17 ms·

Can a 32-bit OS machine use up all 8GB RAM + 20GB page file?

67 points by readme 14y ago

zvrba 14y ago
Which myth exactly? Also, swapping memory in and out of the address space is a rather costly operation. And it becomes infeasible if the process's working set grows larger than the available address space [thrashing].
- tonfa 14y ago
  I think that on Linux, the 4G/4G split never made it, so in practice the limit is around 64GB due the limited amount of lowmem available to the kernel in the regular 3G/1G split. https://lwn.net/Articles/39283/ https://lwn.net/Articles/39283/
rbanffy 14y ago
Did everyone forget about bank switching and address extension tricks like PAE?
rplst8 14y ago
There was a myth?
0x0 14y ago
There is also a very recently developed middle ground, the Linux "x32" ABI, since Linux v3.4: http://en.wikipedia.org/wiki/X32_ABI http://en.wikipedia.org/wiki/X32_ABI As I understand it, this mode gains performance by sticking with 32bit pointers in user-space instead of 64bit pointers, while still allowing for taking advantage of all the new registers etc available in x86-64 ("amd64") CPUs. (Many programs doesn't really need more than 4gb virtual address space, and so using full 64bit pointers can cause quite the overhead - imagine doubly-linked lists with small data structures, for example)
- ajross 14y ago
  x32 is clearly a good technical solution. If you're running on a 64 bit kernel, x32 is superior to i686 in every measurable way. But it's also yet another architecture. It's not binary compatible with either i686 or x86_64. You need your whole userspace compiled to use it. Middleware with embedded assembly (there's a surprising amount of this in glibc, and of course things like ffmpeg, libjpeg, etc...) needs to be ported. You can't run 32 bit binaries from proprietary sources. And frankly the benefit over straight x86_64 is quite modest. I don't see x32 taking off. It's just not worth the hassle.
  - 0x0 14y ago
    I thought that the work that goes into multiarch support would allow you to run a single kernel and mix&match x32 and x86_64 binaries on the same system, but I might be wrong (Of course that would require a separate set of any lib/dependency). Some numbers mentioned on the x32abi page hint on anywhere between 4% and 40% performance gains, if that is true then I'd think the benefits would outweigh the hassle of another architecture. (Edit: Most middleware also ship straight-C versions of the routines; whether or not an x32 C compiler can measure up to handcrafted x86 or x86_64 assembly I don't know - but I'm guessing the much higher register count would help a lot. Regarding proprietary software: There are a great many server configurations that do not need anything beyond the standard open source packages available in Debian)
    - sp332 14y ago
      A standalone x32 binary will run fine on an x64 machine. But if you want to link to any libraries, the library will also have to be x32. So an x64 system, which probably has 32-bit legacy libs as well as normal 64-bit ones, will also need a complete set of x32 libs for x32 to be practical.
      - 0x0 14y ago
        Sure. But after the porting work has been done by the distribution vendor, it's done. The package manager software should be able to do whatever is necessary to almost transparently ensure any necessary dependencies are installed for the required sub-architecture. So I would imagine that in most cases, end-users won't notice any hassle except having the option to choose between x32 and x86_64 per package during installation. I think that sounds kind of neat :)
deleted 14y ago
[deleted]
Mara 14y ago
Am I missing something, or does the top answer not address the question? Memory tricks aside, it seems the questioner is asking whether the physical RAM itself will ever be used. Short of PAE, unless my understanding is wrong, I was under the impression that the answer to this is no.
- exDM69 14y ago
  > Short of PAE, unless my understanding is wrong, I was under the impression that the answer to this is no. PAE is used everywhere and has been around since 1995 Pentium Pros. So the answer is yes, a 32 bit OS/CPU can use 8 gigabytes (and more) of physical memory and an infinite amount of swap space. It all depends on the CPUs physical address size, not virtual. My "64 bit" CPU has 36 bit physical addresses and 48 bit virtual addresses.
  - mbell 14y ago
