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Learn Something Old Every Day, Part XVIII: How Does FPU Detection Work?

This post ended up being much longer than originally intended because halfway into writing it, I found that 286 and later CPUs don’t behave the way I had assumed they would… While investigating a bug related to a program using floating-point math on a 386SX system with no FPU, I started pondering how exactly FPU detection works on 286 and newer CPUs. Although math co-processors became standard some 30 years ago, on old PCs they were an uncommon and expensive add-on, and a 66 MHz 486SX2 would still have a usable yet FPU-less processor in the mid-1990s.

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