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Hack 21 Which CPUs Are Hackable?

Know which CPUs can be hacked or overclocked.

With this brief look at the evolution of CPUs, you'll quickly see who makes the most hackable CPU. Armed with positive CPU identification and knowing what you want to accomplish in terms of performance, you'll be able to determine if your present CPU can be hacked or if you need a new CPU to gain better performance.

AMD CPUs are typically deemed more hackable than Intel CPUs as there are usually more system boards for AMD CPUs that provide the option to adjust the base clock speed [Hack#25] and clock multiplier [Hack 27] and [Hack 28]] Intel has typically locked down their CPUs to function at only one or a few clock multiplier settings, and they have usually rated and sold most of their CPUs at the highest speed they can be reliably be run. Table 3-1 provides a quick overview of the most hackable CPU types and methods. Most CPUs can be overclocked by changing the base clock frequency, while some allow multiplier changes. In the case of a number of AMD and a few Intel CPUs, the chip can be modified to support overclocking.

Table 3-1. Overclocking methods by CPU type

CPU	Overclocking options	Overclocking method
AMD Athlon	FSB clock, multiplier	Jumpers, BIOS, chip mod
AMD Duron	FSB clock, multiplier	Jumpers, BIOS, chip mod
Intel Celeron	FSB clock, multiplier	Jumpers, BIOS, chip mod
Intel Pentium I	FSB clock, multiplier	Jumpers, BIOS
Intel Pentium II (pre-8/98)	FSB clock, multiplier	Jumpers, BIOS, chip mod
Intel Pentium II (post-8/98)	FSB clock	Jumpers, BIOS
Intel Pentium III	FSB clock	Jumpers, BIOS
Intel Pentium 4	FSB clock	Jumpers, BIOS

The most significant limiting factors in overclocking potential are the features of the system board. As the Front Side Bus (FSB) speed is increased, the PCI and AGP bus speeds are also increased. So if the clock settings alter the FSB and PCI/AGP speeds in proportion to each other, you may reach the speed limit of the system components before you reach the limits of your CPU. If your system board features provide a variety of FSB speeds with separate PCI/AGP speed options, you can likely increase the CPU speed by 100% and still maintain reliable PCI/AGP bus speeds. The available clock setting options vary from system board to system board, even across boards from the same manufacturer.

According to http://www.sysopt.com, one of the most popular CPU analysis and overclocking sites, which collect data from dozens of real-world users who thrive on overclocking, the most overclockable CPUs are:

• AMD Thunderbird

• Pentium III Coppermine

• Pentium 4

• AMD Duron

Perhaps Intel saw the light in trying to recapture hobbyist market share and decided not to limit the overclockability of the Pentium III and 4 CPUs, a departure from their earlier restrictions on some Pentium II and Celerons chips. Do not let the lack of being able to change the multiplier values for Intel CPUs fool you into thinking they cannot run faster than rated speeds; most do run faster and quite well. While considering CPUs, you have to consider the most popular system boards for overclocking. According to http://www.sysopt.com, the top four overclockable system boards are:

• Asus Tek

• Abit

• MicroStar

• Epox

3.4.1 Intel CPUs

Relatively few Intel CPUs can be hacked because it's not in Intel's best interest to sell lower performance chips that can be made to perform like their higher performance versions. Some Intel CPUs perform better under overclocking conditions than others. Table 3-2 lists the most hackable CPUs based on end-user reports from http://www.sysopt.com.

Table 3-2. Hackable Intel CPUs

Processor	Published speed	Achieved speed	Percent increase
Pentium 4	3.1 GHz	5.0 GHz	+ 163%
Pentium 4	2.7 GHz	3.4 GHz	+ 25%
Celeron II	2.0 GHz	2.9 GHz	+ 45%
Pentium III Tualatin	450 MHz	1.2 GHz	+ 166%
Pentium II	400 MHz	2.6 GHz	+ 550%

While these performance gains are impressive, the claims by users who have achieved these speeds were not accompanied by how-to tips. Intel CPUs can be two to five times more expensive than comparable AMD CPUs, and hacking these CPUs and the system boards that support them is not well documented.

3.4.2 AMD CPUs

AMD CPUs are generally more overclockable than Intel's, with more parameter flexibility in the BIOS, for three reasons:

• The manufacturers of boards that use AMD processors and related chipsets tend to use the hackable Award BIOS.

• Available "white box" system boards are more hackable.

• The flexibility of AMD processors accepts higher clock speeds and various clock-multiplier values.

Intel carries the majority of the CPU and the system board market with reputable OEMs whose products generally use restricted versions of Phoenix BIOS. By contrast, AMD reaches out to a different market that includes AMD for CPUs, Via for supporting chipsets, and Phoenix's Award BIOS division with significant parameter flexibility.

Tweaking AMD CPUs yields impressive results, as seen in the data from http://www.sysopt.com in Table 3-3, feeding the myth that many slower CPU chips are really higher speed devices that failed high-speed tests, were marked as slower speed devices, and were undersold.

Table 3-3. Most hackable AMD CPUs and performance increases

Processor	Published speed	Achieved speed	Percent increase
Athlon MP	1.8 GHz	2.7 GHz	+ 50%
Athlon Thunderbird	1.7 GHz	2.4 GHz	+ 41%
Athlon Thunderbird	1.5 GHz	2.3 GHz	+ 53%
Athlon MP	1.5 GHz	2.1 GHz	+ 40%
Athlon MP	500 MHz	1.8 GHz	+ 260%

As with disclaimers for just about everything else, "your mileage may vary"—and it will—because the various combinations of CPU, chipset, BIOS, and system board design all yield different results. The distribution of overclockable CPUs for retail or online is not predictable; the plant where the chips are made may have had an excess of fast chips and a backlog of orders for slower ones and simply relabeled and shipped the faster CPUs to meet business needs or some monthly shipment quota.

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