    > My "64 bit" CPU has 36 bit physical addresses and 48 bit virtual addresses. Just to clarify for anyone that doesn't catch what you mean and thinks PAE is the devil: x86-64 CPUs must support in PAE when operating in long mode (64 bit mode). Further the version of PAE used is 'classic PAE' with an additional translation layer added to get some more physical address space. In other words even 64bit CPUs don't have a real 64bit physical address space, they still use PAE to access large amounts of ram.
- floody-berry 14y ago
  Even with PAE, many Windows versions hard limit the available RAM to 4GB (or less). So his answer isn't even correct given the reality of many operating systems & hardware. http://en.wikipedia.org/wiki/Physical_Address_Extension#Operating-system_support http://en.wikipedia.org/wiki/Physical_Address_Extension#Oper... http://en.wikipedia.org/wiki/3_GB_barrier http://en.wikipedia.org/wiki/3_GB_barrier
joshka 14y ago
Stop editorialising with titles
iuguy 14y ago
This reminds me of some of the fun and games we used to have with 8-bit computers. 8-bit computers running chips such as the Z80 and 6502 were limited to 16-bit addressing and a total of 64k of addressable RAM. For most computers earlier on this wasn't a problem, but in the later 80s when machines with 128k started to become more common this meant that there were lots of strange quirks to be considered. For example, the Sinclair ZX Spectrum (Timex TS2000 in the US) originally came with 16 or 48k of RAM and 16k of ROM (64k of total addressable memory). When the +128k came out in 1985, Sinclair used a fairly interesting way of getting around the 64k limit. The memory was turned into 16k banks, which were the manipulated via a port. A bit more info on how this worked is available here[1]. Incidentally someone built a 4Mb RAM upgrade for the Spectrum![2] [1] - http://www.worldofspectrum.org/faq/reference/128kreference.htm#ZX128Memory http://www.worldofspectrum.org/faq/reference/128kreference.h... [2] - http://8bit.yarek.pl/upgrade/zx.zx4mb/index.html http://8bit.yarek.pl/upgrade/zx.zx4mb/index.html
- pmarin 14y ago
  http://en.wikipedia.org/wiki/Bank_switching http://en.wikipedia.org/wiki/Bank_switching
virmundi 14y ago
This seems like an appropriate place to ask this question. At work I have an i5 with 8 GB of ram. Unfortunately we're stuck with XP 32. So is there a away for me to use more memory over all? Specifically I have to have Outlook and my VirtualBox Linux machine running at the same. I'd like to use bank switching or PAE to give my VM 4 GB. While all of the other applications just fight out for the remaining 4.
- Symmetry 14y ago
  No, only Windows server editions have PAE enabled and AFAIK this isn't something you can use without kernel support.
  - masklinn 14y ago
    Technically, all modern Windows versions have PAE enabled (the NX bit requires enabling PAE for instance), but Windows adds license-based memory restrictions on top of that. That's how the 2GB limit on Starter, the 4GB limit on non-server and the various limits on server editions are implemented. And there is a kernel hack/patch to lift that restriction, although it is of course fully unsupported: http://superuser.com/questions/52275/how-can-i-enable-pae-on-windows-7-32-bit-to-support-more-than-3-5-gb-of-ram/95309#95309 http://superuser.com/questions/52275/how-can-i-enable-pae-on...
    - virmundi 14y ago
      Thanks for the info. It's sad that I out my employer will allow it.
      - keithpeter 14y ago
        would employer allow a linux install then XP in a VM or several? All problems solved then.
- j_s 14y ago
  Run VMWare vSphere on the bare metal, virtualizing both XP and Linux. Of course, you'd have to re-install everything. http://www.vmware.com/products/vsphere-hypervisor/faq.html http://www.vmware.com/products/vsphere-hypervisor/faq.html
thechut 14y ago
I dual boot Ubuntu 12.04 32-bit and Windows 7 32-bit on my HP laptop, it has 8GB of RAM. When I am running Ubuntu I see 7.8GB (I'm assuming this is 1000 vs 1024 issue and doesn't concern me) in my system settings and resource monitor. When I am running Windows I only see 3.8GB of RAM in resource monitor, but I see the full 8GB if I look in device manager. Can somebody explain this to me?
- untog 14y ago
  I'm not an expert on these matters, but as I understand it, Windows is not ignorant of the fact that you have 8GB of RAM, it is simply unable to address it. So it makes logical sense that it would show you having 8GB, but only 3.8GB of usable RAM.
