The primary benefit of moving to 64-bit is the increase in the maximum allocatable system memory (RAM). Windows XP 32-bit is limited to a total of 4 GB, which is, by default, equally divided between Kernel and application usage. Using the /3GB switch in the boot.ini file forces Windows to limit the kernel to the upper 1GB and provided up to 3GB per application. Windows XP x64 can support much more memory; although the theoretical memory limit a 64-bit computer can address is about 16 exabytes (16 billion gigabytes), Windows XP x64 is currently limited to 128 GB (237 bytes) of physical memory and 16 TB (244 bytes) of virtual memory.
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