New micro-architecture

Core changes

The most talked about feature is probably the processor’s integrated memory controller, a function that was once left up to the North Bridge of the system chipset, an external chip. This dramatically improves the processor’s memory bandwidth, increasing performance. The Athlon 64 FX features the same 128-bit wide interface to its memory controller as AMD’s Opteron processors, while the Athlon 64 relies on a tamer 64-bit interface.

AMD is quick to point out that this new feature makes the processor’s front-side bus speed less important, as the memory data that was once tied to the front-side bus in previous processors now operates at the same clock frequency as the processor. If you recall, the Athlon XP 3200+ featured a 400MHz front-side bus. In addition, the speed at which the processor and chipset communicate is dependant on the processor’s HyperTransport link. Currently, both Athlon 64 processors can communicate to the system chipset at speeds up to 1.6GHz effective.

Like the Athlon XP, the Athlon 64 family features a 2-way set associative 128KB Level one cache, 64K for instructions and 64K for data cache. AMD has doubled the size of the L2 cache from 512K in the latest Athlon XP processors to 1MB in Athlon 64. Increasing the size of the L2 cache is a popular enhancement CPU manufacturers implement to improve performance.

To further improve performance, AMD has improved its branch prediction in Athlon 64 while including larger Translation Look-Aside Buffers (512 entries in Athlon 64 versus 256 in Athlon/Athlon XP) with reduced latencies. The Athlon 64 also incorporates Intel’s SSE2 instructions (in addition to 3DNow! Professional and SSE), first introduced with the Pentium 4.

Finally, to help the Athlon 64 scale to higher clock speeds, two additional stages have been added to its pipeline, bringing the grand total to 12 stages in its integer pipeline and 17 stages in its floating-point pipeline. If you recall the original Pentium 4 launch, the hyper-pipelined nature of its design was one of the more controversial features.

The Athlon 64 is produced at AMD’s 0.13-micron Fab in Dresden Germany. AMD has spent a considerable amount of time refining its 0.13-micron process, which now not only features copper interconnects, but also silicon-on-insulator technology (SOI), licensed from IBM. SOI is a technique that reduces the capacitance of the millions of transistors within the processor, allowing them to run faster and consume less power. The Athlon 64 is composed of approximately 105.9 million transistors; nearly twice the amount of Athlon XP’s 54.3 million.

Athlon 64 FX versus Athlon 64

As we mentioned earlier, one of the key performance differences between the Athlon 64 FX and Athlon 64 is its wider memory interface: 64-bits in Athlon 64 versus 128-bit in the FX model. Essentially, the FX model is nothing more than an Opteron processor; the chip even uses the same 940-pin socket interface.

AMD also separates the Athlon 64 FX-51 from the Athlon 64 3200+ by running it at 2.2GHz, while the Athlon 64 3200+ operates at 2GHz.

Finally, the Athlon 64 FX requires registered DDR memory - the Athlon 64 does not. This is an important distinction for Athlon 64 FX system builders, as registered DDR memory is harder to come by; Kingston recently announced its HyperX registered DDR400 memory modules.