A Primer on Windows' Memory Management

In 1986, Intel released the 386 processor, which offered support for a new instruction set (IA32), that was an extension of the original x86 instruction set. Among the most notable features in IA32 was an increase in the amount of memory the CPU could address, moving from 20bit addressing(1MB) to 32bit addressing(4GB)(ed: this is not including the convoluted mess that was segmented addressing). All x86 CPUs released since then have supported the same instruction set, including the same 4GB limit. Only recently have 64bit x86 CPUs been released, and while they support more than 4GB of memory, still are limited to 4GB when operating in 32bit mode.

As we have mentioned in previous articles, most modern system running in 32bit x86 mode have trouble seeing and using more than roughly 3GB of memory. This is because part of the total 4GB of memory space (not the physical memory) is reserved for various functions, such as computer components transferring data between each other using memory-mapped input-output(MMIO). The textbook example of this is the CPU transferring data to the memory of a video card, where a chunk of the address space equal to the size of the memory of the video card is reserved by the video card, and any data sent to those addresses actually ends up going to the video card. This design has many technical merits, but it makes the consumed memory addresses unavailable for use with physical memory.

Things only get more complex as we start including the operating system (in this case Windows) in to the equation. The above is actually handled by a combination of Windows and the BIOS, meanwhile Windows also needs some address space so that programs can communicate with the Windows kernel, for storing buffers, for storing memory tables, etc; all of which means we have lost even more address space. All of the above besides preventing us from addressing 4GB of physical memory are also the cause of the actual 2GB barrier that is the problem.

Quickly, there is one more pre-requisite piece of information: virtual address space. For a 32bit Windows application(Win32), each application has a full 4GB worth of private addressing space that it can use, which again is 32bits and a result of how a 32bit processor works. How the above exactly works is beyond the scope of this article, but it's sufficient to say that at some point virtual addresses get translated in to other addresses for mapping data between the application and physical memory or the swap file. The important thing to take from this is that each application has its own 4GB virtual address space, regardless of the hardware the computer contains or what applications are running. Now we may begin to understand the 2GB barrier.

Due to design reasons outside the scope of this article, Windows takes a pre-determined portion of each application's virtual address space and reserves it for itself. Windows uses its portion of the virtual address range for all of the address needs listed above. At the end of the day, and what really matters, is that in designing Windows Microsoft opted to split up the virtual address space of an application in half; 2GB goes to Windows (kernel space) and 2GB goes to the application (user space). Under normal circumstances this 2GB of space is all a 32bit application has to work with, this is the 2GB barrier and as we'll see is the cause of the problems with Supreme Commander.

It is also worth noting at this point that virtual address space is not directly correlated to physical memory size. Windows and any applications running under it may use up to all of their allocated virtual address space, with Windows simply swapping data between the hard drive and physical memory if the amount of physical memory needed is in excess of what's available - this is virtual memory, not to be confused with virtual address space. The only meaningful relation between virtual address space and physical memory is that the amount of physical memory an application can use can never be greater than its virtual address space. The user space must take up a larger portion of the virtual address space if an application is to use more than 2GB of physical memory.

Index Removing the 2GB Barrier


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  • BabyBear - Thursday, July 12, 2007 - link

    This also happens with Flight Simulator X by the way.

    Over on Phil Taylor's blog he makes mention of it awhile back

    http://blogs.msdn.com/ptaylor/archive/2007/06/15/f...">Microsoft's Phil Taylors Blog

    There was also some talk that the June 07 DirectX Redist. seemed to 'help' with Out of Memory problems when using the /3gb switch.
  • joetron2030 - Thursday, July 12, 2007 - link

    On "Page 4: A Case Study: Supreme Commander", in the paragraph discussing the first screen shot from Sysinternals' Process Explorer, you refer to the "Virtual Size" column as the "Private Size" column (which doesn't exist in the screen shot).

