Introduction To Overclocking
Another form of processor upgrade is to set the processor speed to run faster than the rating on the chip; this is called overclocking. In many cases, you can get away with a certain amount of overclocking, because Intel, AMD, and others often build safety margins into their ratings. So, a chip rated for, say, 3GHz might in fact run at 3.5GHz or more but instead be down-rated to allow for a greater margin of reliability. By overclocking, you are using this margin and running the chip closer to its true maximum speed. I don’t normally recommend overclocking for a novice, but if you are comfortable playing with your system settings, and you can afford and are capable of dealing with potential consequences, overclocking might enable you to get another 10%–20% or more performance from your system. Overclocking is the process of running a processor at a faster speed then what it was set to run at by the manufacturer. Any processor can be overclocked, and the concepts behind overclocking is to increase system performance at very little cost.
How Is Overclocking Accomplished?
In many ways, overclocking can be done by selecting or deselecting a few jumpers on the motherboard but newer motherboard designs have overclocking settings configurable via the system's Basic Input Output System (BIOS), or through software that can run through a Windows operating system.
Overclocking is usually applied to the processor, but it can also be applied to other components in the system, including memory, video cards, bus speeds, and more.
When chips run faster, they run hotter, so cooling upgrades and modifications usually go hand-in-hand with overclocking. Systems that run cool tend to be more stable and more reliable, so even if you don’t overclock your system, ensuring that it runs cool is essential for trouble-free operation. Many systems are not properly designed or configured for optimal cooling even at their standard speeds, much less when overclocked.
Overclocking PCs dates all the way back to the original 4.77MHz IBM PC and 6MHz AT systems of the early 1980s. In fact, IBM made overclocking the AT easy because the quartz crystal that controlled the speed of the processor was socketed. You could obtain a faster replacement crystal for about a dollar and easily plug it in. The first several editions of this book covered how to perform this modification in detail, resulting in a system that was up to 1.5 times faster than it started out. Modern systems allow overclocking without replacing any parts by virtue of programmable timer chips and simple and easy-to-change BIOS Setup options. Some processors, such as Intel Extreme Edition and AMD Black Edition processors, are especially suited to overclocking because they feature unlocked core multipliers. However, some overclocking is possible with almost any processor.
Overclocking And Warranties
It's also important to note that overclocking voids any warranty you have with your CPU's manufacturer. An incorrect configuration could leave your processor inoperable.
Over Heating Issues
Overheating issues can cause permanent damage the processors so its imperative that appropriate cooling is provided within the case and above the processor within the case. Most power users overclocking their processors use some type of liquid cooling system or other high end, more expensive cooling devices to prevent overheating issues.
When overclocking a processor heat becomes a considerable factor. An appropriate CPU fan must be installed and continously monitored to insure the core temperature of the system remains at an appropriate level. Additionally, a heat sink must also be installed to allow for the proper ventilation of the system. Multiple case fans are also recommended to help dissipate heat within the system. A heatsink is a device that absorbs heat, and is typically attached to the top of a CPU. On newer motherboards a heatsink is generally already soldered onboard, and adding an additional fan to this heatsink can be a simple task provide you have the appropriate parts and tools to complete the job.
To understand overclocking, you need to know how computer system speeds are controlled. The main component controlling speed in a computer is a quartz crystal. Quartz is silicon dioxide (SiO2) in crystalline form. Oxygen and silicon are the most common elements on earth (sand and rock are mostly silicon dioxide), and computer chips are made mainly from silicon. Quartz is a hard, transparent material with a density of 2649 kg/m3 (1.531 oz/in3) and a melting point of 1750°C (3,182°F). Quartz is brittle but with a little bit of elasticity, which is perhaps its most useful attribute.
In crystalline form, quartz can generate regular and consistent signal pulses to regulate electronic circuits, similar to the way a metronome can regulate music. Quartz crystals are used because they are piezoelectric, which is defined as having a property that generates voltage when subjected to mechanical stress. The opposite is also true—that is, quartz generates mechanical stress or movement when subjected to a voltage. Piezoelectricity was discovered by Pierre and Jacques Curie in 1880, and it is the essential feature of quartz that makes it useful in electronic circuits.
