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What I understand about 32-bit OS is, the address is expressed in 32 bits, so at most the OS could use 2^32 = 4G memory space -- I assume the unit is Byte, so 4GB.

Does this mean any 32-bit OS, be it Windows or unix, if the machine has RAM + page file on hard disk more than 4GB, for example 8GB RAM and 20GB page file, there will never be "memory used up"?
- used up means increasing RAM or page file won't help the performance, of course, it's always possible an application keep applying memory from the OS but failed.

Similarly, if this 32-bit OS machine has 2GB RAM and 2GB page file, increasing the page file size won't help the performance. Is this true?

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As a side note, if you happen to have more then 4GB of RAM, and only a 32 bit OS, you can make a RAM Disk. This is a hard drive in your RAM, and you can use the unused/unaddressed RAM to place it in. – Simon Verbeke Dec 13 '11 at 13:33
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You can't. A ramdisk still has to be created throught the OS, if the OS can't use the memory you can't create a ramdisk there either. (Unless your bios can create a ramdisk, which is rare). – AVee Dec 15 '11 at 8:17
haha this is ture... – athos Dec 15 '11 at 17:02
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What I understand about 32-bit OS is, the address is expressed in 32 bits, so at most the OS could use 2^32 = 4GB memory space

The most that the process can address is 4GB. You are potentially confusing memory with address space. A process can have more memory than address space. That is perfectly legal and quite common in video processing and other memory intensive applications. A process can allocated dozens of GB of memory and swap it into and out of address space at will. Only 2 GB can go into user address space at a time.

If you have a four-car garage at your house, you can still own fifty cars. You just can't keep them all in your garage. You have to have auxiliary storage somewhere else to store at least 46 of them; which cars you keep in your garage and which ones you keep in the parking lot down the street is up to you.

Does this mean any 32-bit OS, be it Windows or unix, if the machine has RAM + page file on hard disk more than 4GB, for example 8GB RAM and 20GB page file, there will never be "memory used up"?

Absolutely it does not mean that. A single process could use more memory than that! Again the amount of memory a process uses is almost completely unrelated to the amount of virtual address space a process uses. Just like the number of cars you keep in your garage is completely unrelated to the number of cars you own.

Moreover, two processes can share non-private memory pages. If twenty processes all load the same DLL, the processes all share the memory pages for that code. They don't share virtual memory address space, they share memory.

My point, in case it is not clear, is that you should stop thinking of memory and address space as the same thing, because they're not the same thing at all.

if this 32-bit OS machine has 2GB RAM and 2GB page file, increasing the page file size won't help the performance. Is this true?

You have fifty cars and a four-car garage, and a 100 car parking lot down the street. You increase the size of the parking lot to 200 spots. Do any of your cars get faster as a result of you now having 150 extra parking spaces instead of 50 extra parking spaces?

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Thanks, this clearly pointed out my misunderstanding! – athos Dec 13 '11 at 15:31
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@Rig: Awesome, I am glad to hear your constructive criticism. Now's your chance to write a better answer and show us the sort of answer you think is less deceptive and more pertinent. – Eric Lippert Dec 13 '11 at 20:40
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@BlueRaja: Raymond Chen's article on the subject might help. He shows how to allocate 4GB out of the page file; that is memory that the process owns. The process cannot address the whole thing at once of course, but it can address any page by mapping a page into an unused VM page, using it for a while, and then discarding it. blogs.msdn.com/b/oldnewthing/archive/2004/08/10/211890.aspx – Eric Lippert Dec 13 '11 at 22:59
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@BlueRaja: You're probably used to just calling VirtualAlloc, and of course that gives you memory mapped to virtual memory. But memory-mapped files are memory too, and you can make a memory-mapped file that is only partially mapped into virtual memory. – Eric Lippert Dec 13 '11 at 23:49
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Although some 32 bit operating systems can use more than 4GB memory, not all can. On recent x86 chips it's a restriction of the operating system. Windows XP is limited to a little under 4GB, whereas Windows 2003 Server (also 32 bit) can use much more. But this is all specific to x86 processors. On some chips, there is no support for virtual addressing, so either you resort to hardware hacks or you live within the 32 (or however many) bits limit. – Steve314 Dec 14 '11 at 8:59
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up vote 33 down vote

It is true that the CPU can only address maximum 4Gb of RAM. However, current CPU's use an MMU to translate process-specific memory addresses into physical memory addresses.

This MMU is used for all sorts of different tricks, from memory isolation (process A cannot manipulate memory of process B) to memory sharing (process A can access the same physical memory region as process B and can exchange data this way).

Although 32-bit CPU's only support 4Gb of memory per process, it can address up to 64Gb of RAM when using Physical Address Extension. This allows process A to use the first 4Gb of memory, while process B uses the next 4Gb. In total, more than 4Gb of physical memory is used, but the total amount of memory a single process uses is still capped at 4Gb.

