Cyrix 5x86

The Cyrix 5x86 was a line of x86 microprocessors designed by Cyrix and released on June 5 of 1995. Cyrix, being a fabless company, had the chips manufactured by IBM. The line came out about 5 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 75 MHz, the Cyrix Cx5x86 filled a gap by providing a medium-performance processor option for 486 Socket 3 motherboards (which are incapable of handling Intel's Pentium CPUs, apart from the Pentium Overdrive). The IBM 5x86C is an IBM branded and produced version of the Cyrix-designed Cyrix Cx5x86 CPU. Previous IBM x86 processors, IBM 386SLC and IBM 486SLC, were based on modified Intel designs. The Cyrix 5x86 processor, codename "M1sc", 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. Some performance-enhancing features of the CPU were intentionally disabled due to potentially stability-threatening bugs which were not fixed before release time (these features can be enabled with freely downloadable software utilities; see below). The similarly named SGS-Thomson (STMicroelectronics) ST5x86 and 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 75 MHz edition which Cyrix did not bring to market, and slight differences in voltage requirements.