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The PC's x86 Instruction Set
The instruction set that is used on the vast majority of PCs is often called the x86 instruction set, referring to the entire family of Intel processors whose designations end in "86". In actual fact, the instruction set hasn't remained totally static through all the generations of processors over the past 15 or so years. However, the last significant change to the instruction set (with the exception of the MMX extensions) happened with the introduction of the 80386.
The two processor generations before the 386, the 8088/8086 and 286, had fewer instructions and processor modes. The result is that some software written for 386 and later processors won't work on these earlier CPUs. (Of course they are long obsolete and glacial in speed so this isn't much of an issue). Also, the pre-386 processors were 16-bit internally and so could not handle the modern 32-bit software of today.
The x86 instruction set is CISC in design. It contains a large number of instructions, some of which can perform some rather complicated functions, and can require many clock cycles to execute.