I486SX
The Intel's i486SX was a modified Intel 486DX microprocessor with its floating-point unit (FPU) disconnected. All early 486SX chips were actually i486DX chips with a defective FPU. 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. Computer Manufacturers that used these processors include Packard Bell, Compaq and IBM. Back in the early 1990s it wasn't advantageous for most users to have a FPU. On one hand, many typical applications like word processing and email do not use floating point operations. On the other hand, those involved in heavy computer gaming or mathematical work generally benefit from a FPU.
Some systems allowed the user to upgrade the i486SX to a CPU with a FPU. The FPU upgrade device was shipped as the i487, which was a full blown i486DX chip with an extra pin. The i487 was installed in an upgrade socket and the extra pin was either a power or ground pin that indicated that the i487 was installed. That signal was used to disable the i486SX when the i487 was installed. Although i486SX devices were not used at all when the i487 was installed, they were hard to remove because the i486SX was installed in non-ZIF sockets or in a plastic package that was surface mounted on the motherboard.
References
This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.