No matter the designation or origin, all microprocessors in today's Windows-based computers share a unique characteristic and heritage. All are direct descendents of the very first microprocessor. The instruction set used by all current computer microprocessors is rooted in the instructions selected for that first-ever chip. Even the fastest of today's Pentium 4 chips has, hidden in its millions of transistors, the capability of acting exactly like that first chip.

In a way, that's good because this backward-looking design assures us that each new generation of microprocessor remains compatible with its predecessors. When a new chip arrives, manufacturers can plug it into a computer and give you reasonable expectations that all your old ...

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