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Why does Windows reserve 1Gb (or 2 Gb) for its system address space?

Asked 2009-07-12T00:45:33.807
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It's a known fact that Windows applications usually have 2Gb of private address space on a 32bit system. This space can be extended to 3Gb with the /3Gb switch.

The operating system reserves itself the remaining of the 4Gb.

My question is WHY?

Code running in kernel mode (i.e. device driver code) has its own address space. Why, on top of a exclusive 4Gb address space, the operating system still want to reserve 2Gb of each user-mode process?

I thought the reason is the transition between user-mode and kernel-mode call. For example, a call to NtWriteFile will need an address for the kernel dispatch routine (hence why the system reserve 2Gb in each application). But, using SYSENTER, isn't the system service number enough for the kernel-mode code to know which function/service is being called?

If you could clarify to me why it's so important for the operating system to take 2Gb (or 1Gb) of each user-mode process.

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Windows (like any OS) is a lot more than the kernel + drivers.

Your application relies on a lot of OS services that do not just exist in kernel space. There are a lot of buffers, handles and all sorts of resources that can get mapped to your process' own address space. Whenever you call a Win32 API function that returns, say, a window handle, or a brush, those things have to be allocated somewhere in your process. So part of Windows runs in the kernel, yes, other parts run in their own user-mode processes, and some, the ones your application needs direct access to, are mapped to your address space. Part of this is hard to avoid, but an important additional factor is performance. If every Win32 call required a context switch, it would be a major performance hit. If some of them can be handled in usermode because the data they rely on is already mapped to your address space, the context switch is avoided, and you save quite a few CPU cycles.

So any OS needs some amount of the address space set aside. I believe Linux by default sets only 1GB for the OS.

The reason why MS settled on 2GB with Windows was explained on Raymond Chen's blog once. I don't have the link, and I can't remember the details, but the decision was made because Windows NT was originally targeted at Alpha processors as well, and on Alpha's, there was some REALLY good reason to do the 50/50 split. ;)

It was something to do with the Alpha's support for 32 as well as 64-bit code. :)

answered 2009-07-12T01:34:59.730

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