The intel 8087 was a floating point co-processor to intel's 8088 and 8086 models of single chip computer. Both the 8088 and 8086 implemented the 8086 instruction set, which included a subset of instructions for floating point operations -- arithmetic operations that might include fractions.
Integer arithmetic requires much less silicon real estate to implement than calculations on values that had a fractional component. Intel off-loaded the hardware to perform those instructions to a co-processor -- a support chip devoted solely to that task. Computers could be equipped with an 8088 or 8086, and without an 8087. If the 8087 were present floating point instructions would be off-loaded and executed relatively quickly, by the 8087.
Later chips, lineal descendants to the 8086, the 80186, 80286 and 80386 had a companion floating point co-processor manufactured to accompany them. Intel was to market an intel 80487. But it was not a separate chip, different from the 80486.
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