  - masklinn 14y ago
    That is not correct, Windows is actually fully aware of the physical RAM on the machine and is technically able to use it, but has a license-based restriction scheme only letting a subset of the RAM be used.
- zokier 14y ago
  Most likely your Ubuntu uses PAE, and Windows does not. NT kernel certainly supports PAE, but likely MS has disabled it in client systems for some reason.
  - masklinn 14y ago
    > NT kernel certainly supports PAE, but likely MS has disabled Nope. They can't actually disable it since the NX bit requires that PAE be enabled. The limitation comes from licensing and tiering, and is handled in the kernel independently from PAE.
- 3amOpsGuy 14y ago
  >> I'm assuming this is 1000 vs 1024 issue It'll likely be your graphics card uses a portion of your main RAM for its memory. This is common.
AD1066 14y ago
Holy Baader-Meinhof phenomenon. I ordered 8 GB of RAM for my HP laptop last night and spent well over two hours reading about this exact subject to see if I needed to install Windows 64-bit when it arrives. I think I even stumbled upon this same page. That being said, I still don't fully understand; Once I install the new RAM, will I only receive the benefits if I perform a new 64-bit install of Windows 7, or will upgrading just amplify the benefits? (I currently have 3 GB in 1 GB + 2 GB as the default factory configuration.) Sorry if this question is dense of me, but my field is economics and the more technical aspects of HN are often lost on me
- ThePherocity 14y ago
  Yea, although there appear ways around it though hackery, for all intents and purposes, you need a 64bit OS to take advantage of the extra memory.
- gergles 14y ago
  You need to install 64-bit Windows to use anything beyond ~3.5GB of RAM.
  - icebraining 14y ago
    This is not true. Did you read the answers? 32-bit OSs with PAE support can use all the RAM. Most processes will only be able to use 4GB, but that's per process, not for the whole system. So AD1066 doesn't need a 64-bit Windows, just any 32-bit Windows newer than XP.
    - masklinn 14y ago
      > So AD1066 doesn't need a 64-bit Windows, just any 32-bit Windows newer than XP. No. Non-server 32b Windows will never give access to more than 4GB RAM to the user unless you patch the kernel. XP 32b, Vista 32b and 7 32b — including Ultimate for the latter 2 — all restrict "available physical memory" to 4GB in all situations. So do many server editions as well, for Windows Server 2008 x86 for instance, only the Enterprise and Datacenter editions will give access to more than 4GB RAM (up to 64). A 2008 Enterprise license is ~$1500. I doubt ad1066 has any desire to pay such a price.
3amOpsGuy 14y ago
The top answer seems wrong to me. >> a process can have more memory than address space How does that work? The best I can come up with is the author is referring to mmap? In which case the address space is still limited to 4Gb although I concede that with the OS pagefile it would be possible to have more than 4Gb in core at a time via judicious use of mmap.
- icebraining 14y ago
  You need to ask the OS to remap your address space to different area of physical memory. Windows has the Address Windowing Extensions[1], which allow processes to reserve multiple blocks of memory and switch between them. [1]: http://en.wikipedia.org/wiki/Address_Windowing_Extensions http://en.wikipedia.org/wiki/Address_Windowing_Extensions
ralish 14y ago
Most of the responses to the question seem to be "sort of" correct but missing important parts of the overall picture. Further, a complete answer to the question is going to be subject to the operating system the given application is running under as the specifics are going to be heavily dependant on OS implementation. I can't speak for Unix systems, but on the Windows side, it's complicated and messy: PAE (Physical Address Extensions) Client editions of Windows do not support PAE while server editions do. If you want to physically address more than 4GB of RAM you need PAE. Realistically, you're unlikely to be able to address even 4GB under non-PAE x86 due to reserved memory ranges from hardware taking a chunk. Funfact 1: As for why PAE isn't supported on client editions, this isn't a licensing problem (you can buy x64 editions for the same price that have massively increased RAM limits), but rather, Microsoft playing it safe with driver