Intel 80486SX
The Intel 80486SX is an Intel 486DX microprocessor with its floating-point unit (FPU) disconnected. All 486SX chips were fabricated with FPUs. If testing showed that the central processing unit was working but the FPU was defective, the FPU's power and bus connections were destroyed with a laser and the chip was sold cheaper as an SX; if the FPU worked it was sold as a DX. Some have claimed that DX chips with working FPUs were turned into SX chips to meet demand for lower-cost chips. This needs to be confirmed. --
Two Intel SX chips
Some systems allowed a DX chip to be plugged into an expansion socket. A board jumper would disable the SX chip, which was hard to remove because it was inserted in a non-ZIF socket.
Based on material from FOLDOC, used with permission.