Released in August 1995, four months before the more famous Cyrix 6x86, the Cyrix Cx5x86 was the fastest CPU 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 aging 486 Socket 3 motherboards (which are incapable of handling Intel's Pentium CPUs, apart from the Pentium Overdrive).

The 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 6x86, the 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 potentionally 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 ST5x86 and IBM IBM5x86C 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. 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.

Specifications

• iDX4WB pinout, 168 pins
• Socket 3
• 2.0 million transistors on 0.65 micrometre process
• 144mm² die
• 3.45 volt electricity usage
• 16kilobyte unified level-one cache

100mhz (http://www.cpushack.net/chippics/Cyrix5x86-100GP-HS.jpg) capable edition for 25MHz (25x4), 33MHz (33x3), and 50MHz (50x2) front side bus
120/133mhz (http://www.cpushack.net/chippics/Cyrix5x86-120GP.jpg) capable edition for 40mhz (40x3) and 33mhz (33x4) front side bus. The 133 MHz version is very rare, however, and producers of upgrade kits were given preferential access to it.

External Links

• Comparative performance benchmarks (http://www.interwb.com/486bench.txt)
• Cyrix 5x86 (http://www.pcguide.com/ref/cpu/fam/g4C5x86-c.html)
• Cyrix 5x86 Processor Brief (http://www.worldwide-web-design.com/5x-tb.html)
• Entry in 486 processors chart (http://users.erols.com/chare/486.htm)

• Performance-enhancing utility to enable 5x86 "register bits" (http://www.simtel.net/pub/pd/50153.html)
• Information on write-back cache performance-enhancing utility from Evergreen Tech (http://homepage3.nifty.com/sandy55/PowerBoard/PowerBoard.html)(see "Cyrix5x86" section in the middle of the page and "et9603.exe" hyperlink)