Intel 80486

The intel 80486 was a single chip processer ancestral to intel's Pentium family of single chip computers.

Like earlier CPUs in this lineage intel produced a premium model, the Intel 80486dx, and lower performance model, the intel 80486sx, which, they said, required an expensive Intel 80487 floating point coprocessor.

The 80486dx was intel's first CPU that devoted silicon real estate to on-chip floating point operations.

The two model required two separate incompatible kinds of motherboards. Motherboards designed for the 80486sx shipped with an empty socket where the user could decide to boost performance, by plugging in an 80487 floating point coprocessor.

However, this was all a charade. CPUs are manufactured on wafers, with dozens, of nominally identical CPUs. During the manufacturing process, temporary leads are connect to the CPUs, extensive pinouts, and each CPU is extensively tested, to see if it can execute all instructions. CPUs which cannot perform properly are discarded.

Well, approximately half of the silicon real estate in the 80486dx was devoted to floating point operations. So, half of the chips that failed, were completely operational, except for the inability to process floating point arithmetic. These CPUs were packaged as 80486sx, and were mounted on motherboards intended for them. Completely working CPUs were packaged as either 80486dx, or 80487 chips.

When a consumer bought a computer with a 486sx motherboard, and an empty 487 socket, just like with earlier CPUs, the floating point operations would trigger an interrupt, and the interrupt handler would process the floating point operations. However, if they plugged in a 487, it would completely replace the 486sx, as the 487 was identical to the premium 486dx, in a different package.