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The Power (and Curse) of Compatibility
The compatibility of the instruction sets from the 386 class chips up to the most modern processors, with only a couple of special exceptions, is a testament to the importance that PC owners place on backward compatibility. The large amount of software available for the 386 processors led to the 486 being introduced without changes to the instruction set. This in turn led to more compatible software being written, with in turn created more reason to keep the instruction set compatible in future CPUs.
This is a sort of positive feedback loop, and is one of the strengths of the PC platform (in many different areas). It is also one of its weaknesses, because it makes it extremely difficult for chip makers to increase performance by making major changes to system architecture. Any changes they make must still preserve compatibility with the older processors. While certain extensions are being made to the x86 instruction set, the base set is likely to continue to be the industry standard for some time to come.