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The Athlon XP at a 333MHz CPU Bus

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The Athlon XP at a 333MHz CPU Bus

AMDs latest Athlon XP processors, the 2600+, 2700+ and 2800+, differ from their Athlon XP predecessors in two important ways:

  • higher clock speeds
  • support for a faster 333 MHz CPU bus

How much better are these processors than their predecessors, and how do they compare with their rivals, Intels 2.53 GHz, 2.66 GHz and 2.8 GHz Pentium 4?

Athlon XP with a Thoroughbred Flavor

The 2600+, 2700+ and 2800+ processors are the latest members of AMDs third version of its Athlon XP processor family. While the original Athlon XP processors (1500+ through 2100+) used the 0.18 micron Palomino core, AMD has switched to a more efficient 0.13 micron process for current (1700+ and above) and forthcoming Athlon XP processors. This core, code-named Thoroughbred, runs cooler and is more efficient. However, after AMD introduced the initial Thoroughbred-based processors, it tweaked the design to add a ninth layer for better signal quality and to allow for higher processor speeds. All AMD Athlon XP processors with performance ratings of 2400+ and above use the revised core (sometimes referred to as Thoroughbred-B), while slower processors will be switched to this newer core as stocks of the original Thoroughbred-core processors in these speeds are exhausted.

Palomino or Thoroughbred?

AMD refers to processors using the Palomino core as Athlon XP Model 6 processors, whereas they refer to processors which use the Thoroughbred core as Athlon XP Model 8 processors. AMD used the Model 6 core for Athlon XP processors from 1500+ through 2100+. Model 8-based processors range from 1700+ through 2800+ and beyond. If you want to overclock a processor, Athlon XP Model 8 processors (particularly those with the Thoroughbred-B core) are a better choice. Since Athlon XP processors with performance ratings from 1700+ to 2100+ could be based on either design, you might want to determine which AMD ordering part number (OPN) a particular processor has before you buy it.

All Athlon XP processors based on the Model 6 (Palomino) design have AMD OPNs starting with AX, while Athlon XP processors based on the Model 8 (Thoroughbred) design have AMD OPNs starting with AXDA. For example, a Model 6 2100+ has an AMD OPN starting with AX2100, while a Model 8 2100+ has an AMD OPN starting with AXDA2100. Additional letters and numbers follow to indicate specifications for voltage and die temperature.

Chapter 3 of the 14th Edition of Upgrading and Repairing PCs provides a detailed comparison of Athlon XP processors from 1500+ up through 2200+, including their actual clock speeds, along with a discussion of AMDs QuantiSpeed architecture. Note that processors with AMD performance ratings from 1700+ through 2100+ could use either the older, hotter-running Palomino or the newer, cooler-running Thoroughbred design.

Comparing The Latest Members of the Athlon XP Family

The following table provides information about the latest members of the AMD Athlon XP processor family.

AMD Athlon XP Model Number

Compares to Pentium 4 running@

Actual Clock Speed of Athlon XP

Front Side Bus (FSB) Speed


2.4 GHz

2.0 GHz

266 MHz


2.6 GHz

2.13 GHz

266 MHz


2.6 GHz

2.08 GHz

333 MHz


2.7 GHz

2.16 GHz

333 MHz


2.8 GHz

2.25 GHz

333 MHz

Note that the 2600+ processor is available in two versions, one that supports the slower 266 MHz FSB speed and one that supports the faster 333 MHz bus. I recommend the 333 MHz bus version: although its actual clock speed is a tiny bit slower than the 266 MHz FSB version, the faster FSB speed enables it to have faster throughput for better system performance.

Note that the FSB speed listed in the previous table is twice the BIOS reported speed used to configure the processor (Athlon XP processors perform two data accesses per clock cycle). Thus, you select a frequency of 133 MHz in your BIOS setup to select the proper speed for processors with a 266 MHz FSB speed (133 times 2 equals 266); select 166 MHz to select the proper speed for processors with a 333 MHz FSB speed (166 times 2 equals 333).

For more information on PC2700 DDR SDRAM memory, which run at 333 MHz, see my previous article.

The Effect of the 333 MHz FSB on Performance

As you can see from the preceding table, theres little difference in actual clock speed between the 2400+ and the faster members of the Athlon XP family. However, the FSB speed is much faster. How does the increase in FSB speed affect performance? While the difference in actual clock speed between the 2600+ (2.13 GHz) and 2800+ (2.25 GHz) is just 6%, an average of 28 different benchmark tests performed by Anandtech and Toms Hardware show an average improvement in performance of about 13%. The 13% difference in overall performance between the 2600+ and 2800+ processors is about double what could be expected from the difference in clock speeds alone. The improved performance comes from the faster 333 MHz CPU bus (FSB), and the consequent chipset support for 333 MHz DDR SDRAM. The Athlon XP 2700+ (which also supports the 333 MHz CPU bus) is just a bit slower than the 2800+ version.

