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From this post, I read the following comment.

If you only have 2GB, I recommend you install 32 bit version, 64 bit will be much slower (because of several technical reasons about 64 bit architecture)

I think that 64bit OS uses more memory to make available memory less, and the performance could be worse. However, that should not be the only reason.

What would be the technical reasons about 64 bit architecture?

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migrated from Jan 27 '11 at 17:06

This question came from our site for professional programmers interested in conceptual questions about software development.

up vote 3 down vote accepted

The old logic about 64 bit code was based on code size, well actually pointer/data size. Both pointers and long integers are now twice as big (8 bytes vs 4 bytes). All this expanded memory usage uses more memory bandwidth, may make you page to disk more, may make less efficient use of caching, etc.

Even saying that, even in the beginning of the x86 move to 64 bit, there were advantages to going to 64 bit. The 32 bit ia32 architecture was always resource constrained, and had a very small number of registers (the places where things are added and moved around from). When AMD invented its 64 bit mode, they added a lot of registers (since they didn't have to be backwards compatible) so this may have increased speed in some circumstances, or at least offset some of the speed loss from the increased code size.

BTW: Intel didn't come up with the 64 bit extensions; it was betting hard on Itanium, which was a brand new architecture with a radical new design. What helped Intel before (x86 backwards compatibility) hurt Intel this time (Itanium x86 compatibility sucked at the beginning) and now Itanium is a niche product. Intel was able to copy the AMD64 extensions because of previous licensing agreements, rebadged them EM64t, and here we go.

Now, every chip has 64 bit, so if you want to get a 32 bit only chip, you're using old tech. And a lot of new OSes need 64 bit to use all the RAM in systems.

TL;DR version: The warning against 64 bit mode is based on looking at one thing only (code size inflation) and ignores other advances (better instruction set). You won't see much of a difference, feel free to worry about other things

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It isn't, really. In the forum thread you linked, there's a link to some benchmarking: TL;DR: It doesn't seem like modern operating systems have any significant performance differences between x32 and x64.

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For most programs you run, there is no real difference. The difference comes when you run programs that use a lot of memory such as CAD, video/DVD editing, programming and the like. In these areas, 64 bit gives you the advantage because you can use more memory in your system, achieving more speed for these apps. But that's about the only place you will see any real difference. But that supposes that you install a 64 bit system with more than 3.5 GB of RAM – BBlake Jan 27 '11 at 17:38

Keep in mind that forum post is 3 years old. A lot has changed since then, for one many computers come with 4+GB of RAM by default. If you only have 2GB of RAM, there really isn't a whole lot of reason to migrate to 64bit, particularly if your OS limits any 32bit process to 2GB of RAM max (i.e. Windows).

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In Windows, a risk is potential increased memory usage, especially when running many simultaneous applications. This is due to the fact that Windows libraries and DLLs must be loaded in memory twice to suit distinct 32bit and 64bit applications which would be the same library in a 32bit OS.

Of course, this risk is mitigated as soon as one has 4GB of RAM, where the x64 version provide greater benefits.

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