compatibility. Apparently, during testing a non-trivial number of drivers were found that made assumptions that don't hold under PAE and thus broke horribly. Things like using the high bit of a 32-bit pointer for data storage. Rather than risk a tsunami of bluescreens caused by poorly written drivers (which people would blame MS for out of naivety) the decision was made to simply not support PAE on client editions. Server editions do as there's the reasonable expectation that servers should be running higher quality drivers. Funfact 2: Technically, PAE is supported on client editions, as it's required for hardware DEP support, so it's more a hard limit on addressable physical memory than it is a lack of support for PAE itself. AWE (Address Windowing Extensions) This is a Windows-specific capability (unlike PAE, which is a hardware capability provided by modern CPUs) which allows a process to address more memory than it has virtual address space. Further, it works hand-in-hand with PAE as you by definition need PAE support in order for the OS to address memory higher than 4GB in the first place. However, in order to use AWE, the application has to be explicitly written to utilise it as the usage of AWE requires the application itself to take a far more active role in managing its memory. AWE support is indicated via a flag in the PE header of the application binary. Needless to say, a tiny minority of applications have this support. The only one I could name right now would be Microsoft's SQL Server. 4GT (4-Gigabyte Tuning) This is one other Windows-specific capability that isn't really related to a discussion on 4GB limitations but inevitably will be raised regardless. While PAE provides for the OS to address more than 4GB of physical memory, and AWE provides for an application to address more memory than in its virtual address space, neither of these features increase the amount of addressable memory at any given time in an application (ie. AWE allows you to change your view of memory, but not address more than 2GB of memory simultaneously). A Windows process in 32-bit has a 2GB virtual address space, with the remaining 2GB mapped to the kernel address space. 4GT, in contrast to PAE and AWE does allow you to modify the address space available to a user-mode application. By providing the appropriate bootloader parameter you can modify the usermode/kernelmode split to skew it up to a maximum of 3GB/1GB user/kernel-mode respectively. This used to be done quite a lot on large Terminal Servers for example, or to optimise database servers, however, the ramifications of doing so need to be carefully understood. You are reducing the amount of memory available to the kernel, and so, memory hungry drivers may now fail to load, or worse, outright crash the system if they haven't been coded to properly handle low memory conditions. EDIT: I forgot to mention, that like AWE, an application won't even use the increased address space 4GT provides unless it is marked as capable in its PE Header. Merely enabling the setting won't get you anything unless your application supports this capability. Firefox, as an example, is large address aware and being 32-bit, can thus address up to 3GB instead of 2GB. While on 32-bit systems this capability is not likely used (how many 32-bit Windows users have modified their bootloader parameters to support 4GT?) it is fully used on 64-bit Windows and can use the full 3GB address space out of the box. And god knows Firefox needs it... In summary, in the age of x64, I tend to view all of the above as far more trouble than it's worth. The virtual address space of a Windows 64-bit process is currently 8TB. Windows 7 Professional and up give you 192GB of physical memory, while Windows Server 2008 R2 Enterprise and up give you up to 2TB. So if at all possible, avoid 32-bit, 64-bit is the solution; PAE, AWE & 4GT were only ever workarounds. If any of the above is incorrect, please correct me, as it's been a while since I studied much of the above, and if anyone can shed more light on the Unix situation, I'd be interested (in a very nerdy way).
jiggy2011 14y ago
How does one swap memory in and out of address space? I understand this can be done with files via mmap but not sure about RAM, since in that case would you not need to know the physical memory addresses of what you wanted to swap in or out? Isn't this information deliberately hidden from userspace applications?

