Hack 25 Control CPU Clock Speed from the BIOS

Increase your CPU speed with parameters controlled by your BIOS.

After you've determined your present CPU speed [Hack #6] and[Hack #24]

Some system boards give you everything you need to adjust clock speed from within the BIOS setup. Figure 3-21 shows very basic CPU speed control available in the Award BIOS of an ECS system board for an AMD Athlon CPU.

Figure 3-21. This Award BIOS setup program provides only CPU clock control with a default of 166 MHz

Starting with a system with an AMD Athlon XP that clocks at 1.9 GHz, with the original or default speed setting based on a 166 MHz CPU clock and an internal CPU clock multiplier value of 11.5 (166 11.5 = 1.91 GHz), adjust the base clock speed up incrementally until the system fails to boot up or function reliably. Changing the base CPU clock speed to 185 MHz with the same fixed multiplier value (11.5), the CPU makes it up to 2.13 GHz (11.4% faster), runs reliably with no hacking other than this speed setting, and delivers a measured 11.40% performance increase per Sandra benchmarks [Hack #23] .

Changing the CPU clock speed also affects the operating speed of other system components, most notably the Front Side Bus (FSB) that interfaces with the system's chipset and memory. If an overclock attempt fails, check the speed of your RAM and consider the limitations of the system board design and chipset. If you start out with 333 MHz DDR RAM it may be limited to a mere 10-20% overclock rate and heat up considerably. Changing to 400 MHz RAM may allow you to further increase the CPU or FSB speed.

As shown in Figure 3-22, the CPU clock can be adjusted in 1 MHz increments up to 199 MHz. A setting of 185 MHz with a CPU multiplier of 11.5 causes the CPU to clock at 2.13 GHz.

Figure 3-22. CPU clock settings in 1 MHz increments with 185 MHz chosen in the process of overclocking in small steps

When adjusting the CPU speed, you must be aware of one other clock value that may increase or decrease as the main CPU/FSB clock is adjusted, that of the PCI bus clock. System boards use various divider and multiplier logic circuits to keep the PCI and memory bus in sync with CPU operations. Like the CPU, ramping up these clock speeds too high (too low never seems to be a problem) can cause the system to function unreliably. The default FSB speeds for popular CPUs are shown in Table 3-4.

Table 3-4. FSB speeds for various common CPUs

FSB speed	CPUs
66 MHz	Intel Celeron Pentium I
100 MHz	AMD Thunderbird (early versions) Intel Pentium II (early versions) Intel Celeron Intel Pentium III (early versions)
133 MHz	AMD Thunderbird (C versions) Intel Pentium II (later versions) Intel Celeron Intel Pentium III
200 MHz	AMD Athlon XP (early versions)
266 MHz	AMD Athlon XP (mid-versions)
333 MHz	AMD Athlon XP (mid-versions)
400 MHz	AMD Athlon XP (late versions) Intel Pentium 4 1.3-2.6 GHz Intel Celeron 1.7-2.8 GHz
533 MHz	Intel Pentium 4 2.26 GHz and higher (Northwood and Prescott)
800 MHz (200 MHz 4)	Intel Pentium 4 (with Hyper-Threading Technology) 2.4 GHz and higher

Both AMD and Intel update CPU versions and capabilities within major and minor product revisions. Table 3-4 is not intended to be a complete or exhaustive reference to every CPU or production version.

You can typically get away with allowing the default 33 MHz PCI bus speed to increase 10-20%, letting it run between 36 and 40 MHz, but this will depend on whether or not your peripheral cards (video, LAN, sound) can handle the higher speed.

I also tried overclocking an older Pentium III chip but ran into problems. The first sign of trouble when overclocking appeared in the P.O.S.T. phase. The Memory Test indicated that there were 524288 K of RAM (512 MB) when the system was running with the stable "124/31" overclocked settings. When I used the available 124/41 setting, the CPU still ran at 744 MHz, but the system became unstable once the operating system was loaded and running. Pushing the CPU even further with a selection of 133/33 allowed the system to run at 800MHz, but the memory test stopped testing at 360MB of RAM, as shown in Figure 3-23.

Failures in the P.O.S.T. typically indicate that the chipset, memory, or I/O bus cannot handle the higher speeds. It is recommended that you use clock settings that keep the PCI and AGP bus speeds within a few percent of their normal specifications.

Figure 3-23. Memory test misses some RAM when overclocked