Sign in with
Sign up | Sign in
Your question

[RES!] Does 32bit Vista suffer from the not-quite 4GB limit?

Tags:
  • Memory
  • Windows Vista
  • Windows XP
  • Product
Last response: in Memory
Share
February 12, 2007 5:04:35 PM

I understand that typical 32 bit OS's like XP can't access the full 4 GB or RAM that I have installed (it shows up at about 3.25 GB), but can Vista Ultimate, 32 bit? Does it matter if you have a 64 bit processor like Core 2 Duo?
Thanks for any help!

More about : res 32bit vista suffer 4gb limit

February 12, 2007 5:11:51 PM

I have Vista Home Premium 32 bit and 4 gigs of ram. Vista only recognizes 2.75 gigs of it. I am trying to get the 64bit Vista to work, but to no avail.

I have a 64 bit system too. However, in my case, I may have a stick of bad ram, so hopefully someone can either counter or confirm this.
February 12, 2007 5:25:35 PM

I tried 32bit XP and Vista with 4GB and got the same 2.xGB ram available.

I moved to Vista x64 and now running with full 4GB usuable.

Vista a little crash but blame that mostly on the nvidia 8800 drivers.
Related resources
Can't find your answer ? Ask !
February 12, 2007 5:28:21 PM

Quote:
I understand that typical 32 bit OS's like XP can't access the full 4 GB or RAM that I have installed (it shows up at about 3.25 GB), but can Vista Ultimate, 32 bit? Does it matter if you have a 64 bit processor like Core 2 Duo?
Thanks for any help!


I've said this many times and I will say it again... XP CAN ACCESS 4GB. Here is why you see less:

This is a limitation of a 32-bit architecture. The system can only address 4 GB of allocated memory. Allocated memory is made up of physical RAM, and any I/O space needed by devices. The way memory is allocated is that starting at 4 GB, the system allocates device I/O addresses working its way down. Normally this is not a problem, but when systems have 4 GB of physical memory, the addresses needed to address RAM overlap the space needed for I/O. In this case, the need for I/O space takes precedence, and the amount of RAM visible to the operating system and applications is limited to 4 GB minus I/O space. Examples of devices that consume I/O space are:

System BIOS
PCI Express configuration space and memory for PCI Express device(s)
Memory mappy I/O
Motherboard Resources (I/OxAPIC)
Chipset
PCI Enumeration

For example: If you have 4GB of system memory, an Intel 915g Express chipset, Windows XP with Service Pack 2, and a PCI Express graphics card the remaining system memory as reported by System Information would be ~3.25GB. The same configuration but with 2GB of system memory would result in all 2GB being available. This is due to the limited capability of memory mapping (or limited amount of addresses) on 32-bit architecture systems.

EDIT:

Windows XP can utilize more than 2GB of memory as well. However, 2GB of memory is the most that XP can allocate to a particular task.
February 12, 2007 5:53:17 PM

I've actually been wondering why this was a problem. Nice explanation.

:D 
February 13, 2007 12:32:49 PM

Thanks for the info, but maybe I'm a bit dense on this... So how does this actually affect the usage of 4 GB or RAM? Is the system actually really using all 4 GB of RAM, even if it is only showing 3.25 GB?
February 13, 2007 1:14:38 PM

You may have to change the memory config in your bios. On my Asus board I have to turn on memory remapping to see all the ram in Vista 64.
February 13, 2007 1:23:49 PM

No, it's not using all of the 4gb.

Let's say you have 2gb of memory.

Your Hardware drivers will actually have memory pointers to pseudo memory locations in the 3gb-4gb range. These are just for logical and not physical use.

When you increase from 2gb to 4gb, these drivers still need this logical space. As a result, the physical ram in that location can't really be used by Windows.

Vista would have the same type of limitation.
February 13, 2007 2:39:08 PM

Weird that it shows 2.7gb - My desktop has 3GB and it shows up as 3GB so 2.7 isnt some kind of limitation in the OS.
February 13, 2007 2:44:22 PM

The maximum usable value varies depending upon the motherboard, various hardware drivers, and the configuration thereof.

My system has 4gb and 3.5gb is usable.
That tends to be the max.

The amount you are seeing is towards the bottom end.
There is a chance some BIOS settings may free up more memory.

The OS limitation comes in that the drivers need to have "scratch" memory pointers within the 4gb range. This reduces the physical memory usable by Windows from 4gb to
(4gb-(Scratch Space for Drivers)).
February 13, 2007 3:14:22 PM

Quote:
I have a 64 bit system too. However, in my case, I may have a stick of bad ram, so hopefully someone can either counter or confirm this.


Memtest
February 13, 2007 4:14:24 PM

Quote:
I tried 32bit XP and Vista with 4GB and got the same 2.xGB ram available.

