Hello everyone!

I'm posting here hoping to find a definitive answer to an ancient question.

The question many people often ask me is "how much RAM can Windows XP Pro 32-bit detect?".
Some time ago I had asked myself the same question. I'm no pro, so I did a little research on the internet and read a couple of interesting articles on the "historical" reason behind the limits on how much RAM can an OS address (the whole HIMEM issue in old computers etc..).
It was my initial undertanding that it depends on the processor architecture. So I did a couple of quick calculations.
If I undestood what I read correctly, the maximum amount of addressable RAM by a 32-bit system is 2^32 addresses = approx 4 GB RAM.

So, by simply using brute calculations, 4 GB seems to be the limit. Which appears to not be the case... so, back to the internet! After some further research, I found out that while 4 GB is indeed the limit, the "upper" 1 GB is reserved for PCI I/O. And here my understanding comes to an end.
So, the original question now seems to have evolved to:

- What is the discriminant that decides how much of the upper 1 GB is visible to the OS?

For example:

I have 4 GB installed, and my OS detects 3.5 ([pic] which, as a handy side effect, demolishes the vox populi I've often heard that "3 GB are the limit, period"). My (PCI) graphics card has 512 MB. So, is the last half GB reserved to represent the 512 MB of the graphics card? Is that how it works? If 4 GB are installed, the RAM detected by the OS is the difference between those 4 GB and the video card's RAM? So, supposing my video card had only 128 MB, would the OS detect 3.968 GB?
Vice-versa, if I had a video card with 1 GB of its own RAM, would the OS detect only 3 GB of system RAM?

Note how, up to now, I've ignored the implications in terms of performance.
Which brings me to the second question:

- Does the amount of memory seen by the OS beyond the 3 GB mark have any effect on system performance?

My immediate answer would be that it doesn't.
Considering the previous example, in fact, it would appear that video card memory "steals" the upper 1 GB to the OS. Which would yield the conter-intuitive conclusion that a card with more video memory will leave less RAM to the system, which is something I refuse to believe.
So "ceteris paribus" (ie. if all other hardware components are exaclty the same) a computer with 3 GB installed and one with 4 GB installed (both running Win XP 32-bit, and in both cases RAM modules are installed in the proper slots allowing dual channel mode) will perform exactly in the same way?

Again, this is all speculation from a non professional. I'm just an amateur computer user who could use with some clarifications!
Sorry for the long list of questions, but this is an issue I'd really like to settle once and for all.

Interesting question. Basically the limit has to do with the 32-bits used for memory addressing [a location in RAM] & because of this XP [as well as Vista] 32-bit can only recognize and use about 3.5 GB of RAM. To use anymore would require XP or Vista 64-bit. It has nothing to do with the graphics RAM because those cards actually have their own RAM and don't need to "steal" memory from the system. Now I've been told by a member of this forum that Windows 7 32-bit can use 4 gb of RAM but don't ask me how. Maybe he'll comment.

Performance wise I really don't see how anyone would need even close to 4 gb of RAM in Vista and especially XP unless they are running a lot of little stuff at once, or are running some really insanely resource intensive game [and I might be wrong but I don't think that any of them require that much], or they are in heavy OS virtualization [running a fake system on top of your real system]. The closest I get to using up my 2 gb is when I'm using VMWare with a couple of fake systems consuming my memory. In my opinion if you turn off all the unnecessary crap that windows, as well as other programs, like to run all of the time that you generally don't need more than 2 gb, at least most shouldn't. Watch your task manager and see how much you use. The important thing is to have enough RAM that the OS doesn't need to use the hard drive excessively for virtual memory which is what really hurts system performance. The only downside to having 4 gb of RAM and XP or Vista 32-bit is that you can't use all of your RAM but again, do you really need all of it?

I just thought of a downside: Windows likes to reserve 1.5 times the amount of RAM that you have for virtual memory on your hard drive. That means that on a system with 3.5 gb of RAM windows is reserving 5.25 gb of space on your hard drive. I think thats really wasteful on a system that has that much ram because it probably doesn't use virtual memory all that much anyway.

Originally Posted by townsbg

Basically the limit has to do with the 32-bits used for memory addressing [a location in RAM] & because of this XP [as well as Vista] 32-bit can only recognize and use about 3.5 GB of RAM. To use anymore would require XP or Vista 64-bit.

Ah yes, but, see, as I said in my OP, using 32 bits to describe memory addresses would add up to approx 4.3 billion (2^32) possible combinations, and, assuming that our computer has byte-addressable memories, each address describes a byte. And 4.3 billion byte would mean 4.3 GB (decimal, ie. 4 GiB binary) of addressable memory!
Why the limit to 3.5 then? And this does not appear to be a constant, because I'm quite sure to have read of people lamenting of having systems detect only 3.1 GB or 3.2 GB... etc..

Originally Posted by townsbg

It has nothing to do with the graphics RAM because those cards actually have their own RAM and don't need to "steal" memory from the system.

Oh yes, I was aware of that. But I was wondering if along having their own RAM built on themselves, the system was also "reserving" the same amount of proper RAM. I'm relieved to learn it is not the case. Thanks for confirming it!

Originally Posted by townsbg

Performance wise I really don't see how anyone would need even close to 4 gb of RAM in Vista and especially XP.

The only downside to having 4 gb of RAM and XP or Vista 32-bit is that you can't use all of your RAM but again, do you really need all of it?

Ah yes, but you see, I too am in the habit of using virtual machines often. I use VMWare as well. Especially to test OS installations CDs, or as sandbox in case I suspect something may be infected by a virus.

Originally Posted by ZeframCochrane

Ah yes, but, see, as I said in my OP, using 32 bits to describe memory addresses would add up to approx 4.3 billion (2^32) possible combinations, and, assuming that our computer has byte-addressable memories, each address describes a byte. And 4.3 billion byte would mean 4.3 GB (decimal, ie. 4 GiB binary) of addressable memory!
Why the limit to 3.5 then? And this does not appear to be a constant, because I'm quite sure to have read of people lamenting of having systems detect only 3.1 GB or 3.2 GB... etc..

Well I had never done the math before so maybe all of the windows systems have some kind of flaw that they patched in windows 7.

[S]Oh yes, I was aware of that. But I was wondering if along having their own RAM built on themselves, the system was also "reserving" the same amount of proper RAM. I'm relieved to learn it is not the case. Thanks for confirming it![/S]
That really wouldn't make any sense because that would be more work for the OS and it wouldn't be as efficient.

Lol... I just found this article:

4GB RAM in 32-bit Windows XP and Vista

An excerpt:

The first thing that I see on this list is “Graphics card”. Video cards nowadays commonly have one or two hundred MB of RAM on them. That video RAM takes up part of the 4GB address space, which then is not available for your physical RAM.

...turns out my hypothesis wasn't as crazy as it seemed!