Cyrix 6x86 - Performance

Performance

It has been speculated by experts that 6x86 was designed to perform well specifically on business-oriented benchmarks of the time, most notably Ziff-Davis' Winstone benchmark. Winstone ran various speed tests using several popular applications. It was one of the leading benchmarks during the mid-'90s and was used in some leading magazines, such as Computer Shopper and PC Magazine, as a deciding factor for system ratings.

Cyrix used a PR rating (Performance Rating) to relate their performance to the Intel P5 Pentium (pre-P55C), because a 6x86 at a lower clock rate outperformed the higher-clocked P5 Pentium. For example, a 133 MHz 6x86 will outperform a P5 Pentium at 166 MHz, and as a result Cyrix could market the 133 MHz chip as being a P5 Pentium 166's equal. A PR rating was also necessary because the 6x86 could not clock as high as P5 Pentium and maintain equivalent manufacturing yields, so it was critical to establish the slower clock speeds as equal in the minds of the consumer. However, the PR rating was not an entirely truthful representation of the 6x86's performance.

While the 6x86's integer performance was significantly higher than P5 Pentium's, it's floating point performance was more mediocre—between 2 and 4 times the performance of the 486 FPU per clock cycle(depending on the operation and precision). The FPU in the 6x86 was largely the same circuitry that was developed for Cyrix's earlier high performance 8087/80287/80387-compatible coprocessors, which was very fast for its time—the Cyrix FPU was much faster than the 80387, and even the 80486 FPU. However, it was still considerably slower than the new and completely redesigned P5 Pentium and P6 Pentium Pro-Pentium III FPUs. During the 6x86's development, the majority of applications (office software as well as games) performed almost entirely integer operations. The designers foresaw that future applications would most likely maintain this instruction focus. So, to optimize the chip's performance for what they believed to be the most likely application of the CPU, the integer execution resources received most of the transistor budget.

The popularity of the P5 Pentium caused many software developers to hand-optimize code in assembly language, to take advantage of the P5 Pentium's tightly pipelined and lower latency FPU. For example, the highly anticipated first person shooter Quake used highly-optimized assembly code designed almost entirely around the P5 Pentium's FPU. As a result, the P5 Pentium significantly outperformed other CPUs in the game. Fortunately for the 6x86 (and AMD K6), many games continued to be integer-based throughout the chip's lifetime.