I moved to Vista x64 and now running with full 4GB usuable.

Vista a little crash but blame that mostly on the nvidia 8800 drivers.


I'm getting my dual-boot system at the end of the week with XP-32bit and Vista-64bit. I'll only have 2GB to start but will upgrade to either 4 or 6GB in the summer when price of DDR2-800 will go down.

I know this isn,t the point of the tread, but did you get any incompatibility or trouble with regular apps (not AV or the like) compare to XP or is it quite good? Just curious about how often I'll have to use XP.

Thanks.
February 13, 2007 4:34:38 PM

Quote:
Thanks for the info, but maybe I'm a bit dense on this... So how does this actually affect the usage of 4 GB or RAM? Is the system actually really using all 4 GB of RAM, even if it is only showing 3.25 GB?


Windows will not utilize the full 4GB because the the I/O space has to be allocated first.
EDIT:
Technically it will not utilize the full 2GB either would it? The only reason you see 4GB in 64 vs 32 is the addressing scheme in a 64 vs 32 bit OS. Even though the I/O space needs to be allocated first on a system with 2GB you still see 2GB of memory.

Also, some people will say that anything over 2GB is a waste. Yes and no, it depends on your applications you are running. Windows can only assign 2GB of memory to a particular task if I remember correctly.
February 13, 2007 4:48:33 PM

Quote:
Just add the /PAE to the correct like in BOOT.INI


I really appreciate everyone being patient with my denseness on this!

I read the articles, and they are a bit confusing. Let me see if I get this:

By design, VIRTUAL memory pointers are created for hardware that are listed as being in the hexadecimal addresses that actually reside in the 3-4 GB address range?

First, by virtual, does this mean that they are allocated to virtual memory (page file)? It doesn't sound like that is the case; it sounds like it is non-existent memory addresses that they never really thought would be used in a 32 bit OS. Is this correct, or close? If these are non-existent memory addresses, why are they needed? How would the OS use pointers to memory that it does not actually fill and manipulate? What are these pointers for?

Second, if I use the /PAE switch, does the OS then assign the virtual memory pointers to higher hex addresses? How does it actually free up the memory (the articles weren't clear for us non-engineer types).

Third, I want to ensure I have a stable and fast system. Will I be able to count on this after I enable /PAE?

Thanks for the help!
February 13, 2007 5:20:59 PM

Quote:
This is on a production server, that is up 24x7. It is actually my main production server and the one I (and 3600 other people) rely on the most. Therefore, stability is not an issue.


Thanks for the stability clarification! Does it affect performance for intensive gaming, either for better or for worse? The reason I ask is that it seems you often have a trade off.

Also, can anyone comment on the other questions I had listed about the virtual pointers and all? (second post up)
February 13, 2007 5:24:36 PM

/PAE will not help you with current versions of XP. This feature was removed to prevent certain issues. It is still supported on most or all of the Server versions of Windows.

Quote:
Just add the /PAE to the correct like in BOOT.INI


I really appreciate everyone being patient with my denseness on this!

I read the articles, and they are a bit confusing. Let me see if I get this:

By design, VIRTUAL memory pointers are created for hardware that are listed as being in the hexadecimal addresses that actually reside in the 3-4 GB address range?

First, by virtual, does this mean that they are allocated to virtual memory (page file)? It doesn't sound like that is the case; it sounds like it is non-existent memory addresses that they never really thought would be used in a 32 bit OS. Is this correct, or close? If these are non-existent memory addresses, why are they needed? How would the OS use pointers to memory that it does not actually fill and manipulate? What are these pointers for?

Second, if I use the /PAE switch, does the OS then assign the virtual memory pointers to higher hex addresses? How does it actually free up the memory (the articles weren't clear for us non-engineer types).

Third, I want to ensure I have a stable and fast system. Will I be able to count on this after I enable /PAE?

Thanks for the help!
February 14, 2007 2:58:04 AM

S-
The first ref you listed says that even with PAE, WinXP is still limited to a 4GB physical address space. It also implies that the only function of PAE on Win XP is to enable DEP. Thus, it seems we are still stuck with the 3.2GB or so practical limit of usable memory.
February 14, 2007 3:15:47 AM

The /PAE switch in XPSP1 and earlier will actually get you more usable memory than the /PAE switch with SP2 due to changes in the OS.

It's highly unlikely, however, that the gain would be worth it.
February 14, 2007 4:21:18 PM

Are you saying that XP SP1 doesn't have a 4GB address limit with PAE?
February 15, 2007 10:55:47 AM

Quote:
If you only have 4GB of memory you have to understand that you cant see all of it and it is not the fault of the os. even using PAE you wont see all of the memory. the reason is because of your system bios. the system bios takes a certain amount of memory for your system and the devices that can be plugged into it. there are certain memory requirments for systems and the bios has to make sure that there is memory available for the devices you plug into it. thats where the PCI config space comes from. most bios's are optimized for 2GB and not for 4GB.


