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I understand that due to physical construction, CPUs come in different sizes 16-bit, 32-bit, 64-bit. But what does this means " Windows XP is 32-bit OS"? what is the limitation on a software to limit itself to to 32-bit or 64-bit?

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The operating system must be compiled with a certain assumption about the size (and thereby range) of pointers that it manages. Bear in mind that it's also the operating system's task to implement the virtual memory abstraction, and deal with the hardware side of that (pages, swapping, paging in and out, and so on). So the OS most certainly must be written to take into account architectural details like those.

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@unwind so can we simply say that 'memory management capability' determines whether an OS is 32-bit or 64-bit? – Saurabh Feb 4 '11 at 12:01
@Saurabh: hm ... I think that is a bit too narrow. I guess there are also other considerations, OS:es for processors that are switchable between 32 and 64 bits need to support that, and the programming model must be supported of course (more registers, calling conventions, etc). – unwind Feb 4 '11 at 16:24
@user5627 that sounds pretty logical now. Long before being an operating system, it is a software created in some language. And as all the softwares are some ISA specific, so does the operating system also. It took me a while to grasp the depth of your reply, but in the end I have better understanding. Thanks a lot – Kawai Kx Feb 21 '11 at 15:45
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