   The board supports 4GB with no problem ... the recommendation to use no more than 3GB is simply a reflection of the reserved address spaces that will result in much of the last GB being unavailable to the OS>

MarcoVestAuthor Commented:
Do you need to match the 2 x 512MB modules with the 2 x 1GB modules for speed and brand, when installing 3GB of RAM?  Thanks.
Gary CaseConnect With a Mentor RetiredCommented:
The brand doesn't matter; but the speed and timing characteristics (CAS latency, etc.) need to match ==> otherwise the motherboard may not run your memory in dual channel mode, since the SPD info wouldn't match for all modules.
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