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In The Lab
Introduction to Overclocking
by chris

What is overclocking?
When designing processors and other components, the manufacturer must design them to operate under a variety of thermal, electrical and other environmental conditions. This builds in a "safety margin" that is necessary to ensure proper operation under these extremes, and where overclocking can take advantage of the operational range of the device. Overclocking usually refers to operating the processor at a faster speed than it is rated for, but may also refer to adjusting or "tweaking" the speed at which the chipset, memory, expansion bus, or the video card operate at as well. The operating speed of a CPU is calculated from the speed of the system clock and a multiplier which is typically "locked" during the manufacturing process. If the CPU clock is operating at 100 MHz (Megahertz = 1 million cycles per second) and the internal multiplier is 20, then the CPU's operating frequency would be 100*20 = 2000 MHz or 2.0GHz (Gigahertz = 1 billion cycles per second.)

Because the multiplier value is usually "locked," the typical method of overclocking a CPU is by adjusting the bus speed of the system clock producing the timing pulses for the processor. With many systems, the processor clock is directly tied to the system bus. This means that if you increase the frequency to the CPU, you also increase the frequency to the memory, the video, the system chipset, and maybe your expansion slots. Even if the CPU can function at the higher speed, other components in the system may not be able to operate properly; this can cause the system to lock-up or crash frequently, assuming it will start at all.

Locking of the CPU multiplier does not always mean that you cannot change the clock multiplying value, but more than likely you can only decrease the value, not increase it. What options you have to adjust the multiplier, how much you can change the clock speed, and what other settings are available is a feature in the system BIOS Setup menus. Most OEM manufacturers can and will hide settings such as this, mainly to prevent changes that could damage the system, or create unstable results that the company cannot easily identify or resolve the cause, and are not willing to support.

Why overclock your system?
There are a variety of reasons why someone would overclock a system, but the main one seems to be because low performance processors generally cost less, and by overclocking, may perform like a high-performance processor that can cost hundreds of dollars more.

Potential overclocking issues
The worst possible result of an overclocking failure would be catastrophic failure of the CPU which could also result in damage to the system board and other components. Most of the current generation processors have some level of thermal protection integrated into the CPU. For example, if a Pentium 4 processor starts to exceed the predetermined operating temperature range, then the CPU will either throttle back (reduce its operating speed) or shut down the system to prevent damage or catastrophic failure.

Running the CPU faster then what it was designed for will generate more heat in the process. Like the processor itself, a standard boxed heat sink will have a range that it should able to function within and still keep your processor cool enough to prevent damage or slow down. However, if the processor is generating more heat as a result of overclocking, then your heat sink safety margin is also being reduced. The simple solution is to replace the boxed heatsink with a high-performance heat sink or with water cooling.

Another setting used to stabilize an overclocked CPU is the core voltage level. As the processor runs at higher speed, more current is required for the operation (and more heat is generated). Increasing the processor voltage can help stabilize power levels to the CPU when under stress, but there are physical design specifications that limit the voltage levels the CPU can operate with and not be damaged or "burn out." Adjusting voltage levels must be done carefully, never to exceed the specified maximum of the device. Unlike clock speeds, voltage levels should be increased in small steps and testing system stability (preferably under stress) needs to be done after each change.

Because changing the system clock can impact other components, system instability can show up as: failure to POST or boot; corrupted or unreadable video display; random system hangs, intermittent restarting or crashing; data corruption of the drive(s); and memory errors or random diagnostic failures. The primary cause can usually be tied to changing the FSB (Front Side Bus) speed to increase the number of operations the CPU can perform. This often has the effect of increasing the timing used for memory access, or I/O transfer to your other components and system slots. The solution to these issues is to look for system boards that support separate settings for the memory and PCI bus timing, so you can minimize the impact on the rest of the system.

The Overclocking Process:

  1. Select and use high-performance components. Even if you only plan to overclock the CPU, choosing high-performance memory makes it more likely that you will not develop timing instability between the processor and RAM. When choosing video cards and other hardware, get the best and fastest components within your budget. The point of having the equivalent of a high-performance processor makes little sense if the graphics performance is only average. Keep in mind that some of your components such as hard drives and network adapters may create bottlenecks that will exist no matter what the operating performance of your system is like. The type of applications you run may disregard any overclocking performance gain you can observe in benchmarks. How can you tell if the system responds faster if your primary activities include Internet surfing, word processing, or database access?

    If you are building a system from parts, compare motherboard features, and look for ones that are designed for gaming and/or overclocking. These system boards will usually offer more options when attempting to change the overclock-related settings, and some, like the high-end Asus boards even have overclock "profiles" that apply a variety of predetermined settings to achieve a specific percentage boost in performance. Asus also includes software for monitoring temperatures and voltage levels and allows you to make small adjustments from within Windows without continuously restarting. Overclocked video cards are available from some vendors, giving you a manufacturer-tested performance boost right out of the box.

    Don't forget to have a good power supply to handle the increased load, and to provide stable power to the processor and other components. Additional fans to improve airflow through the case are recommended, even if using water cooling on the primary components.

  2. Assemble and test the system with default settings first. Before overclocking the CPU or the rest of the system, make sure everything you have assembled works at normal settings and conditions. You want to make sure that the system exhibits no instability or timing issues at default settings before trying to troubleshoot what fails at non-standard ones. You also want to make sure that there are no defective components, especially memory, video or drives, and that the cooling solution is installed correctly and is operational.

