Closer Look at FX-8120, FX-6100 and FX-4100
Besides the top FX-8150 processor, there are three more FX CPUs available today: FX-8120, FX-6100 and FX-4100.
We had the opportunity to test all four processors that is why besides the official specifications we can also offer you CPU-Z screenshots:
All CPUs use the same B2 processor stepping and they differ by the number of active modules/cores, clock frequency and TDP. Both eight-core processors belong to 125 W thermal envelope, FX-6100 and FX-4100 are more energy-efficient and their TDP is set at 95 W. I would like to point out that eight-core processors will not always have higher TDP than the rest of the models. Very soon AMD is going to release their FX-8000 series models with 95 W TDP. They will most likely be built on the new B3 processor stepping, which will be designed to lower Bulldozer’s thermal and energy appetites.
The eight- as well as six-core FX processors have two-step Turbo mode. The first step called “All Core Boost” allows increasing the processor clock frequency even if all cores are utilized, but the processor’s heat dissipation and power consumption remain thin acceptable limits. The second step called “Max Turbo Boost” allows increasing the frequency when no more than half of all the cores are utilized. As you can see from the table above, All Core Boost may increase the CPU clock speed only by 100-300 MHz, while Max Turbo Boost offers much more dramatic automatic overclocking. However, Turbo mode will have real practical value only for eight and six-core CPUs models. As for quad-core Bulldozer based processors, they have seriously limited automatic overclocking potential and its benefits will be barely noticeable.
Moreover, another thing that makes six- and quad-core FX processors inferior to their eight-core brothers is the North Bridge frequency. AMD’s lower-cost CPU models with fewer cores have this frequency lowered by 10%, which in its turn affects the memory and L3 cache performance. We can see the practical repercussions of this change in the AIDA64 cache & Memory Benchmark, which estimates the practical bandwidth and latency of the memory sub-system. For an even more illustrative picture we ran this test on FX-8150 and FX-4100 processor working at the same 3.6 GHz clock frequency with DDR3-1867 SDRAM with 9-11-9-27 timings.
We do see slight difference in actual memory speed and L3 cache performance by different Bulldozer modifications, but it is not critical at all. Moreover, we shouldn’t forget that all FX processors are Black Edition ones. It means that the multipliers for the clock frequency and the frequency of the built-in North Bridge are unlocked, which gives the users freedom to configure and easily overclock their processors. Some mainboards take advantage of this AMD FX feature and by default set the processor North Bridge frequency to 2.2 GHz for all Bulldozer CPUs.