67 points by readme 14y ago

zvrba 14y ago

Which myth exactly? Also, swapping memory in and out of the address space is a rather costly operation. And it becomes infeasible if the process's working set grows larger than the available address space [thrashing].

tonfa 14y ago
I think that on Linux, the 4G/4G split never made it, so in practice the limit is around 64GB due the limited amount of lowmem available to the kernel in the regular 3G/1G split. https://lwn.net/Articles/39283/ https://lwn.net/Articles/39283/

rbanffy 14y ago

Did everyone forget about bank switching and address extension tricks like PAE?

rplst8 14y ago

There was a myth?

0x0 14y ago

There is also a very recently developed middle ground, the Linux "x32" ABI, since Linux v3.4: http://en.wikipedia.org/wiki/X32_ABI http://en.wikipedia.org/wiki/X32_ABI As I understand it, this mode gains performance by sticking with 32bit pointers in user-space instead of 64bit pointers, while still allowing for taking advantage of all the new registers etc available in x86-64 ("amd64") CPUs. (Many programs doesn't really need more than 4gb virtual address space, and so using full 64bit pointers can cause quite the overhead - imagine doubly-linked lists with small data structures, for example)

ajross 14y ago
x32 is clearly a good technical solution. If you're running on a 64 bit kernel, x32 is superior to i686 in every measurable way. But it's also yet another architecture. It's not binary compatible with either i686 or x86_64. You need your whole userspace compiled to use it. Middleware with embedded assembly (there's a surprising amount of this in glibc, and of course things like ffmpeg, libjpeg, etc...) needs to be ported. You can't run 32 bit binaries from proprietary sources. And frankly the benefit over straight x86_64 is quite modest. I don't see x32 taking off. It's just not worth the hassle.
- 0x0 14y ago
  I thought that the work that goes into multiarch support would allow you to run a single kernel and mix&match x32 and x86_64 binaries on the same system, but I might be wrong (Of course that would require a separate set of any lib/dependency). Some numbers mentioned on the x32abi page hint on anywhere between 4% and 40% performance gains, if that is true then I'd think the benefits would outweigh the hassle of another architecture. (Edit: Most middleware also ship straight-C versions of the routines; whether or not an x32 C compiler can measure up to handcrafted x86 or x86_64 assembly I don't know - but I'm guessing the much higher register count would help a lot. Regarding proprietary software: There are a great many server configurations that do not need anything beyond the standard open source packages available in Debian)
  - sp332 14y ago
    A standalone x32 binary will run fine on an x64 machine. But if you want to link to any libraries, the library will also have to be x32. So an x64 system, which probably has 32-bit legacy libs as well as normal 64-bit ones, will also need a complete set of x32 libs for x32 to be practical.
    - 0x0 14y ago
      Sure. But after the porting work has been done by the distribution vendor, it's done. The package manager software should be able to do whatever is necessary to almost transparently ensure any necessary dependencies are installed for the required sub-architecture. So I would imagine that in most cases, end-users won't notice any hassle except having the option to choose between x32 and x86_64 per package during installation. I think that sounds kind of neat :)

deleted 14y ago

[deleted]

Mara 14y ago

Am I missing something, or does the top answer not address the question? Memory tricks aside, it seems the questioner is asking whether the physical RAM itself will ever be used. Short of PAE, unless my understanding is wrong, I was under the impression that the answer to this is no.

