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    <blockquote> <p>"If we now decide to use only 48 of the 64 bits for addressing". Why? &amp; Why only 48bits? Why not some other number?</p> </blockquote> <p>System architects make tradeoffs. 256TB seems like more than enough room for 1 process's address space. Remember virtual address != physical address, and generally speaking, each process has its own address space.</p> <p>As long as pointers are 64 bits, this is more of a performance capability issue than anything else. If &amp; when 48 bits becomes a limitation, the OS could be tweaked to use more bits of the 64-bit address space without breaking application incompatibility. For now, the architects are just buying themselves a very comfortable amount of time.</p> <p>It may have to do with processor-side virtual addressing capabilities, as many processors now have <a href="http://en.wikipedia.org/wiki/Memory_management_unit" rel="nofollow noreferrer">memory management units</a> to handle the virtual -> physical memory mapping.</p> <blockquote> <p>How to know the number of address pins (= size of address bus) for a processor. <a href="http://ark.intel.com" rel="nofollow noreferrer">http://ark.intel.com</a> specifications of a processor doesn't include this spec.</p> </blockquote> <p>This is for the most part irrelevant. It's a way for a processor to implement various physical addressing schemes. A 64-bit processor could achieve external address/data buses for its complete address space with 64, 32, 16, 8, 4, 2, or 1 address pin if the bus is synchronous and the address bits get multiplexed in time. Again, virtual address != physical address; 64-bit virtual addressing could be implemented with 48-bit or 32-bit physical addresses (just that you would be limited to 2<sup>48</sup> or 2<sup>32</sup> words of memory).</p> <p><strong>update:</strong> if you really want to know, you have to look at the datasheet of each processor in question. E.g. <a href="http://www.intel.com/design/core2duo/documentation.htm#datashts" rel="nofollow noreferrer">Intel Core 2 Duo</a> -- section 4.2 of the datasheet talks about the signals -- the address bus is 36-bits wide (but is really 33 signal lines; the data width is 64-bit = 8 bytes so the other 3 lines are probably unnecessary with proper data alignment)</p> <blockquote> <p>Well, I'm just a regular PC user &amp; programmer. Its just hard to believe for me that 32-bit addressing ie.. 4GB (2GB/3GB to be more correct) address space per process is a limit. If you really encountered this limit. Please give me example.</p> </blockquote> <p>Two words: <em><a href="http://en.wikipedia.org/wiki/Memory-mapped_file" rel="nofollow noreferrer">memory-mapped files</a></em>.</p>
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    1. POWhy can't OS use entire 64-bits for addressing? Why only the 48-bits?
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    1. POWhy can't OS use entire 64-bits for addressing? Why only the 48-bits?
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    1. CO`Remember virtual address != physical address`. Yeah, Now I recite it in my dreams too. :P. Yeah, 256TB seems to be large but its seems large for now. Once 1MB thought to be more than enough & 32-bit IP addresses exhausted in no time. When this 256TB will be a limit. People say, "now its time to move on to 64-bit". Damn! I hate it.
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    2. CO:-) From a programmer's standpoint, as long as the pointers are 64-bits it doesn't matter.
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    3. COJason: From a recompilation standpoint, sure. But it's only *really* true as long as the programmer didn't make any assumptions that are true at 48 but not at 64. :-) It sounds crazy that anyone would write code that works at 48 but not 64, but then, I've thought that same thing for every word-length and endianness change ever.
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