Piezoelectricity works two ways, meaning that if a voltage is generated when you bend a crystal or apply voltage to a crystal, it bends (contracts, expands, or twists) in a similar fashion. Although the crystal is mostly brittle in nature, it is still somewhat elastic, such that any deformation tends to snap back and then occur again, resonating or vibrating at a natural frequency as long as the voltage is present. Much like a tuning fork or the pipes in an organ, the natural resonant frequency depends on the size and shape of the crystal. In general, the smaller and thinner it is, the faster it vibrates.
The actual movement is exceedingly small, on the order of 68 nanometers (billionths of a meter) per centimeter, which in a normal crystal is only a few atoms in length. Although the movement is small, it is also quite rapid, which means tremendous forces can be generated. For example, the surface acceleration of a 50MHz crystal can exceed five million times the force of gravity.
Crystal resonators are made from slabs of quartz sawed from raw quartz crystal stock. The raw stock slabs are cut into squares, rounded, and ground into flat discs called blanks. The thinner the disc, the higher the resonant frequency; however, there are limits as to how thin the discs can be made before they break. The upper limit for fundamental mode resonators is approximately 50MHz. At that frequency, the discs are paper thin and are generally too fragile to withstand further grinding. Still, higher-frequency crystals can be achieved by using harmonics of the fundamental frequency, resulting in crystals of up to 200MHz or more. Even higher frequencies can be achieved by using frequency synthesizer circuits, which use a base crystal-generated frequency fed to a circuit that then generates multiples of frequency that can extend well into the gigahertz or terahertz range. In fact, crystal-based frequency synthesizer circuits generate the high operating speeds of modern PCs.
The crystal packages, as well as the shape of the actual quartz crystals inside the packages, can vary. The packages are usually a metal can that is either cylindrical or oblong in shape, but they can also have other shapes or be constructed of plastic or other materials (see Figure 3.40).
Figure 1.1 - Crystal packages of varying shapes.
The sliver of quartz inside the package is normally disc shaped, but it is shaped like a tuning fork in some examples. Figure 3.41 shows a cylindrical crystal package with the cover removed, exposing the tuning fork–shaped sliver of quartz inside.
Figure 1.2 - Crystal interior showing the quartz tuning fork.
Most crystals use a disc-shaped sliver of quartz as a resonator. The disc is contained in a hermetically sealed evacuated enclosure. Figure 3.42 shows the interior view of a typical crystal with a disc-shaped resonator inside. The quartz disc inside has electrodes on each side, allowing voltage to be applied to the disc. The details are shown in Figure 3.43.
Figure 1.3 - Figure showing the disc-shaped resonator.
Figure 1.4 - Disc-shaped quartz resonator details.
Walter G. Cady was the first to use a quartz crystal to control an electronic oscillator circuit in 1921. He published his results in 1922, which led to the development of the first crystal-controlled clock by Warren A. Marrison in 1927. Today, all modern computers have multiple internal oscillators and clocks, some for controlling bus and processor speeds and at least one for a standard time-of-day clock.
Modern PC Clocks
A typical PC has at least two crystals on the motherboard; the main crystal controls the speed of the motherboard and motherboard circuitry, and the other controls the real-time clock (RTC). The main crystal is always 14.31818MHz (it might be abbreviated as 14.318 or just 14.3), and the RTC crystal is always 32.768KHz.
The original 1981 vintage IBM PC ran at 4.77MHz, a speed derived by taking a 14.31818MHz crystal and using a divider circuit to divide the frequency by 3 to get 4.77MHz. Many people were confused as to why IBM chose to run the processor at 4.77MHz; after all, the 8088 processor they used was rated for 5MHz, and all they would have had to do to run it at that speed was change the main crystal from 14.318MHz to 15MHz instead. Well, the truth is that if they did that, they would have had to add more crystals to the design. You see, the same 14.318MHz crystal that was divided by 3 to run the processor was also divided by 4 to get 3.58MHz, which is the exact frequency needed for the NTSC color video modulation signal to be compatible with color TV.