exDM69 14y ago
> Short of PAE, unless my understanding is wrong, I was under the impression that the answer to this is no. PAE is used everywhere and has been around since 1995 Pentium Pros. So the answer is yes, a 32 bit OS/CPU can use 8 gigabytes (and more) of physical memory and an infinite amount of swap space. It all depends on the CPUs physical address size, not virtual. My "64 bit" CPU has 36 bit physical addresses and 48 bit virtual addresses.
- mbell 14y ago
  > My "64 bit" CPU has 36 bit physical addresses and 48 bit virtual addresses. Just to clarify for anyone that doesn't catch what you mean and thinks PAE is the devil: x86-64 CPUs must support in PAE when operating in long mode (64 bit mode). Further the version of PAE used is 'classic PAE' with an additional translation layer added to get some more physical address space. In other words even 64bit CPUs don't have a real 64bit physical address space, they still use PAE to access large amounts of ram.
floody-berry 14y ago
Even with PAE, many Windows versions hard limit the available RAM to 4GB (or less). So his answer isn't even correct given the reality of many operating systems & hardware. http://en.wikipedia.org/wiki/Physical_Address_Extension#Operating-system_support http://en.wikipedia.org/wiki/Physical_Address_Extension#Oper... http://en.wikipedia.org/wiki/3_GB_barrier http://en.wikipedia.org/wiki/3_GB_barrier

joshka 14y ago

Stop editorialising with titles

iuguy 14y ago

This reminds me of some of the fun and games we used to have with 8-bit computers. 8-bit computers running chips such as the Z80 and 6502 were limited to 16-bit addressing and a total of 64k of addressable RAM. For most computers earlier on this wasn't a problem, but in the later 80s when machines with 128k started to become more common this meant that there were lots of strange quirks to be considered. For example, the Sinclair ZX Spectrum (Timex TS2000 in the US) originally came with 16 or 48k of RAM and 16k of ROM (64k of total addressable memory). When the +128k came out in 1985, Sinclair used a fairly interesting way of getting around the 64k limit. The memory was turned into 16k banks, which were the manipulated via a port. A bit more info on how this worked is available here[1]. Incidentally someone built a 4Mb RAM upgrade for the Spectrum![2] [1] - http://www.worldofspectrum.org/faq/reference/128kreference.htm#ZX128Memory http://www.worldofspectrum.org/faq/reference/128kreference.h... [2] - http://8bit.yarek.pl/upgrade/zx.zx4mb/index.html http://8bit.yarek.pl/upgrade/zx.zx4mb/index.html

pmarin 14y ago
http://en.wikipedia.org/wiki/Bank_switching http://en.wikipedia.org/wiki/Bank_switching

virmundi 14y ago

This seems like an appropriate place to ask this question. At work I have an i5 with 8 GB of ram. Unfortunately we're stuck with XP 32. So is there a away for me to use more memory over all? Specifically I have to have Outlook and my VirtualBox Linux machine running at the same. I'd like to use bank switching or PAE to give my VM 4 GB. While all of the other applications just fight out for the remaining 4.

Symmetry 14y ago
No, only Windows server editions have PAE enabled and AFAIK this isn't something you can use without kernel support.
- masklinn 14y ago
  Technically, all modern Windows versions have PAE enabled (the NX bit requires enabling PAE for instance), but Windows adds license-based memory restrictions on top of that. That's how the 2GB limit on Starter, the 4GB limit on non-server and the various limits on server editions are implemented. And there is a kernel hack/patch to lift that restriction, although it is of course fully unsupported: http://superuser.com/questions/52275/how-can-i-enable-pae-on-windows-7-32-bit-to-support-more-than-3-5-gb-of-ram/95309#95309 http://superuser.com/questions/52275/how-can-i-enable-pae-on...
  - virmundi 14y ago
    Thanks for the info. It's sad that I out my employer will allow it.
    - keithpeter 14y ago
      would employer allow a linux install then XP in a VM or several? All problems solved then.
j_s 14y ago
Run VMWare vSphere on the bare metal, virtualizing both XP and Linux. Of course, you'd have to re-install everything. http://www.vmware.com/products/vsphere-hypervisor/faq.html http://www.vmware.com/products/vsphere-hypervisor/faq.html

thechut 14y ago

I dual boot Ubuntu 12.04 32-bit and Windows 7 32-bit on my HP laptop, it has 8GB of RAM. When I am running Ubuntu I see 7.8GB (I'm assuming this is 1000 vs 1024 issue and doesn't concern me) in my system settings and resource monitor. When I am running Windows I only see 3.8GB of RAM in resource monitor, but I see the full 8GB if I look in device manager. Can somebody explain this to me?

