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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  • brink - Thursday, July 12, 2007 - link

    it's a 2 prong solution for WinXP, you have to set a /3GB flag in your BOOT.INI file for the instance of windows you're booting. Additionally since Supcom doesn't have the LARGEADDRESSAWARE flag, you have to patch the EXE using the tool mentioned in the article ((someone created a batch that uses the tool to patch the supcom exe, easily found in the official supcom forums)

    I mention this since the article doesn't really say how they did it (and they used Vista, which is another boc to contend with) Since HardOCP did a comparison a while back between supcom perf in XP and Vista, I've really only installed/used supcom in XP still. With our fix for a 4GB machine (the machine I regularly use still has 2GB, I just stay away from 81KM maps) XP has still remained stable, but we did have one crash in a 40KM map game on Gentleman's Reef.

    I don't like the article's preference on FPS in Supcom, mainly because I don't look at Supcom as a FPS centric game at all. If you've played, you know when a slow computer enters the game (or you have 7 computers each with 1,000 units on a 81KM map) the in-game timer will start to crawl. 1 second of game time will take 2 seconds, or much much worse. It would have been approx 100x cooler if the bench was "it took this much before the timer started to skew".
  • jay401 - Thursday, July 12, 2007 - link

    lol funny, now that I am paging through the article I see you mention this very issue. Good! Reply
  • jay401 - Thursday, July 12, 2007 - link

    Well you didn't address the 'WHY' - why the game uses so much memory. Hopefully I provided a little light on that subject.

    Also on Page 5 none of your graphs are labeled as to what they are measuring. Please note if they're measuring fps, which is my gut but I'm not sure because they are unlabeled.

  • MadBoris - Thursday, July 12, 2007 - link


    Well you didn't address the 'WHY' - why the game uses so much memory. Hopefully I provided a little light on that subject.

    Although you are right in part, the lead engineer did mention the units are one reason for large memory consumption. (BTW, I had heard that they are all being/were rerendered for November). There is another issue beyond that though that becomes obvious. The initial virtual address space at the beginning of a game between a 20k map and 81k map is only about 150MB difference. But as unit count climbs, the larger map gap grows somewhat exponentially compared to the smaller. So something else is askew.

    As Ryan mentioned the whys and wherefores aren't really the point, this issue is a global one and 2GB is a real hard limit now for games since we have the horsepower(CPU & GPU) for larger memory consuming texture maps, larger resolutions, yet the 2GB memory limit for a game is a definitive roadblock to forward progress so I am glad the issue is coming to the forefront.

    As much confusion and fear there is on this /3GB subject for the laymen, this is still a great rabbit in the hat for us with 32 bit OS's if more driver writers get on the ball, fears can subside. Hopefully devs like Crytek can continue to push demand for 64 bit with a nice 64 bit Crysis patch too, and we can start making the transition leaving 32 bit behind as drivers/apps also make the transition.

    I think articles like these help the cause.
  • Ryan Smith - Thursday, July 12, 2007 - link

    To be honest, we didn't address why because it really isn't relevant. Even if Supreme Commander was done perfectly in every way, the result would have been the same once it reached the 2GB barrier. Reply
  • gigahertz20 - Thursday, July 12, 2007 - link

    They should have made Vista only 64 bit to put pressure on the transition. Reply
  • johnsonx - Friday, July 13, 2007 - link

    The problem with going 64-bit only this time around is that there are too many 32-bit programs around that simply won't run on 64-bit windows. I have several myself that I depend on daily. They are slightly older programs that the developer doesn't intend to upgrade to be 64-bit, but that doesn't change the fact that I need them. If these apps didn't work with Vista because it was released in 64-bit only form, then I wouldn't be running Vista. Millions of others are (or would be) in this same situation, which would significantly harm Vista sales.

    If the next version of Windows were made 64-bit only, around the 2010 time frame, I think that would be quite reasonable. By then most 32-bit only programs will have been replaced or rendered obsolete.

    I think Microsoft has handled the 32/64-bit issue correctly so far, for the most part. XP64 should have been ready sooner and should have been better supported though.

    Related question for anyone who knows: I know retail Vistas include 32-bit and 64-bit on the same disc, and the user is free to install either. I also know that OEM Vistas include only the one version on the disc. What about the OEM keycodes though? Can you install a 64-bit Vista using the keycode that came with a 32-bit disc? Or has MS limited the keycode as well?
  • StygianAgenda - Thursday, June 05, 2008 - link

    To answer your question about the Windows Vista Retail package, I have 2 copies of Vista Ultimate retail, and it was packed with 2 DVDs, 1 is 32bit, the other is the 64bit build.

    The set comes with a single key, and the key is bound to the 64bit version, so if you opt to use the 32bit version instead, you'll most likely have to call into Microsoft's activation center and manually activate your copy. I've had to do this 4 times now, due to hardware changes, because Vista detects system changes, so if you remove 1 or 2 boards and boot into Vista, the system will automatically de-activate. Now, granted, the call to MS was fairly painless, but it's annoying all the same.

    Out of the 4 Vista systems I own currently (3 of which are laptops), I've had great success with the OS, itself. Unfortunately for me, the motherboard I've been using on my custom built workstation is flaky... I've done my research though (tonight), and might have a fix in the works, if it works, that is. Otherwise, I'll be ordering a new motherboard, and calling Microsoft yet again to transfer my license to the new configuration. By the way, they always ask "Is this copy running on more than one PC?". In light of all the hoopla over the licensing scheme in Vista, I would hope that no one is stupid enough to try to use a Vista Retail license on multiple PCs, because it'll cause all of them to be blacklisted. Oh, and the new Vista Enterprise edition is only available in lots of 25 licenses or higher, and requires a licensing server on the LAN with the deployed workstation licenses. It's either that, or expect to have a couple of extra hundred MB or so of net traffic from all of the Vista Ent. workstations checking in with MS everytime the systems are booted. Makes me glad that I also work with Linux very heavily, and all things considered, if Linux + WINE can run all of my criticle Win32 apps, then this will be the last Vista Licenses that I buy. I'll still keep Vista on my laptops, and I'll continue to run my XP workstations, and 2K3 servers, but MS is going to have to really do some impressive work to get me convinced to migrate to their next platform... such as maybe... a *real* 3D desktop... which is already available, stable and totally badass on Linux (check out Kubuntu + compiz).

    (btw: Sorry if I seem like I'm on a rant here... no offense intended toward the readers, at all... it's just that when you work with OS's at the level that I do, after a while stupid mistakes made by OS vendors start to get beyond aggrivating.)
  • instant - Saturday, July 21, 2007 - link

    And when we are talking about GAMES, how if at all is this relevant to the current discussion?

    x64 has been the way to go ever since it was released.
  • miahallen - Saturday, July 14, 2007 - link

    That is incorrect, Vista x64 will run x86 apps without problem (so will XP x64), that's the nice thing about it. I ran x64 for quite a while, and ran almost nothing but x86 apps on it.

    The keycodes for x86 do not chnge for x64 installs...they use the same key. And the retail versions I bought did not have both versions on one disc, I had to order a x64 disc online (and pay $10 for S&H).

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