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owl [soylentnews.org] writes:

In the 1980s, if you wanted your IBM PC to run faster, you could buy the Intel 8087 floating-point coprocessor chip. With this chip, CAD software, spreadsheets, flight simulators, and other programs were much speedier. The 8087 chip could add, subtract, multiply, and divide, of course, but it could also compute transcendental functions such as tangent and logarithms, as well as provide constants such as π. In total, the 8087 added 62 new instructions to the computer.

But how does a PC decide if an instruction was a floating-point instruction for the 8087 or a regular instruction for the 8086 or 8088 CPU? And how does the 8087 chip interpret instructions to determine what they mean? It turns out that decoding an instruction inside the 8087 is more complicated than you might expect. The 8087 uses multiple techniques, with decoding circuitry spread across the chip. In this blog post, I'll explain how these decoding circuits work.

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