untog 14y ago
I'm not an expert on these matters, but as I understand it, Windows is not ignorant of the fact that you have 8GB of RAM, it is simply unable to address it. So it makes logical sense that it would show you having 8GB, but only 3.8GB of usable RAM.
- masklinn 14y ago
  That is not correct, Windows is actually fully aware of the physical RAM on the machine and is technically able to use it, but has a license-based restriction scheme only letting a subset of the RAM be used.
zokier 14y ago
Most likely your Ubuntu uses PAE, and Windows does not. NT kernel certainly supports PAE, but likely MS has disabled it in client systems for some reason.
- masklinn 14y ago
  > NT kernel certainly supports PAE, but likely MS has disabled Nope. They can't actually disable it since the NX bit requires that PAE be enabled. The limitation comes from licensing and tiering, and is handled in the kernel independently from PAE.
3amOpsGuy 14y ago
>> I'm assuming this is 1000 vs 1024 issue It'll likely be your graphics card uses a portion of your main RAM for its memory. This is common.

AD1066 14y ago

Holy Baader-Meinhof phenomenon. I ordered 8 GB of RAM for my HP laptop last night and spent well over two hours reading about this exact subject to see if I needed to install Windows 64-bit when it arrives. I think I even stumbled upon this same page. That being said, I still don't fully understand; Once I install the new RAM, will I only receive the benefits if I perform a new 64-bit install of Windows 7, or will upgrading just amplify the benefits? (I currently have 3 GB in 1 GB + 2 GB as the default factory configuration.) Sorry if this question is dense of me, but my field is economics and the more technical aspects of HN are often lost on me

ThePherocity 14y ago
Yea, although there appear ways around it though hackery, for all intents and purposes, you need a 64bit OS to take advantage of the extra memory.
gergles 14y ago
You need to install 64-bit Windows to use anything beyond ~3.5GB of RAM.
- icebraining 14y ago
  This is not true. Did you read the answers? 32-bit OSs with PAE support can use all the RAM. Most processes will only be able to use 4GB, but that's per process, not for the whole system. So AD1066 doesn't need a 64-bit Windows, just any 32-bit Windows newer than XP.
  - masklinn 14y ago
    > So AD1066 doesn't need a 64-bit Windows, just any 32-bit Windows newer than XP. No. Non-server 32b Windows will never give access to more than 4GB RAM to the user unless you patch the kernel. XP 32b, Vista 32b and 7 32b — including Ultimate for the latter 2 — all restrict "available physical memory" to 4GB in all situations. So do many server editions as well, for Windows Server 2008 x86 for instance, only the Enterprise and Datacenter editions will give access to more than 4GB RAM (up to 64). A 2008 Enterprise license is ~$1500. I doubt ad1066 has any desire to pay such a price.

3amOpsGuy 14y ago

The top answer seems wrong to me. >> a process can have more memory than address space How does that work? The best I can come up with is the author is referring to mmap? In which case the address space is still limited to 4Gb although I concede that with the OS pagefile it would be possible to have more than 4Gb in core at a time via judicious use of mmap.

icebraining 14y ago
You need to ask the OS to remap your address space to different area of physical memory. Windows has the Address Windowing Extensions[1], which allow processes to reserve multiple blocks of memory and switch between them. [1]: http://en.wikipedia.org/wiki/Address_Windowing_Extensions http://en.wikipedia.org/wiki/Address_Windowing_Extensions

