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 OK 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.
This needs to be confirmed. --STG
Some systems allowed a DX to be plugged into an expansion socket. A board jumper would disable the SX which was hard to remove because it was surface mounted.
Some SX chips only had a 16-bit wide external data bus. The DX has a pin to select the data bus width (16 or 32). On the smaller SX, that line is hard-wired to 16 inside the package.