Different instruction sets. You know processors have instruction sets (or at least I hope you know that)
the x86 instruction set (most common processors these days) is 32-bit. The 32-bit OS's are complied to use the x86 instruction sets
the new processors supports new instruction set, that is 64-bit. Pentium processors that are 64-bit (and just 64-bit) must have an operating system complied that supports these new instruction sets.
however AMD 64-bit processors support both 32, and 64 bit instruction sets, so you can use 32-bit (98, 2k, 32-bit XP) and 64-bit OS's
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