Finnish overclocker The Stilt managed to create a BIOS tweak that improves x87 performance for AMD's processors. The overclocker discovered that the poor performance of AMD's processors in SuperPI is partly due to lack of BIOS tuning. By enabling certain BIOS registers and disabling a certain black called NRAC, The Stilt managed to boost the performance of the AMD A10-6800 APU by 18-30% in the SuperPI benchmark:
Now I roughly know what it is and what it does, but still some questions remain: Why does this "feature" exist in the first place and why it is activated on all 15h family parts. I would normally assume it is a workaround for some errata, however no bulletin exists for this one either. Also this feature does not exist in any documentation, or it does but only AMD has access to the required level. I find it hard to believe that it would be a design issue as the affected instructions work fine (but slowly) and it existed since early Zambesi revisions and, currently is still present in Richland and probably beyond (within family 15h)...
I'd say it is either a errata fix or a errata fix gone wrong. If it is a programming mistake which has gone un-noticed during the last two years ... That would make me just sad.
Effect: A massive performance hit in application heavily utilizing x87 instructions.
Negative effects: TBD, none found yet. The performance in non x87 applications remains the same or improves very slightly. No instability, increased power consumption, reduced overclockability or anything else abnormal has been observed. However the final conclusion requires far more extended testing than I am able to do myself.
After the fix has been applied SuperPI shows 18-30% improvement in performance. Bigger the calculation, bigger the improvement. Since this kind of fix is quite unheard of, I knew that I would be crucified if I would make such claims without any providing evidence.
X-bit Labs tested the patch on AMD's A10-6800K APU and found a 14.7 percent performance increase in SuperPI 1M and a 18.7 percent performance increase in SuperPI 8M. The performance increases are only found in SuperPI, a benchmark that relies heavily on the outdated x87 instruction set, other benchmarks show no difference.