Technically the 64-bitness refers only to addressing capability. In modern machines, it nearly always matches the register size.
There is not much point is going to 64-bit software unless you have 4+ GIG of RAM. If you are thinking of experimenting in this direction, make sure you buy a CPU and motherboard that supports at least 4 GIG. You can experiment with less, but you will make less efficient use of your resources than had you used 32-bit software.
On the other hand, adding RAM to a 32-bit system would speed it up too. 64-bit architectures will tend to do better on computationally intensive tasks, where 32-bit architectures will tend to do better on RAM-intensive tasks. Of course 64-bit architectures will handle massive virtual address spaces that 32-bit can’t computers even attempt. The best way to find out which will better and by how much is to benchmark your application in both 32 and 64-bit mode.
The main reason then to go for a 64-bit machine is when you have bought 4+ gigabytes of real RAM, and you want massive virtual RAM space so that you can memory-map huge files (i.e. treat them as part of the vidual memory) or deal with millions of Java objects. The other reason is for improved floating point performance. This has nothing to do with 64-bitness per se, just AMD ’s redesign of the floating point part of the architecture.
Oracle’s Java supports the 64-bit Opteron under W2K, XP, W2003, Vista, W2008, W7-32, W7-64, W8-32, W8-64, Linux, Ubuntu and OSX
AMD ’s Athlon, Turion, Sempro and Opteron series use a different 64-bit architecture than the Intel Itanium(IA-64) called AMD64 or x-64. The architecture features 64 general purpose registers (vs only 8 in 32-bit mode), and an address space of 248 bytes or 281,475 gigabytes. The architecture is designed to be theoretically extended to support 264 bytes of virtual RAM. There is a x-64 version of Windows Vista at nominal extra charge. Because of AMD ’s success with this instruction set, and the slow Itanium sales, Intel implemented AMD64 too, (renaming it Intel-64, IA-32e, EMT64T… to disguise its origin) and used it in their newer Pentium, Celeron D, Xeon, and Core 2 processors.
64-bit is still largely experimental since only a subset of the device drivers have been ported to 64-bit. When you run in 64-bit mode, you may find not all of your peripherals will work. 64-bit in production is limited to number crunching machines and servers without any fancy peripherals.
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