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 Post subject: 4 Gig Of Ram Under XP 32-Bit
 Post Posted: Thu Oct 16, 2008 10:16 am 
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Scott,

Windows XP can address up to 4 gigabyte's of ram, yet when 4 gigabyte's of memory is used under the 32-bit OS not all of the installed memory is displayed on the system properties sheet. With that been said, how is the remaining memory utilized, and what if any impact does that have on performance?


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 Post subject: Re: 4 Gig Of Ram Under XP 32-Bit
 Post Posted: Thu Oct 16, 2008 4:33 pm 
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Scott will undoubtedly give a better explanation because he knows a lot more than me on this ... and a search would bring up a lot of information on this memory phenomenon already on this site. But with respect to performance with the memory that is available for your OS/apps in XP: Generally XP SP2 is its fastest with 2 or 3 Gb of ram. (I do not know how SP3 may have effected this rule of thumb). 4Gb can actually slow performance down somewhat, but give you more room if you open and run a lot of apps at once or use a memory hog -- so here is the tradeoff, lose a slight amount of performance to gain more space for apps to operate in faster RAM than being swapped out for disk. Presumably the RAM performance loss lies with it searching the RAM before it goes to the hard drive cache, and 4Gb takes longer than 2Gb (2X's ?).

Vista32 is a hog... go with 4Gb.

Scott?


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 Post subject: Re: 4 Gig Of Ram Under XP 32-Bit
 Post Posted: Thu Oct 16, 2008 6:40 pm 
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32-bit editions of Windows support 4GiB of physical memory (RAM). What many don't realize is that the PC system hardware uses some or all of the 4th GiB for things like the the BIOS, motherboard resources, memory mapped I/O, PCI configuration space, device memory (graphics aperture), VGA memory, etc.

This means that if you install 4GiB (or more) RAM, none of it past 4GiB will be seen at all, and most or all of the 4th GiB (i.e. the RAM between 3GiB and 4GiB) will be disabled since it is already occupied by other system hardware. This is called the 3GiB limit, which is analogous to the 640K memory limit we had on 16-bit systems in the '80s. The 16-bit addressing supported 1MiB, but the upper 384K was already in use by the system hardware (BIOS, video, adapter ROM, etc.).

Can any of that unused memory between 3GiB and 4GiB be reclaimed? For those running a 32-bit OS the answer is no. However, when running a 64-bit OS on systems that support memory remapping (primarily a function of the motherboard chipset and BIOS), then the answer is yes. Most newer motherboard chipsets have a feature that can remap the otherwise disabled RAM in the 4th GiB to the 5th (or higher) GiB, where it will be both visible to and usable by a 64-bit OS. Note however that if the motherboard doesn't support remapping, then even when running a 64-bit OS, the memory will be lost.

Note that the 3GiB limit is not as strictly defined as it was with the 640K limit. This means that if you do install 4GiB, you might get to use as much as 3.5GiB of it, or possibly as little as 2.85GiB or less. It depends largely on the types of buses in the system as well as the type and number of video cards installed. With a single low-end video card you may have access to 3.5GiB, however on a newer system with two or more PCIe x16 slots, and especially with two or more high-end PCI Express video cards installed, you may drop the usable limit to something even below 3GiB.

The following figure shows the memory map for a modern system using an Intel G45 chipset. Note that the PCI memory range, FLASH, APIC and Reserved areas take up a total of 770MiB of the memory below 4GiB. You can also see the 384K (0.375MiB) of memory below 1MiB that is used by the system as well. For a 32-bit OS, the line labeled "Top of usable DRAM (memory visible to the OS)" is at 4,096MiB - 770MiB - 0.375MiB, which is 3,325.625MiB or about 3.24GiB (rounded down). The latter is what a 32-bit version of Windows (XP or Vista) would report if you installed 4GiB of RAM in the system.
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Bottom line: When running 32-bit editions of Windows 2000, XP or Vista, I used to recommend installing a maximum of 3GiB RAM, since most if not all of the 4th GiB will be unusable. However today, with memory prices so low, it is often just cheaper to install two 2GiB modules than it is to install two 1GiB modules and two 512MiB in order to get 3GiB. That's because on desktop systems that support dual-channel memory, you would not want to install three 1GiB modules as in that case not all of the memory would run in dual-channel mode.

