Quote Originally Posted by informal View Post
It's Cyrix and I'm not sure I've heard that their x86 implementation had any issues.
Well, among other issues, there was one bug that caused a 20%-30% performance degradation with a Cyrix 6x86 chip under Windows NT:

http://www.realworldtech.com/altcpu/...aq/faq4of7.htm
Microsoft did, however, discover a bug in the Cyrix 6x86 processor during stress testing of NT 4.0 that caused the operating system to crash. To guarantee Windows NT 4.0 stability, a workaround was developed by both companies. In the workaround, the chip's internal cache operating mode is changed from write-back to write-through when a problematic 6x86 is discovered. The result is a performance degradation of approximately 20%-30%.
Also, the Cyrix 6x86 by default did not respond to the CPUID instruction. To detect it properly (and identify it as a Pentium-class processor) programmers had to use special code for Cyrix processors, which many programmers opted not to do.

Though the 6x86 had many architectural features matching a P6 Pentium Pro such as out-of-order execution, it did not implement certain Pentium (P5)-supported instructions, which caused it to not truly be Pentium-compatible. This caused issues with software that relied on these instructions, including the Creative Soundblaster AWE64 drivers. Also, while its integer execution was much stronger than a Pentium of similar clockspeed, its FPU was much weaker, which became important when Quake became a popular game.