Introduction

While AMD got off to a slow start with its Thoroughbred core earlier this year, it appears they’ve got things under more control now as today they’re unveiling two new processors – the Athlon XP 2700+ and the Athlon XP 2800+. Like the Athlon XP 2400+ and Athlon XP 2600+, these new chips are based on the second revision of AMD’s 0.13-micron Thoroughbred core, which features an additional metal layer, optimized circuit paths, and additional decoupling capacitors.

All of these enhancements allow the Thoroughbred core to scale to higher clock speeds than its predecessor, which pretty much hit the ceiling at 2GHz. In comparison, today’s Athlon XP 2700+ clocks in at 2.17GHz while the Athlon XP 2800+ runs at 2.25GHz. If you’re familiar with the specs on the Athlon XP 2600+, you realize that its 2.13GHz clock speed is awfully close to the core frequency of the Athlon XP 2700+. “How’d they do that?” you’re probably wondering.

Cranking up the bus

That’s because besides the new clock speeds, today’s new Athlon XP processors also get a new system bus speed: 333MHz! With the higher bus, bandwidth to the processor jumps from 2.1GB/sec to 2.7GB/sec, right in synch with PC2700 (DDR333) memory. This allows the Athlon XP 2700+ to perform faster than the Athlon XP 2600+ even though it’s running at roughly the same clock speed. Earlier this year Intel increased the Pentium 4’s bus speed from 400MHz to 533MHz, bringing a 5% performance boost a 2.4GHz.

While this may not sound like a lot at first, keep in mind that as the clock speed of the processor increases, so does the performance benefit of moving to the faster bus. This is because as processors get faster, they’re left spending more time waiting on other components within the system.

With its 16.0 multiplier, the Athlon XP 2600+ can only access the rest of the system every sixteenth clock cycle. If the processor needs to retrieve data from main memory it often has to wait several clock cycles before it can access the data it needs. CPU manufacturers get around this by implementing larger caches on the CPU itself. For instance, earlier this year, Intel doubled the L2 cache size on Pentium 4 from 256K to 512K. But this solution can get expensive. Another solution is to increase the speed of the system bus.

In comparison to Athlon XP 2600+, Athlon XP 2700+’s 13.0 multiplier yields a similar clock speed (13.0 multiplier x 166MHz system bus = 2.17GHz) but greater performance. Again, with the greater memory bandwidth (provided by the higher bus speed) available to the processor, the CPU is better fed with the data it needs. With DDR333 and, more recently, DDR400 memory being offered for the Athlon platform, the memory was actually running faster than the processor itself. Now that AMD has upped the Athlon’s system bus to 166MHz (effectively 333MHz since the bus transmits data two times per clock cycle), the platform is more balanced and end users such as you and I are happy because we get more performance to run the games and other applications we crave so dearly.