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What we are getting if someone says "my machine is 64-bit computer" . What is the difference between 64-bit computer (I mean the CPU architecture) and operating system (Windows 64-bit OS). Please explain me how these terms are related with processor architecture.

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See 32-bit vs. 64-bit systems. – Mehper C. Palavuzlar Oct 10 '10 at 8:49
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A 64-bit processor is one which handles (or can handle) 64 bits of data in a single operation (for example addition of two registers each containing 64-bit integers, or accessing a memory location via a 64-bit pointer).

A 64-bit operating system is one which is compiled to take advantage of the 64-bit instructions provided by a 64-bit processor and as such will not work on a processor that does not support these instructions. One advantage of 64-bit Windows for example is that it can take advantage of more physical RAM than the 32-bit version of Windows.

Many mainstream 64-bit processors also have 32-bit instructions so that they are able to run either a 32-bit or a 64-bit operating system.

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I would like to further add that most 64-bit operating systems can run 32-bit code. They still require 64-bit drivers, however. – Darth Android Oct 10 '10 at 9:06
Also, there are 64-bit chips that can't run 32-bit code without emulation (such as Itaniums) as well as 64-bit chips that can run 32-bit code but must run a 64-bit OS (such as some SPARC architectures). However, neither shows up in processors that can really be considered "mainstream". – Ignacio Vazquez-Abrams Oct 10 '10 at 9:43
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