Cyrix Cx5x86

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 Intel Pentium Pro. The chip featured near-complete support for i486 instructions, and 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 IBM 5x86 was a licenced rebranding of the Cyrix design (IBM 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/IBM 5x86, 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 millions transistors on 0.65 micron 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.

External Links:

• Comparative performance benchmarks (http://www.interwb.com/486bench.txt)
• Cyrix 5x86 (http://www.pcguide.com/ref/cpu/fam/g4C5x86-c)
• Cyrix 5x86 Processor Brief (http://www.worldwide-web-design.com/5x-tb)
• 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)
• Information on write-back cache performance-enhancing utility from Evergreen Tech (http://homepage3.nifty.com/sandy55/PowerBoard/PowerBoard)(see "Cyrix5x86" section in the middle of the page and "et9603.exe" hyperlink)