Theoretical Comparison

The basics

If you're interested in an Athlon vs. P3 article, chances are you've already seen the FiringSquad Athlon 600 Preview, which details the architectural details of Athlon technology. Instead of opening that huge can of worms, what we'll provide here is a brief rundown of what to expect.

Bus speed and architecture

The biggest difference between the Athlon and the Pentium III is their respective bus architectures. Intel's standard architecture, used in most current P3 and Xeon architectures is the Gunning Transceiver Logic (GTL+) bus. It's currently rated for 100MHz operation, and is synchronous with main memory. In November, Intel's Camino 820 chipset will up the bus to 133MHz, at which point Intel will consider RAMBUS, DDR SDRAM, or even PC133.

The Athlon piggybacks on the advanced Alpha EV6 bus architecture, which is rated at 200MHz, and can theoretically scale up to 400MHz. Technically, the EV6 bus holds a number of advantages over GTL+, including point-to-point topology (each CPU in a multiprocessor system gets its own 200MHz pipe to the chipset, whereas with GTL+ every CPU shares the same 100MHz line), and asynchronous memory interface (RAM speed is independent of bus speed, and is determined solely by the chipset).

Processor core

All modern x86 CPUs take complex CISC operations and decode them to more compact RISC ops for efficient decoding. The Athlon has the theoretical advantage here as well, decoding up to 3 x86 operations per cycle. The Athlon is also designed to be a superscalar processor with deep internal buffers. While both processors currently share a 512KB ½ speed cache, the Athlon quadruples the size of the Pentium III's 32KB L1 cache, all the way to 128KB.

The Pentium III has 2 floating point execution units, one of which is fully pipelined, the other partially pipelined. The Athlon expands on this, with 3 fully pipelined execution units, capable of greater floating point throughput. AMD bills the Athlon as a true 7th generation processor, whereas the Pentium III incorporates additional SIMD technology onto the existing P2 core.

SSE vs. 3DNow!

Speaking of SIMD, the Athlon builds upon the 3DNow! technology first introduced with the K6-2. Included are new instructions to speed up integer operations, as well as DSP instructions which can be used to accelerate functions such as mp3 decoding, modem functions, and more.