Comparing the Athlon XP to the Pentium 4

Even though Athlon XP processors have much slower clock speeds than comparable Pentium 4 processors, they have performed quite well in comparisons, and this trend continues in the latest tests.

While the 2.53 GHz Pentium 4 is 19% faster in clock speed than the Athlon XP 2600+ processor, its average performance improvement is only about 6%. This version of the Pentium 4 has a faster FSB (533 MHz) than the 2.6 GHz version of the Pentium 4 (400 MHz).

The 2.8 GHz Pentium 4 has a 24% advantage in clock speed, but, on the average, performs at virtually a dead heat with the Athlon XP 2800+ in the benchmarks we examined. However, note that more and more applications offer Pentium 4-specific enhancements, so if your application has been optimized for the Pentium 4, it will run faster if you use a Pentium 4 than if you use an Athlon XP processor. Also note that the Pentium 4 is also available in a 3.06 GHz version, and that version also has Hyper-Threading support, which allows one processor to operate as two, with a resulting 10 to 20% overall improvement in performance.

For more information Hyper-Threading technology, see my article "Breaking the 3 GHz Barrier with the Pentium 4 Processor".


While the support for 333 MHz CPU bus helps the new Athlon XP processors to keep pace in most respects with the much faster Pentium 4 processors they compete with, you should check the following before you upgrade:

  • BIOS support
  • 333 MHz (166 MHz x 2) CPU bus support
  • Thermal protection

Because of the differences in core design between the Palomino (Model 6) and Thoroughbred (Model 8) processors, motherboards built for Palomino require a BIOS upgrade before you can install a Thoroughbred processor.

If you want to use processors which support the 333 MHz CPU bus (2700+, 2800+ and the 2.08 GHz version of the 2600+), you must have a motherboard with a chipset that supports 333 MHz FSB. These chipsets currently include VIAs KT333 and KT400 (which also supports AGP 8x), NVIDIA's nForce2 and SiSs 746.

A third concern, thermal protection, affects Thoroughbred-B-based processors (Athlon XP 2400+ and higher-rated models) in particular. As I have demonstrated in one of the videos included on the DVD with the 14th edition of Upgrading and Repairing PCs, older AMD Athlon processors can fail in just a few seconds if the cooling fan built into the heatsink fails. As a consequence, AMD developed a thermal diode for Athlon XP-compatible motherboards. However, tests performed by Toms Hardware have revealed that the changes in design from the original Thoroughbred-A core to Thoroughbred-B render current thermal diode designs useless for detecting overheating. Youll need to rely on processor fan monitoring or internal case temperature monitoring to determine if youre having a problem until AMD revises its thermal diode design and motherboard makers incorporate the change.

If youre moving from an older Athlon processor, you will also need a better heatsink if you prefer OEM processors (which require you to add your own heatsink). Because of the high surface temperature, heatsinks should use a copper transfer surface rather than the aluminum surface which has been common in the past. AMDs list of approved heatsinks and the processor models they should be used with is available at http://www.amd.com/us-en/Processors/TechnicalResources/0,,30_182_869_4348^4356,00.html.


The latest Athlon XP processors benefit from an improved version of the 0.13 micron Thoroughbred processor core, and continue to be less expensive than comparable Pentium 4 processors. However, most existing motherboards dont support the faster 333 MHz CPU bus needed to achieve the full benefit from these new processors. Also note that if you change motherboards to one supporting the 333 MHz bus, you will also need to purchase all new PC2700 memory, which also runs at 333 MHz. You might find it necessary to swap motherboards and RAM in order to upgrade to the latest processors, but if you prefer AMD, the 2800+ and the slightly slower 2700+ are the fastest Athlon XP processors yet.

For Additional Information

AMDs official Athlon XP 2700+ and 2800+ Web site is located at

Toms Hardwares head-to-head comparisons of the latest Athlon XP processors to their Pentium 4 rivals is located at http://www.tomshardware.com/cpu/02q4/021001/index.html

Anandtechs tests of the Athlon XP and Pentium 4 processors is located at

VIA Technologys official Web site for the KT333 chipset is located at
, and its KT400
Web site is lo cated at

SiS Technologys official Web site for the 746 chipset is located at

Copyright©2003 Pearson Education. All rights reserved.

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