But that’s not all: Another circuit divided the crystal frequency by 12 to get 1.193182MHz, which was used by an 8253 programmable three-channel 16-bit interval timer/counter chip. Each channel could be used to take an input clock signal and produce an output signal by dividing by an arbitrary 16-bit number. Channel 0 was used to make the time-of-day clock ticks. It was programmed by the BIOS to call INT 08h every 65,536 ticks, which was about 18.2 times per second (or about every 55 milliseconds). The software routines linked to INT 08h caused the time-of-day clock to be updated and could chain to any other activities that needed to be done periodically. Channel 1 was used to tell the DMA to refresh the dynamic RAM every 72 cycles (about 15 microseconds), and channel 2 was used to make an audio signal for the speaker (beeps)—different tones could be made by changing the divisor.
So by carefully choosing a 14.318MHz crystal instead of 15MHz or some other speed, the IBM engineers were able to design a motherboard in which a single crystal could run the processor, video card, time-of-day clock, memory refresh, and even beep tones. The single-crystal design allowed the motherboard to be manufactured with fewer parts and at a lower cost.
As a testament to their foresight, all modern PCs are still controlled by a 14.318MHz crystal! This crystal, in conjunction with a frequency timing generator chip, derives virtually all the frequencies used on a modern motherboard by the CPU, buses, memory, and more.
PCs don’t run at 14.318MHz, so how can a crystal of that speed be used? And what happens when you install a different processor? How does the system adjust the bus and other speeds to accommodate the new chip? The answer is that a special chip called a frequency timing generator (FTG) or frequency synthesizer is used in conjunction with the crystal to derive the actual speeds of the system. Figure 1.5 shows a portion of a motherboard with an FTG chip and a 14.318MHz crystal below it.
Figure 1.5 - An ICS 9250 frequency timing generator chip with a 14.318MHz crystal.
The RTC in the original PC was notoriously inaccurate, so starting with the IBM AT in 1984, IBM added a separate 32.768KHz crystal to count time independent from the speed of the system. This crystal is used on all modern motherboards as well. Figure 3.45 shows a 32.768KHz crystal next to a chipset South Bridge or I/O controller hub, which contains the RTC circuitry and CMOS RAM.
Figure 1.6 - Chipset South Bridge (I/O controller hub) incorporating an RTC, along with the 32.768KHz clock crystal.
Most frequency synthesizer chips used on PC motherboards are made by a handful of companies, including Integrated Device Technology (www.idt.com; formerly Integrated Circuit Systems) and Cypress Semiconductor (www.cypress.com; formerly International Microcircuits Inc. [IMI]). These chips use phased locked loop (PLL) circuitry to generate synchronized processor, PCI, AGP, and other bus timing signals that are derived from a single 14.318MHz crystal. The crystal and frequency synthesizer chip are usually situated near the processor and main chipset component of the motherboard.
The amazing thing about these chips is that most of them are programmable and adjustable, so they can change their frequency outputs via software, which results in the system running at different speeds. Because all CPUs are based on the speed of the CPU bus, when you change the CPU bus speed generated by the frequency synthesizer chip, you can change the speed of your processor. Because the PCI, AGP, and memory buses are often synchronized with the speed of the processor bus, when you change the processor bus speed by a given percentage, you also change the speed of those other buses by the same percentage. The software to accomplish this is built into the BIOS Setup menus of most modern motherboards.
Most modern motherboards automatically read the CPU and memory components to determine their proper speed, timing, and voltage settings. Originally, these settings were controlled by jumpers and switches, but in most modern boards you can enter the BIOS Setup to change these settings to manual and then use the menu options in the BIOS Setup to alter the speed of the system. Because such alterations can make the system unstable, most systems are designed to boot into the BIOS Setup at a default low speed so you are not locked out from making changes in the future. This makes overclocking as simple as changing a few menu options and then rebooting to test the selections you’ve made.