  3. Overclock in stages, and verify stability at each stage. When building a system from parts, I always recommend that you assemble basics first and then test in stages as you add additional hardware. This allows you to focus only on the changes if you add something that fails to work with the rest of the system. By the time you get to the operating system setup, it means that installing the necessary drivers is straightforward because you know for sure which device is being detected.

    You can perform some simple tests and look for obvious issues by using this approach. You don't need a diagnostics program to do the testing, although it can help isolate specific problems. The computer has a BIOS level self-test routine it runs at every power-on start-up; the POST (Power On Self Test) is your first indication that a new overclock configuration might work. One approach is to start with the default speed value and increase it in jumps of 5% or 10%. When you reach a value where the system is obviously unstable, reduce the setting until you identify a stable speed. Then test everything you can to verify the system is truly stable for applications, transferring data to and from drives, and that video and peripheral devices all work.

Since communication between the CPU and the system memory is critical to your computer operating correctly, a basic memory test utility like MemTest86 is useful. You can download a copy of this free utility from www.memtest86.com; this memory-only diagnostic is very useful for identifying defective memory or just intermittent timing-related issues in a system. You may be able to find a number of benchmarks and diagnostic utilities on the Internet, and a good place to start looking is C|net's Download.com. Some programs that are favored by overclockers include:

  • SuperPi - A utility to calculate Pi to a variety of decimal places which also reports the time to calculate the result. SuperPi can be used as a simple benchmark and test CPU and memory stability in the process.
  • 3DMark - A graphical benchmark program from Futuremark. Note: this requires a 3D capable video card and uses Microsoft DirectX to perform all tests)
  • Prime95 - A program is used to find Mersenne Prime numbers, but serves to act as a sort of stress test of the system.
Many other stress, benchmark and diagnostic programs can be found, some free, some trial versions, and some only available for purchase. You will have to determine what suits your needs best. Some overclock enthusiasts will recommend playing Doom, Quake, or other resource-demanding game for several hours to test system stability.

Real-world examples
The most direct way to overclock a CPU is to increase the system clock speed from which it operates. With a normal factory-setting startup, the default value is usually set the first time the system starts up with a new or different CPU installed. On older system boards, it may be a physical setting made by jumpers on the system board itself, although most of the current systems handle this through software settings in BIOS Setup.

Scenario 1
MachSpeed PM800 system board, Intel Pentium D 830 (3.0GHz), 1GB dual-channel DDR 400 memory.

This initial POST screen shows that the system board detects the CPU as 3.06GHz and memory as DDR400. Notice the speed is calculated from a 133MHz clock with a multiplier of 23. Finish installing hardware before changing system settings. If you want to run benchmarks and other high-end stress tests or diagnostic programs you will probably need your operating system and all drivers installed as well. It is recommended to do this at the default  settings rather than later, although any instability in the system will probably trash a typical OS install.

After a successful POST, press the Delete key to enter Setup. In the "Miscellaneous" menu, there is a setting for "Host Clock at Next Boot." This BIOS only allows the user a narrow range from between 133 and 166MHz.

Just to see what happens, I entered the maximum (166) and restarted. At which point, the system locked up, forcing me to power off the power supply and set the jumper or pull the CMOS battery to reset to factory defaults. Some trial and error efforts are necessary just to find a value where the system will even POST.
Eventually, a bus speed of 145MHz is found to be stable for POST and testing under MemTest86. Note that the RAM speed is now reported as DDR436 and the CPU is reported running at 3.33 GHz. In this system board, changing the CPU clock also directly impacted memory and probably the PCI bus. This means that instability in video, and other I/O is a definite possibility as well as intermittent memory problems. If you also compare the PC Health values displayed at the bottom, you will notice the CPU is running about 2 deg. C warmer at idle, just starting up.

Scenario 2
Asus PP5WD2 system board, Intel Pentium D 805 (2.66GHz), 1GB dual-channel DDR2 800 memory.

The Asus system board has many settings that are used for overclocking the CPU and the rest of the system. CPU, memory, and PCI bus speeds can be individually adjusted, and CPU multipliers can also be changed if the processor is an "unlocked" extreme edition version. In this image, you can see the overclock profile selections found in the "JumperFree" menu setup. The Intel version of the Asus Platinum board had profiles from 5-30% faster, while the AMD version went to 10%.

To achieve overclocking results greater than 30%, manual settings must be used.  (Yes, it is possible with some CPUs and providing you have adequate power and cooling.)

In this example, I managed to push an Intel Pentium 4 805 CPU from 2.66 GHz to 4 GHz, although I had to nudge the CPU core voltage up a bit for it to remain stable during testing.

Symptoms of unstable overclocking

When the change in clock speed affects the memory and expansion bus clock speed, you can get problems as shown to the right. The system did POST without error, but the video card is obviously not operating correctly at the new bus frequency; the result is scrambled video memory and an unreadable display.

The Asus board will recover from most overclocking experiments and revert to default settings after an unsuccessful POST. The BIOS displays a message indicating that the previous settings failed.

If you manage to get past the POST and actually start running Windows or another OS, you can still get intermittent errors that cause the system to "crash" with the infamous "Blue Screen Of Death." If the system actually halts and displays the cause, you will probably see something to do with memory or I/O operations. This particular error occurred when testing the 2.66 GHz processor at 4.0 GHz before adjusting the core voltage.

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