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The x86 architecture is the generic name for a multi-vendor 16-bit, 32-bit, and most recently 64-bit architecture. It was originally developed by Intel in the 8086 series of CPU, extended to 32-bit by Intel in the 80386 CPU, and extended by AMD to 64 bits in the Opteron and Athlon 64 CPU lines. While as of 2007, Intel and AMD are the highest volume manufacturers of x86 CPUs, many other vendors have also manufactured x86 CPUs. Generally the manufacturers have cross-licensed (or copied) major improvements to the architecture, but there are some unique features present in many of the implementations.
The x86 architecture has a variable instruction size that allows for moderate code compression while also allowing for very complex operand combinations as well as a very large instruction set size with many extensions. Instructions generally vary from zero to three operands with only a single memory operand allowed.