Cyrix 5x86 processors
Introduction: August 1995
Released in August 1995, four months before the more famous Cyrix 6x86, the Cyrix 5x86 was one of the fastest CPUs ever produced for Socket 3 computer systems. With better performance in most applications than an Intel Pentium processor at 75MHz, the Cyrix Cx5x86 filled a gap by providing a medium-performance processor option for aging 486 Socket 3 motherboards (which are incapable of handling Intel's Pentium CPUs, apart from the Pentium Overdrive).
The Cyrix 5x86 processor was based on a scaled-down version of the "M1" core used in the Cyrix 6x86, which provided 80% of the performance for a 50% decrease in transistors over the 6x86 design. It had the 32-bit memory bus of an ordinary 486 processor, but internally had much more in common with fifth-generation processors such as the Cyrix 6x86, the AMD K5, and the Intel Pentium, and even the sixth-generation Intel Pentium Pro. The chip featured near-complete support for i486 instructions, but very limited support for Pentium instructions. Interestingly, some performance-enhancing features of the cpu were intentionally disabled due to potentially stability-threatening bugs which were not fixed before release time.
The similarly-named SGS-Thomson ST 5x86 and IBM IBM 5x86C were licensed rebrandings of the Cyrix design (IBM and ST physically produced Cyrix's CPUs for them), marketed separately but identical for practical purposes, apart from the availability of a 75MHz edition which Cyrix did not bring to market, and slight differences in voltage requirements. The Cyrix 5x86 design, however, should not be confused with the similarly-named AMD Am5x86 which was essentially a fast 486 (not an all-new design like the Cyrix part) but which had broadly similar performance, used the same Socket 3, and was introduced at the end of the same year.
Source: Wikipedia, the free encyclopedia.