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.
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).