Thei486SX was amicroprocessor originally released byIntel in 1991. It was a modified Inteli486DX microprocessor with itsfloating-point unit (FPU) disabled. It was intended as a lower-cost CPU for use in low-end systems—selling forUS$258—adapting theSX suffix of the earlieri386SX in order to connote a lower-cost option. However, unlike the i386SX, which had a 16-bit external data bus and a 24-bit external address bus (compared to the fully 32-biti386DX, its higher-cost counterpoint), the i486SX was entirely 32-bit.[1]: 117 The Intel486 SX-20 CPU can perform up 20 MIPS at 25 MHz while this can also perform 70% faster than the 33 MHz Intel386 DX with external cache.[2]
In the early 1990s, common applications, such as word processors and database applications, did not need or benefit from a floating-point unit, such as that included in thei486, introduced in 1989. Among the rare exceptions wereCAD applications, which could often simulate floating point operations in software, but benefited from a hardware floating point unit immensely.[3]AMD had begun manufacturing itsi386DX clone, theAm386, which was faster than Intel's.[4] To respond to this new situation, Intel wanted to provide a lower costi486 CPU for system integrators, but without sacrificing the better profit margins of a full i486. Intel were able to accomplish this with the i486SX, the first revisions of which were practically identical to the i486 but with its floating-point unit internally wired to be disabled.[1]: 116–117 [5]: 162–164 [a] The i486SX was introduced in mid-1991 at 20 MHz, one core with 8kb of cache in apin grid array (PGA) package.[3] There were low-power version of 16, 20, and 25 MHz Intel486 SX microprocessors. They were available USD $235, USD $266, and USD $366 for these frequency range respectfully. All pricing were in quantities of 1,000 pieces.[7] Later versions of the i486SX, from 1992 onward, had the FPU entirely removed for cost-cutting reasons and comes in surface-mount packages as well.[1]: 117
The first computer system to ship with an i486SX on its motherboard from the factory wasAdvanced Logic Research's Business VEISA 486/20SX in April 1991.[8] Initial reviews of the i486SX chip were generally poor among technology publications and the buying public, who deemed it an example ofcrippleware.[1]: 117
Many systems allowed the user to upgrade the i486SX to a CPU with the FPU enabled. The upgrade was shipped as thei487, which was a full-blown i486DX chip with an extra pin. The extra pin has no electrical connection; its purpose is to physically prevent the chip from being installed incorrectly ("keying").[b] The choice of keeping an inactive i486SX is because i486SX was physically hard to remove, being typically installed in non-ZIF sockets or in a plastic package that was surface mounted on the motherboard. Lateri486 OverDrive processors also plugged into the 169-pin socket (since namedSocket 1) and offered performance enhancements as well.[9]
