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What's the difference between 32 bit OS and 64 bit OS? Can a 32 bit OS run in a 64 bit processor?

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migrated from stackoverflow.com Jul 20 '09 at 9:52

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If you're talking about Windows (and the Win32 tag seems to indicate that), then yes, absolutely - Vista and Win7 64-bit support almost all 32-bit apps through a compability layer - I haven't found anything yet that doesn't work just fine on Vista 64-bit (except for system-level drivers) –  marc_s Jul 20 '09 at 11:15
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The question seems more whether you can install say Win Vista x86 onto a box that have a 64-bits processor, rather than whether x86 apps run on say Win Vista x64. –  KTC Jul 26 '09 at 0:14
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3 Answers

Your question is architecture specific. x64 is essentially an extension to x86 architecture. It supports a 64 bit address space. It provides some new instructions and new registers.

You can run 32-bit x86 Windows on an x64 machine. Note that you can't do this on Itanium 64-bit systems.

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A 64 bit processor can run both 32 and 64 OS (at least an x64 can). A 32 bit processor can run only 32 natively.

The difference is mostly about the size of a Pointer/Reference. On 64 bit machines, you can reference an address in a 64 bit address-range (thus giving you 2^64 bytes of memory). On 32 bit you can only address 2^32 bytes (=4 GB). Now if you look at current computers it is obvious why the world is moving to 64 bit: 32 bit can't easily address all the RAM anymore.

On x64 (AMD/Intel) you have additional benefits of 64 bit. The CPU has more registers and thus allows more efficient code.

On other architectures the differences between 64 and 32 bit are less obvious. For example the Nintendo 64 (remember that?) was a 64 bit machine but most of its code was 32 bit. So in that case 64 bit served more as a marketing trick.

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Both a 32 and 64 bit OS can run on a 64 bit processor, but the 64 bit OS can use full-power of the 64bit processor (larger registers, more instructions) - in short it can do more work in same time. A 32 bit processor supports only 32 bit Windows OS.

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This is incorrect generally, though correct for x64 specifically. –  ChrisInEdmonton Jul 30 '09 at 18:28
    
A 32 bit processor will run any operating system designed to run on such a processor architecture. –  Waxhead Nov 29 '11 at 18:48
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