i486SX

The Intel i486SX, 25-MHz version

Pin side of an i486SX

The i486SX was a microprocessor originally released by Intel in 1991. It was a modified Intel i486DX microprocessor with its floating-point unit (FPU) disabled. It was intended as a lower-cost CPU for use in low-end systems—selling for US$258—adapting the SX suffix of the earlier i386SX 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-bit i386DX, 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 the i486, introduced in 1989. Among the rare exceptions were CAD applications, which could often simulate floating point operations in software, but benefited from a hardware floating point unit immensely.^[3] AMD had begun manufacturing its i386DX clone, the Am386, which was faster than Intel's.^[4] To respond to this new situation, Intel wanted to provide a lower cost i486 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 a pin 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 was Advanced 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 of crippleware.^[1]: 117

Embedded SQFP version of the i486SX

BQFP version of the i486sx

Many systems allowed the user to upgrade the i486SX to a CPU with the FPU enabled. The upgrade was shipped as the i487, which was a full-blown i486DX chip with an extra pin. The extra pin prevents the chip from being installed incorrectly.^[b] Although i486SX devices were not used at all when the i487 was installed, they were hard to remove because the i486SX was typically installed in non-ZIF sockets or in a plastic package that was surface mounted on the motherboard. Later OverDrive processors also plugged into the socket and offered performance enhancements as well.^[9]

• Intel 80486SX images and descriptions at cpu-collection.de

Intel datasheets

• Embedded i486SX
• Embedded Ultra-Low Power i486SX