Then why don't you see a loss of reported RAM when you have 2 GB installed? The motherboard would still have devices to configure, wouldn't it?
February 16, 2007 4:39:28 PM

Quote:
If you only have 4GB of memory you have to understand that you cant see all of it and it is not the fault of the os. even using PAE you wont see all of the memory. the reason is because of your system bios. the system bios takes a certain amount of memory for your system and the devices that can be plugged into it. there are certain memory requirments for systems and the bios has to make sure that there is memory available for the devices you plug into it. thats where the PCI config space comes from. most bios's are optimized for 2GB and not for 4GB.


Then why don't you see a loss of reported RAM when you have 2 GB installed? The motherboard would still have devices to configure, wouldn't it?

yes but the bios guys have different optimizations with 1GB vs 2GB and 4GB vs 2 GB. most of the time 4GB isnt optimized at all. depends on the system / bios you have.

Ok, I am not getting this... So it's just a matter of the bios being optimized is why the hardware memory pointers are not in the 1-2 GB address spaces?

Sounds like someone would have changed the bios'es by this point, since more than 2 GB of RAM is not uncommon anymore.

None of this makes sense, and it looks like no one else really knows, seeing no one responded to my earlier example/ question...
February 16, 2007 5:34:35 PM

Quote:
...
None of this makes sense, and it looks like no one else really knows, seeing no one responded to my earlier example/ question...

Perhaps you missed the replies on this thread?

WinXP only has 4GB of physical memory addresses. Some of those addresses are taken up by hardware interfaces (the number depends on the MB design, expansion cards, etc). Only the leftover addresses are available for accessing actual physical memory, hence the less-than-4GB value you see.

If you have an AMD 64, 64 x2, or recent Pentium or Pentium D, not just a C2D, you already have a 64-bit CPU, and most likely can run a 64-bit OS if you want to (you can try 64-bit Ubuntu Linus for free).

It looks like 32-bit editions of Windows Vista will have the same 4GB limit (see e.g. http://msdn2.microsoft.com/en-us/library/aa366778.aspx ).
February 20, 2007 10:46:01 AM

Quote:

WinXP only has 4GB of physical memory addresses. Some of those addresses are taken up by hardware interfaces (the number depends on the MB design, expansion cards, etc). Only the leftover addresses are available for accessing actual physical memory, hence the less-than-4GB value you see.



Ohhh... So you are saying that the OS was designed to only look for memory addresses in the 2-4 GB hex address range, where they didn't expect to even have actual RAM, and it really has nothing to do with the BIOS? I think I get it now, thanks!

Still seems like a silly oversight...
February 20, 2007 7:37:38 PM

Not exactly. Standard x86 CPU/chipset designs have (until recently) limited the standard range of physical memory addresses to 0-4GB (2^32). These limits were designed-in many years ago when tens of megabytes of RAM seemed like a lot of RAM. Standard OSs (32-bit OSs like Win XP) written for these CPUs have incorporated this limit by being written to work on this standard hardware.
Part of this standard hardware design is that certain hardware components/devices are assigned one or more memory addresses (not actual memory, just the addresses) out of the 4GB "universe" of addresses. For example, an I/O port might be assigned a specific memory address for its output. Whenever the CPU writes data to that address, instead of being stored in memory, the data will be sent out the I/O port. This doesn't use any main RAM, but does use up an address.
Whatever addresses are not used by the hardware are then available to be assigned to the main system RAM. Any RAM without an address cannot be accessed, and so is not usable and not visible to the OS. Until recently, installed system RAM amounts were small enough that there were always plenty of addresses for both the system RAM and the hardware.
During the boot process, the BIOS program identifies what hardware is present and allocates memory addresses to it as needed. Some BIOSes may "waste" addresses during this allocation process for reasons of performance, hardware design, or even BIOS design. One can also inactivate certain hardware in the BIOS, thus reducing the addresses allocated, but in practice for a standard general-purpose system, this won't save many addresses for main RAM.
In practice, the number of addresses gained by re-writing BIOSes and re-designing hardware to be more efficient in the allocation of addresses for hardware would not significantly delay the need to move to 64-bit OSs, and might result in significant hardware incompatibility. Thus, the practical limit of usable system RAM for a standard Win XP (and presumably, 32-bit Vista) system is around 3GB.
There are hardware extensions that allow accessing more than 4GB of physical addresses with a 32-bit OS, but both the OS and apps (and often hardware drivers) need to be specially designed to make use of them.
February 20, 2007 9:37:45 PM

Mondoman, that was an excellent and clear explanation! Especially about the processor "writing" to a RAM address for the serial port actually just sends that data out the serial port.

Thanks!
!