    Had me briefly confused.
  • quanta - Thursday, July 12, 2007 - link

    Although Windows 2000/2003/Vista server versions aren't exactly designed for gaming, did anyone tested game titles on these server OS to see if these large address aware titles will use the Physical Address Extension feature? Reply
  • Ryan Smith - Thursday, July 12, 2007 - link

    The short answer is no.

    The longer answer is that due to a combination of chipset support, software support, performance, and driver support, it's not really usable outside of a server environment and shouldn't be used on a consumer system.
  • brink - Thursday, July 12, 2007 - link

    Wouldn't matter, WinXP SP2 uses PAE, why would you want to install it on a server OS? Only 2003 is a server OS of the ones you mentioned, and I think only Windows 2003 Enterprise is the only 32-bit OS that has the ability to use more than 4GB of memory (8GB seems to be the limit for what modules/mobos are available right now) Reply
  • TA152H - Thursday, July 12, 2007 - link

    I'm just reading the remarks about the 386 pushing the memory from 1 MB, to 4 GB. It's patently untrue, unless you think the 386 succeeded the 8086, and not the 286. The 286 had 24 bit addressing (in 64K segments) for 16 MB of memory. This is not only what OS/2 used, but extended memory as well. So, it's not academic. Reply
  • Ryan Smith - Thursday, July 12, 2007 - link

    Just hearing the words "segmented memory" gives me flashbacks - and they're not the good kind. For this article we're talking strictly about flat addressing since I could write a small book on just segmented addressing, but I've updated the article to make this clear. Reply
  • TA152H - Thursday, July 12, 2007 - link

    Either way, you're using the wrong values. Even on the 8086 the memory was segmented. Actually, so was the 386, but it could handle segments up to 4 GB, so it wasn't important. So, using 1 MB and saying you're not referring to segmentation is inaccurate; that is segmented memory.

    Segmentation was not a bad thing, particularly back then. It saved memory space, and back in 1978 that was a big thing. Addresses were given in 16 bits, you didn't have to specify the rest, and the net result was you had shorter code. You would have to change the registers to point to the next segment from time to time, but overall it saved a lot of memory. You also could protect apps from each other, but that wasn't really used.

    I was an OS/2 developer, and it was a pain sometimes for sure. By the time the 286 came out, which I think was the best processor Intel ever made considering the timing, and the incredible capabilities it had over it's predecessor (the 8086, not the 80186 which was made in parallel with the 286), the extra memory saving was not worth the nuisance of having to deal with, at most, 64K segments. But it really wasn't possible for Intel to do it any other way, as I'll explain below.

    Motorola even added some external memory management unit that added segmentation, which today seems strange. It's widely viewed now as just a bad thing, but back then, it wasn't. Motorola really had a choice though, since the 68K was part 32-bit, and part 16-bit, and part 24-bit (addressing). Although the 286 was a more powerful processor than the 68000, it was pure 16-bit, except for the oddity of the 24-bit addressing. Consequently, allowing flat 24-bit addressing would not have been feasible. So, they dealt with it by just adding more segments. Considering the absolutely incredible improvement in this processor in just about every way, it was not such a bad tradeoff.
  • BitJunkie - Friday, July 13, 2007 - link

    Hiya Bill. Reply
  • jay401 - Thursday, July 12, 2007 - link

    Just FYI, SupCom has already exhibited this problem and crashes once it breaks the 2GB barrier, which can happen easily in longer games with a high unit limit.

    This is mostly due to the poor efficiency in their code and models, something GPG (the developer, Gas Powered Games) reported could not easily be fixed in a patch because they basically have to re-render every unit in the game so they take up less memory. Due to this, it's likely to remain unfixed until the expansion pack (now a standalone game) Forged Alliance comes out in November, if it is fixed at all

    One forum member developed a way to increase the addressable memory to around 3GB on 32bit WinXP and Vista, so if you're running 4GB, this provides a sort of band-aid solution in the meantime.

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