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Thursday, January 29, 2009- Overclocking – Theory
Overclocking depends on several factors. Cooling, voltage, architecture, and hardware limits all determine how much a device can overclock.
What is a ‘clock cycle’?
A ‘clock cycle’ is the time it takes for an instruction to be processed by a device. These devices are measured in Hz (e.g. MHz or GHz) which describe how many clock cycles are performed per second. For example, a 3GHz processor performs 3,000,000,000 clock cycles per second.
Intel CPUs
Intel CPUs are overclocked via the FSB (Front Side Bus) and CPU multiplier. The FSB controls the communication between the CPU, RAM and PCI busses. There are two types of FSB; one determined by the CPU called the ‘Rated FSB’ and the other is the ‘Bus Speed’. ‘Bus Speed’ is what is used to calculate clock speed. When you hear/read people referring to FSB, it is most likely referring to the Bus Speed. ‘Rated FSB’ (or ‘effective FSB’) is a little more confusing. Rated FSB is the name given to the result of the Bus Speed when a CPU uses double or quad pumping. Double pumping is double the Bus Speed and Quad Pumping is four times the Bus Speed.
IMPORTANT : Overclocking PCI busses is not recommended as it can easily cause system instability. Most motherboards lock the PCI bus so it cannot be altered.
The ‘multiplier‘ is a number which is multiplied by the Bus Speed to determine the CPU clock speed. The multiplier is often locked, meaning it cannot be changed. Sometimes the multiplier is partly locked, which only allows a smaller number, or completely unlocked (quite rare these days).
Let’s examine a practical example:
Let’s say the Bus Speed of a computer is 300MHz and the CPU multiplier is 11x. To determine the clock speed that the CPU is running at, we multiply the FSB by the multiplier.
11 (Multiplier) x 300 (Bus Speed) = 3300MHz (3.3Ghz)
However, Intel CPUs from Pentium 4 upwards are ‘quad pumped’. This means that the 300MHz Bus Speed is effectively running at 1200MHz. This, however, does not affect overclocking. When adjusting values we are only adjusting the Bus Speed.
You’re probably thinking right now, “Why do I need to know about the Rated FSB if I never use it?” Truth be told, you don’t. It does help, however, when looking at Intel’s specifications for their CPUs. Intel often advertises their CPU FSB speeds in Rated FSB instead of Bus Speed. So knowing how to find the Bus Speed from this data can help.
AMD CPUs
Although the technology used in AMD CPUs differ from Intel processors, the technique for overclocking is still very similar. The main difference is that there is another factor to consider called the LDT Bus. This separate bus allows communication between the CPU and the motherboard chipset.
The Technique Behind HyperTransport
HyperTransport can be considered as AMD’s version of the FSB. All AMD CPUs with HyperTransport are treated as ‘double pumped’. With Intel CPUs, the FSB is on the motherboard, but with AMD CPUs, the FSB is on the actual CPU. The Bus Speed is called the ‘HT Bus’ when working with AMD processors. There are two busses in an AMD CPU, the LDT Bus (Lightning Data Transport) which communicates with the motherboard chipset, and the HT Bus which communicates with the RAM and determines the clock speed of the CPU. The LDT Bus is sometimes known as the ‘HT Link’. The LDT Bus Speed is derived from the HT Bus and ‘HT Multiplier’. Confusing? Here are some examples:
Let’s say the HT Bus is 200MHz, the multiplier is 11x, and the HT multiplier is 5x for an AMD CPU.
To find the LDT Bus frequency we find the product of the HT multiplier and the HT Bus:
200 (HT Bus) x 5 (HT Multiplier) = 1000MHz (LDT Bus)
To find the clock speed of the CPU we find the product of the HT Bus and the CPU multiplier:
200 (HT Bus) x 11 (CPU Multiplier) = 2200MHz (2.2GHz)
As you can see, the method is similar to Intel CPUs except with the addition of the LDT Bus.
The Technique Behind the LDT Bus
The trick with the LDT Bus is to keep it as close to stock as possible. Most CPUs come with a stock HT Multiplier of 5x when the HT Bus is 200MHz (200 x 5 = 1000MHz). Essentially, you should try and keep the LDT around 1GHz as most motherboards do not allow too much overclocking of this bus. When overclocking, the HT Multiplier may have to be lowered to 4x so that the HT Bus can be raised without making the LDT Bus unstable. 1100MHz is the maximum frequency the LDT Bus should reach before dropping the HT multiplier (this can vary depending of the model of motherboard). Because the LDT Bus communicates with other busses such as PCI, SATA, USB, it is essential that they are locked to prevent instability. Most modern motherboards lock these busses automatically.
Let’s say you want to get you HT Bus to 250MHz so your CPU with 11x multiplier can reach 2.75GHz. If the HT Multiplier is set to 5x, the result will be the following:
CPU clock speed: 250 (HT Bus) x 11 (CPU Multiplier) = 2750MHz (2.75GHz) LDT Bus: 250 (HT Bus) x 5 (HT Multiplier) = 1250MHz (1.25GHz)
Notice how the LDT Bus is set too high. To fix this, just lower the HT Multiplier:
LDT Bus: 250 (HT Bus) x 4 (HT Multiplier) = 1000MHz (1GHz)
This result is much better. If you are overclocking and over the limit for the LDT Bus, yet lowering the HT Multiplier results in a frequency lower than stock, you have two options. Either do a stress test (explained later in the tutorial) with the higher HT Multiplier and check if it’s stable, or lower the HT Multiplier anyway. Underclocking the LDT Bus is not as bad as it sounds, as long as it stays close to 1GHz. Further HT Bus overclocking will bump the LDT up anyway.
For example, let’s say the same CPU is using a HT Bus of 225MHz instead:
LDT Bus: 225 (HT Bus) x 5 (HT Multiplier) = 1125MHz (1.125GHz)
This is just above the LDT Bus recommended limit, so you can either stress test it to check stability, or lower the HT multiplier:
LDT Bus: 225 (HT Bus) x 4 (HT Multiplier) = 900MHz
900MHz is still efficient enough for the CPU to run without a bottleneck, but if stable, the 5x HT Multiplier is recommended. How to determine stability will be discussed later.
`updated on- 8:19 AM
