AMD Athlon XP 2600/2400+ - Introducing Thoroughbred Revision B

A Modified Core

As we discovered when we reviewed the first 0.13-micron Thoroughbred based Athlon XP, there didn't seem to be much headroom in the process. Overclocking the XP 2200+ was pretty much a lost cause, and we attributed this to AMD's 0.13-micron process.

It turns out that there were limitations with the Thoroughbred core and it order to reach speeds as high as and beyond 2.13GHz AMD had to do a little fine tuning with the Thoroughbred core.

AMD added another metal layer to the 0.13-micron Thoroughbred core, giving the chip a total of 9 metal layers. The purpose behind adding an additional metal layer is to normally deal with increased transistor counts and although there's a slight increase in the number of transistors with this new core, that wasn't AMD's purpose behind adding a 9th metal layer.

The addition of the 9th metal layer was made from a purely routing standpoint; to put it simply, one limitation of increasing clock speed is the physical distance between blocks on the CPU's die. Adding another metal layer gives you additional room to work with optimizing electrical paths within the processor so that those physical distances don't limit the clock speed of the CPU.

The biggest downside to adding additional metal layers to a process is of course increased manufacturing complexity and thus increased manufacturing costs. To give you an idea of where the competition is at, the current Northwood core features 6 metal layers and it won't be until the release of the 90nm Prescott chip next year that Intel moves to a 7 layer design. To put it plainly - 9 metal layers is a lot. Luckily the Thoroughbred die is small enough that offsetting the extra costs introduced by the additional metal layer should be manageable, but with a design already at 9 metal layers it will be interesting to see where the Athlon XP tops out at. The Pentium 4 can get away with using fewer layers because of its longer pipeline; remember that some of the stages in the Pentium 4 pipeline are there for the sole purpose of moving data across the chip.

We also mentioned that the transistor count of these new Thoroughbred cores (dubbed Revision B) has increased slightly from the now old Revision A cores; these additional transistors go to implementing additional decoupling capacitors that help reduce EMI.

The results of the additional metal layer (and its subsequent electrical path optimizations) and the new decoupling capacitors are immediately evident; the Thoroughbred is now easily able to run at 2.33 - 2.40GHz with relatively little effort. All Thoroughbred cores will begin being manufactured to the new Revision B specifications, unfortunately there's no way of telling which core you're getting without looking at the CPUID string reported by the processor. Obviously if you purchase an Athlon XP 2600+ or 2400+ (or faster, once they're released) you're getting the new core, but if you buy slower Thoroughbred cores there's no way of knowing until you can test the CPU yourself.

Although the actual CPUIDs that correspond to the new Revision B core are not listed on AMD's website yet, you can expect them to appear in an update to the Athlon XP datasheets here.

The modifications to the Thoroughbred core do require BIOS support but rest assured that any motherboard that currently supports the Thoroughbred processors will also support the Revision B CPUs.