The concept for overclocking is simple: You change the settings to increase the speed of the processor, memory, buses, and so on, until the system becomes unstable. Then you can go back in and reduce the settings until the system is stable again. In this manner, you find the maximum sustainable speed for a system. Because each processor is different, even ones with the same ratings can end up allowing different maximum stable speeds.
Why can some chips be clocked faster than others? The reason is in how they are manufactured and marked. As an example, the first Pentium 4 chips based on the Prescott core used die that were 112 square mm on 300mm wafers, resulting in a maximum of 529 full die per wafer. Due to defects, many of those die wouldn’t work, but let’s say that 423 (about an 80% yield) were good. Intel initially sold the Prescott core processors at speeds from 2.4GHz through 3.4GHz, which meant that all the die on each wafer were designed to potentially run at the highest rated speed. However, out of the good (working) die, testing would show that although some of those would indeed run at the highest 3.4GHz rated speed, others would work reliably only at lower speeds. The finished chips would have been sorted into bins according to their speed test results.
Early in manufacturing a given processor design, the sorted bins of chips at the end of the line would contain more that passed only the lower speed tests, and fewer that ran at the highest speeds. This is why the fastest chips are the most expensive—generally fewer of the chips produced on a given day will pass the testing at that speed. Eventually, however, as the manufacturing processes and chip design are tweaked, more and more of the finished chips end up passing the higher-speed tests. But because lower-speed chips are priced less and sell more, the manufacturer might have to dip into the faster bins and mark those chips at the lower speeds to fill the larger number of orders.
Essentially what I’m saying is that chipmakers such as Intel and AMD make all the chips on a wafer identically and try to make them so they will all run at the highest speeds. If you purchase one of the lesser-rated chips, you really have the same chip (die) as the higher-rated versions; the difference is the higher-rated ones are guaranteed to run at the higher speeds, whereas the lower-rated ones are not. That is where overclockers come in.
The current speed of a processor might not be its actual rated speed, either because of overclocking or because some recent systems reduce processor speed when the system is not heavily tasked. Both Intel and AMD have developed software tools that can properly identify the rated speed of a processor.
For newer Intel processors, use the Intel Processor Identification Utility; for older chips, use the Intel Processor Frequency ID Utility. Both of these are available fromwww.intel.com/support/processors/sb/CS-015477.htm.
For AMD processors, use either the AMD CPU Info program or the AMD Clock program. To find these, visithttp://support.amd.com, select Drivers and Downloads, All Processors. Then search for CPU info andAMD Clock.
One drawback of the Intel and AMD programs is that they only work on their respective brands of chips. Another excellent utility that works on both Intel and AMD processors is the CPU-Z program available from www.cpuid.com. I routinely install this program on any systems I build or service because it provides universal processor (and chipset) identification.
Users who overclock their systems purchase chips rated at lower speeds and essentially do their own testing to see if they will run at higher speeds. They can also start with the highest-rated chips and see whether they can run them even faster, but success there is much more limited. The most successful overclocking is almost always with the lowest-rated speed of a given design, and those chips are also sold for the lowest price. In other words, statistically you might be able to find many of the lowest-speed grade chips that are capable of running at the highest-speed grade (because they are essentially identical during manufacture); however, if you start with the highest-speed grade, you might be able to increase the speed only a small percentage.
Just remember that a difference exists between the rated speed of a chip and the actual maximum speed at which it runs. Manufacturers such as Intel and AMD have to be conservative when they rate chips, so a chip of a given rating is almost always capable of running at least some margin of speed faster than the rating—the question is, how much faster? Unfortunately, the only way to know that is by trying it out—that is, by testing chips individually.
A variation of overclocking is the unlocking of disabled cores on AMD Phenom II and Athlon II processors for better performance in single-threaded and multithreaded applications and when multitasking. As Table 3.24indicates, many of AMD’s X3 and X2 K10-based processors are based on X4 designs that have one or two cores disabled.
