Intel 486SX

processor 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)