ralish 14y ago

Most of the responses to the question seem to be "sort of" correct but missing important parts of the overall picture. Further, a complete answer to the question is going to be subject to the operating system the given application is running under as the specifics are going to be heavily dependant on OS implementation. I can't speak for Unix systems, but on the Windows side, it's complicated and messy: PAE (Physical Address Extensions) Client editions of Windows do not support PAE while server editions do. If you want to physically address more than 4GB of RAM you need PAE. Realistically, you're unlikely to be able to address even 4GB under non-PAE x86 due to reserved memory ranges from hardware taking a chunk. Funfact 1: As for why PAE isn't supported on client editions, this isn't a licensing problem (you can buy x64 editions for the same price that have massively increased RAM limits), but rather, Microsoft playing it safe with driver compatibility. Apparently, during testing a non-trivial number of drivers were found that made assumptions that don't hold under PAE and thus broke horribly. Things like using the high bit of a 32-bit pointer for data storage. Rather than risk a tsunami of bluescreens caused by poorly written drivers (which people would blame MS for out of naivety) the decision was made to simply not support PAE on client editions. Server editions do as there's the reasonable expectation that servers should be running higher quality drivers. Funfact 2: Technically, PAE is supported on client editions, as it's required for hardware DEP support, so it's more a hard limit on addressable physical memory than it is a lack of support for PAE itself. AWE (Address Windowing Extensions) This is a Windows-specific capability (unlike PAE, which is a hardware capability provided by modern CPUs) which allows a process to address more memory than it has virtual address space. Further, it works hand-in-hand with PAE as you by definition need PAE support in order for the OS to address memory higher than 4GB in the first place. However, in order to use AWE, the application has to be explicitly written to utilise it as the usage of AWE requires the application itself to take a far more active role in managing its memory. AWE support is indicated via a flag in the PE header of the application binary. Needless to say, a tiny minority of applications have this support. The only one I could name right now would be Microsoft's SQL Server. 4GT (4-Gigabyte Tuning) This is one other Windows-specific capability that isn't really related to a discussion on 4GB limitations but inevitably will be raised regardless. While PAE provides for the OS to address more than 4GB of physical memory, and AWE provides for an application to address more memory than in its virtual address space, neither of these features increase the amount of addressable memory at any given time in an application (ie. AWE allows you to change your view of memory, but not address more than 2GB of memory simultaneously). A Windows process in 32-bit has a 2GB virtual address space, with the remaining 2GB mapped to the kernel address space. 4GT, in contrast to PAE and AWE does allow you to modify the address space available to a user-mode application. By providing the appropriate bootloader parameter you can modify the usermode/kernelmode split to skew it up to a maximum of 3GB/1GB user/kernel-mode respectively. This used to be done quite a lot on large Terminal Servers for example, or to optimise database servers, however, the ramifications of doing so need to be carefully understood. You are reducing the amount of memory available to the kernel, and so, memory hungry drivers may now fail to load, or worse, outright crash the system if they haven't been coded to properly handle low memory conditions. EDIT: I forgot to mention, that like AWE, an application won't even use the increased address space 4GT provides unless it is marked as capable in its PE Header. Merely enabling the setting won't get you anything unless your application supports this capability. Firefox, as an example, is large address aware and being 32-bit, can thus address up to 3GB instead of 2GB. While on 32-bit systems this capability is not likely used (how many 32-bit Windows users have modified their bootloader parameters to support 4GT?) it is fully used on 64-bit Windows and can use the full 3GB address space out of the box. And god knows Firefox needs it... In summary, in the age of x64, I tend to view all of the above as far more trouble than it's worth. The virtual address space of a Windows 64-bit process is currently 8TB. Windows 7 Professional and up give you 192GB of physical memory, while Windows Server 2008 R2 Enterprise and up give you up to 2TB. So if at all possible, avoid 32-bit, 64-bit is the solution; PAE, AWE & 4GT were only ever workarounds. If any of the above is incorrect, please correct me, as it's been a while since I studied much of the above, and if anyone can shed more light on the Unix situation, I'd be interested (in a very nerdy way).

jiggy2011 14y ago

How does one swap memory in and out of address space? I understand this can be done with files via mmap but not sure about RAM, since in that case would you not need to know the physical memory addresses of what you wanted to swap in or out? Isn't this information deliberately hidden from userspace applications?