If you unlock these cores using a method such as enabling the Advanced Clock Calibration (ACC) feature in the system BIOS (see http://www.tomshardware.com/reviews/unlock-phenom-ii,2273-5.html for details), one of the following results can take place:
The unlocked core may function perfectly. This is the result if a core were disabled strictly to enable the chip to be sold as an X3 rather than an X4.
Your system boots and runs normally, but the ‘unlocked’ core can’t be detected or used. A core disabled because of moderate problems would result in this problem.
Your system might not boot or might not be able to run Windows until you reset the ACC setting in your system BIOS to its default mode. More serious core stability problems would cause this result.
You could destroy your processor or motherboard. This would be the result if the core were disabled because it contained a short.
To be able to try unlocking disabled cores, you need a motherboard that has an adjustable ACC setting in the system BIOS (many motherboards using the AMD750 South Bridge [SB] feature this option) and a motherboard BIOS that does not include AMD-provided microcode to prevent unlocking via the ACC adjustment routine.
Although the AMD850 South Bridge used on the latest motherboards does not include an ACC option, motherboard vendors such as ASUS and Gigabyte have added an ACC chip to their motherboards and added core unlocking capabilities to their BIOS setups. MSI also offers unlocking, but is using a purely BIOS-based unlock routine for its motherboards with the SB850 processor.
If you do unlock an additional core or two, you might enjoy faster performance and better multithreaded and multitasking operating for free, or you might find your system to be unstable or unable to start. Before you assume that a system that runs with an unlocked core is truly stable, use some of the tests I recommend inChapter 20, “PC Diagnostics, Testing, and Maintenance.”
Bus Speeds and Multipliers
Modern processors run at a multiple of the motherboard speed, and the selected multiple is usually locked within the processor; therefore, all you can do to change speeds is change the processor bus speed settings. The processor bus is also called the CPU bus, FSB, or processor side bus (PSB), all of which are interchangeable terms.
For example, I built a system that uses an Intel Pentium 4 3.2E processor, which typically runs at 3,200MHz on an 800MHz CPU bus. Thus, the processor is locked to run at four times the speed of the CPU bus. I was able to increase the CPU bus speed from 800MHz to 832MHz, which meant the processor speed increased from 3,200MHz to 3,328MHz, which is 128MHz faster than the rating. This took all of about 60 seconds to reboot, enter the BIOS Setup, make the changes in the menu, save, and reboot again. This was only a 4% increase in overall speed, but it didn’t cost a penny, and testing proved that the system was just as stable as it was before.
Many motherboards allow changes in speed of up to 50% or more, but a processor rarely sustains speeds that far above its rating without locking up or crashing. Also note that, by increasing the speed of the processor bus, you may also be increasing the speed of the memory bus, PCI bus, or PCI Express (or AGP) bus by the same percentage. Therefore, if your memory is unstable at the higher speed, the system will still crash, even though the processor might have been capable of sustaining it. The lowest common denominator prevails, which means your system will run faster only if all the components are up to the challenge.
If you are intent on overclocking, there are several issues to consider. One is that most processors sold since 1998 are multiplier-locked before they are shipped out. Processors that are locked ignore any changes to the multiplier setting on the motherboard. Although originally done to prevent re-markers from fraudulently relabeling processors (creating “counterfeit” chips), multiplier locking has impacted the computing performance enthusiast, leaving tweaking the motherboard bus speed as the only easy way (or in some cases, the only way possible) to achieve a clock speed higher than standard.
Intel’s K-series Core i7 and i5 processors have unlocked clock multipliers, as do AMD’s Black Edition Phenom II. Phenom, and Athlon X2 processors. Choose these processors along with a motherboard that offers adjustable clock settings in its BIOS for easy overclocking.
You can run into problems increasing motherboard bus speed as well. Most older Intel motherboards, for example, simply don’t support clock speeds other than the standard settings. Some newer enthusiast-oriented Intel boards have “burn-in” or “override” features that allow you to increase the default processor bus speed (and the speed of the processor core), voltages, and multiplier (for unlocked CPUs). Most other brands of motherboards also allow changing the bus speeds. Note that small incremental changes in clock multiplier speeds, rather than large jumps, are the best way to coax a bit more performance out of a particular processor. This is because a given chip is generally overclockable by a certain percentage. The smaller the steps you can take when increasing speed, the more likely that you’ll be able to come close to the actual maximum speed of the chip without going over that amount and causing system instability.
