Source : Free On-Line Dictionary of Computing

Intel 486SX
     
         An {Intel 486DX} {microprocessor} with its
        {floating-point unit} disconnected.  All 486SX chips were
        fabricated with FPUs.  If testing showed that the CPU 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.
     
        [Was this true of all 486SX chips?]
     
        Some systems, e.g. Aopen 486SX, 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.  This is similar to the 286 SX, which was a 16-bit
        processor with an 8-bit external data bus.
     
        The {Jargon File} claimed that the SX was deliberately
        disabled {crippleware}.  The German computer magazine, "c't",
        made this same theory the basis of an {April Fools Joke}.
        They claimed that if one drilled a hole of a specified
        diameter through the right point on a SX chip, this would
        brake the circuit that disables the FPU.  Some people actually
        tried (and then bought themselves new processors).
     
        (1997-02-14)