PAE is supported on Linux since kernel version 2.3.23 and on some 32-bit flavours of Windows Server, but not on 32-bit Windows XP, Vista or 7.

If your CPU does not support PAE you will be limited to 4GB of physical memory (or less depending on other factors).

Please note your operating system can still evict parts of physical memory to the disk (page file) regardless of the CPU supporting PAE. This ensures you can start multiple processes who use more than 4Gb combined. The only impact PAE has is whether you can keep the 4Gb of process B in physical memory while running process A.

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does it mean if it does not support, then what i said above it correct? – athos Dec 13 '11 at 13:03
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Windows XP does support PAE (starting with SP2, I think), but ignores RAM beyond 4GB even when the machine supports PAE; see here. It's used for hardware-based Data Execution Prevention — the NX/XD bit is available only in a PAE page table entry, not a "regular" page table entry. – Wyzard Dec 13 '11 at 13:23
Chipset can also be a limiting factor of how much memory can be supported/used regardless of which OS or hack you choose. – Moab Dec 13 '11 at 13:53
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I'd add that the reason why consumer versions of 32bit windows don't support >4GB of ram was that a significant number of hardware drives were found not to work correctly with that much memory; and since at the time >4GB of ram was extremely rare outside of servers/ultra high end workstations that most of the companies who maintained the drivers were uninterested in spending money to update drivers that would only be used by 0.1% of their customer base. – Dan Neely Dec 13 '11 at 14:06
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@DanNeely the OS does have support for the memory, it's limited due to licensing issues. While I agree that this is a third-party hack, it's only for consumer variants of Windows. Server-based Windows distributions, starting from Windows 2000, have had support for more than 4GB of memory (ref. the first link). – Breakthrough Dec 13 '11 at 15:42
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up vote 11 down vote

Speaking specifically about 32-bit Windows variants, they have had support for more than 4GB of RAM since Windows 2003 variants (and you can also get a kernel hack for Windows 7 to allow you to use all of your RAM in 32-bit). However, this comes at a cost, as you outlined in the first part of your question.

In a 32-bit operating system, the size of a pointer (memory address) is the same as the word length of the CPU, 32-bits, which allows (as you mentioned) a 2^32 = 4GB memory space. Windows also takes a "virtual memory" approach for applications, so each application has it's own memory space.

Since each pointer is only 32-bits wide, each application's pointers can only address up to 4GB of memory, even though the system can support more then 4GB of RAM. As far as I know, this is the only caveat to using more than 4GB of RAM in a 32-bit operating system. In total, you can have many applications using more than 4GB of RAM combined, but any one particular process can only allocate/access up to 4GB.

Back to your question, let's say you have a program that uses 2GB of RAM. If you have 10 instances of this program, that's 20GB. All 8GB of your RAM will be used up, as well as another 12GB of the pagefile. So yes, under 32-bit operating systems, it is more than possible to use up this memory.

if this 32-bit OS machine has 2GB RAM and 2GB page file, increasing the page file size won't help the performance. is this true?

Increasing the pagefile size will usually not increase performance (unless your RAM and pagefile are set to the absolute minimum, or set so low your computer constantly thrashes). It will, however, prevent your computer from running out of (virtual) memory. Whenever anything needs to be purged to the pagefile, you're already taking a huge performance hit (since the hard drive is orders of magnitude slower then your RAM).

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Of the 4GB of address space that each process gets, only 2GB is actually available for the program to use; the other 2GB is reserved for use by the kernel. The /3GB boot option can allow some programs to use more RAM. – Wyzard Dec 13 '11 at 13:30
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@Breakthrough: "Increasing the pagefile size will never increase performance" I don't think this is true. For example, if there is more swap, the OS can more aggressively swap out unused program data, and use the free RAM for e.g. disk caching. In some configurations, this may yield better performance. There is a nice answer on ServerFault on why a pagefile is important, which touches on these issues: serverfault.com/questions/23621/… – sleske Dec 13 '11 at 13:47
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Be careful of blanket statements like "Increasing the pagefile size will never increase performance", it is perfectly possible for a page file that is too small to result in more disk thrashing than having a larger page file! See answers and comments in this question. – Mark Booth Dec 13 '11 at 14:21
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@sleske Good point, I updated it to be more of a "usually not" type of thing. While I agree it may make a huge difference in a server, for most people, the point I was trying to get across is that they would be better off just getting more RAM (since when you're paging, performance is pretty much as slow as it can get). – Breakthrough Dec 13 '11 at 15:11
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Short answer. You can tell hardware to switch out the 4G of memory space with another. You can only access 4G at once with 32bits but swapping which memory you are use is possible. That is how it worked in the old days with 8 or 16bit cpus and > 64k memory

How the operating system handles it is its own business.

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