For example, say you have a Socket 775 motherboard running a 2.4GHz Core 2 Quad processor at a CPU FSB speed of 1,066MHz. The motherboard permits 1MHz adjustments of the CPU bus clock speed (which is multiplied by 4 to obtain the FSB) to enable you to fine-tune your processor speed. The base clock frequency is 266MHz and is multiplied by 4 to obtain the motherboard bus (FSB) speed, which is then further increased by the CPU multiplier:
800MHz × 3.5 = 2,800MHz, or 2.8GHz
By adjusting the CPU clock frequency, you can change the FSB and CPU core clock speeds as shown in Table 1.1 - Core Clock, FSB, and CPU Speed Relationships.
Table 1.1 - Core Clock, FSB, and CPU Speed Relationships
|Base Clock Frequency
||Bus Multiplier (Fixed)
||Resulting FSB Speed
||CPU Core Multiplier (Locked)
||Resulting Processor Speed
As you can see in this example, by increasing the base clock from 266MHz to 300MHz, you increase the FSB from 1,066MHz to 1,200MHz, and the CPU core speed from 2.4GHz to 2.7GHz, nearly a 13% increase. Typically, increases on the order of 10%–20% are successful. You might be able to achieve more if your system offers excellent cooling and you can also adjust CPU multiplier, voltage, and other settings.
Overclocking Sand Bridge Processors
The Sandy Bridge Core i-series processors from Intel have made drastic changes in how overclocking works. The clock generator is incorporated into the 6-series chipsets that support Sandy Bridge processors, so that you can no longer independently adjust the speeds of buses such as PCI Express or DMI. The BCLK frequency is also locked at 100MHz (it was 133MHz with adjustments up or down in the Nehalem Core i-series processors).
If you want to have maximum overclock potential for a Core i-series Sandy Bridge processor, look for model numbers ending in K (for example, Core i7-2600K) and choose a motherboard with a chipset designed for overclocking, such as the P67 or Z68.
Core i7 and i5 Sandy Bridge processors without the K suffix allow limited overclocking (“limited unlocking”) up to four speed ranges (bins) above the normal turbo frequency (maximum clock speed). For example, a processor with a turbo frequency of 3.7GHz could be overclocked to 4.1GHz with one core running, 4.0GHz with two cores running, and so on. Again, you need an overclock-friendly chipset such as the P67 or Z68 to make this happen.
Core i3 Sandy Bridge chips don’t include Turbo Boost and thus don’t support overclocking. Consequently, if you want to overclock Sandy Bridge processors, your choice of processor and chipset is more important than ever before.
CPU Voltage Settings
Another trick overclockers use is playing with the voltage settings for the CPU. All modern CPU sockets and slots have automatic voltage detection. With this detection, the system determines and sets the correct voltage by reading certain pins on the processor. Some motherboards do not allow manual changes to these settings. Other motherboards allow you to tweak the voltage settings up or down by fractions of a volt. Some experimenters have found that by either increasing or decreasing voltage slightly from the standard, a higher speed of overclock can be achieved with the system remaining stable. Some motherboards allow adjusting the voltage settings for the FSB, chipset, and memory components, allowing for even more control in overclocking situations.
My recommendation is to be careful when playing with voltages because you can damage the processor or other components in this manner. Even without changing voltage, overclocking with an adjustable bus speed motherboard is easy and fairly rewarding. I do recommend you make sure you are using a high-quality board, good memory, and especially a good system chassis with additional cooling fans and a heavy-duty power supply. See Chapter 18 for more information on upgrading power supplies and chassis. Especially when you are overclocking, it is essential that the system components and the CPU remain properly cooled. Going a little bit overboard on the processor heatsink and adding extra cooling fans to the case never hurts and in many cases helps a great deal when hotrodding a system in this manner.