FascistNation wrote:
Generally XP SP2 is its fastest with 2 or 3 Gb of ram. (I do not know how SP3 may have effected this rule of thumb). 4Gb can actually slow performance down somewhat...

False. (where on earth did you get that idea?) While installing more than 2 or 3GiB might not *increase* performance, it most definitely won't reduce performance. The only thing that might be negatively affected by having more memory is the time it takes for hibernation, which is not an issue on most desktop systems anyway. On the good side, if you do install 4GiB, you might get to use up to 3.24 to 3.5GiB of it depending on what hardware you have, and when running memory intensive applications such as Virtual Machines, that extra quarter- to half-gig can definitely come in handy. Scott.


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 Post subject: Re: 4 Gig Of Ram Under XP 32-Bit
 Post Posted: Fri Oct 17, 2008 9:41 am 
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Scott,

Excellent response as always. With memory prices being quite low nowadays I'm looking to buy some ram for either my main system or for a test system that I may be building with spare parts that I have laying around. I was able to find a 2 GiB kit (2x1GiB) and a 1 GiB kit (2x512MiB) kit for 49.98 + shipping. Both of the above kits are Crucial DDR2 667 (PC5300), CAS 5, 1.8V. If I understood you correctly It seems like the 3GiB route is way the go unless you're planning on running several virtual machines or you do not have a beefy system with more then one video card as the number of components that you have installed will impact the amount of ram that's available to you after the 3GiB.


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 Post subject: Re: 4 Gig Of Ram Under XP 32-Bit
 Post Posted: Fri Oct 17, 2008 10:28 am 
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If you read my last post, you'll see that I *used* to recommend 3GiB, but now I recommend just going with two 2GiB modules (4GiB). The reasoning is that in addition to the extra 0.25 to 0.5GiB you'll be able to use (on most systems) you can get it for nearly the same price as buying 3GiB http://tinyurl.com/66pndc, and you'll only take up 2 sockets, leaving room for future expansion if you install a 64-bit OS. Scott.


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 Post subject: Re: 4 Gig Of Ram Under XP 32-Bit
 Post Posted: Fri Oct 17, 2008 10:56 am 
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Scott,

I must have misread your original post. I totally understand your reasoning for going with a 4 GiB kit (2x2GiB) over a 2 GiB kit (2x1GiB) and a 1 GiB kit (2X512MiB). Thank you again for your help.


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 Post subject: Re: 4 Gig Of Ram Under XP 32-Bit
 Post Posted: Fri Oct 17, 2008 12:38 pm 
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Scott I have read your explanation before on this. While it was clear the first time I read it in a different post I don't remember the picture!. That is an excellent addition to the explanation and makes it even easier to understand!. Now only if you could write a series of books on passing the MCSE. The Authors books that I am reading can't touch the clear and consise explanations you give in your books and on this site. Although I passed my first Microsoft exam awhile ago I still had more trouble than I should have. I'm on the second course now and really wish I could find an technical writer that could explain things as well as you do!.


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 Post subject: Re: 4 Gig Of Ram Under XP 32-Bit
 Post Posted: Tue Oct 21, 2008 8:05 pm 
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Scott wrote:
FascistNation wrote:
Generally XP SP2 is its fastest with 2 or 3 Gb of ram. (I do not know how SP3 may have effected this rule of thumb). 4Gb can actually slow performance down somewhat...

False. (where on earth did you get that idea?) ...SNIP.... Scott.


Well, I thought I read it in a MaximumPC article, but after spending the better part of a couple of days hunting for the reference, I am sure I read it, but no longer think I read it in a MaximumPC article. So I concede your reasoning. And in my searches I came across (PDF) this supporting evidence arguing the superiority of 4G over 2G, by of course someone who wants to sell an extra 2G of memory.

edit -- adding another reference shooting down my position.


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