Cyrix 6×86 CPU – the first Pentium compatible processor

Unveiled in October 1995, the 6×86 was the first Pentium-compatible processor to reach the market and the result of a collaboration with IBM’s Microelectronics Division. Acceptance of the 6×86 was initially slow because Cyrix priced it too high, mistakenly thinking that since the chip’s performance was comparable to Intel’s, its price could be too. Once Cyrix readjusted its sights and accepted its position as a low-cost, high-performance alternative to the Intel Pentium series, the chip made a significant impact in the budget sector of the market. Since a 6×86 processor was capable of an equivalent level of performance to a Pentium chip at a lower clock speed, Cyrix collaborated with a number of other companies to develop an alternative to the traditional clock speed-based rating system. The resulting Processor Performance rating, or P-rating, is an application-based standardised performance measure and Cyrix processors traditionally run at a slower clock speed than their P-rating with no apparent performance degradation. For example, the P133+ runs at a clock speed of 110MHz, while the P150+ and P166+ run at 120MHz and 133MHz respectively. The 6×86’s superior performance was due to improvements in the chip’s architecture which allowed the 6×86 to access its internal cache and registers in one clock cycle (a Pentium typically takes two or more for a cache access). Furthermore, the 6×86’s primary cache was unified, rather than comprising two separate 8KB sections for instructions and data. This unified model was able to store instructions and data in any ratio, allowing an improved cache hit rate in the region of 90%. Indeed, the 6×86 has a number of similarities to the...

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