Understanding CPU frequency
Before we go into how we overclock these CPU’s let us look at what determines how fast your CPU will run. The following simple equation determines the clock speed of the CPU’s cores:
[highlight color=”yellow”]CPU Frequency = Base Clocks x Multiplier[/highlight]
This is a biggest change from the old LGA 775 where FSB and multiplier determined the CPU speed. The base clock is similar to the FSB but also has some key differences. The base clock, also commonly spelled bclocks or bclk in forms, is the foundation around all the other frequencies discussed below.
The CPU speed of the new generation is not the only factor that determines how fast your PC will run, we have a few more definitions such as:
[tabs tab1=”QPI Frequency” tab2=”Uncore frequency” tab3=”Multiplier and Turbo“]
[tab]QPI Frequency – QPI or Quick Path interconnect is the Intel communication path upgrade from the older chipset and front side bus (FSB) communication path, so instead of the CPU communicating with the memory via the LGA 775 Northbridge, there is now a direct link (QPI) that increases efficiency. QPI speeds are a function of base clocks, so as you increase your base clock your QPI speed will also increase, yielding an increase in not only communications speeds but also bandwidth, which leads to an increase in PC performance.[/tab]
[tab]Uncore frequency – This sets the frequency of the on-die memory controller and the L3 cache. Like CPU clock speed, dram speed, and QPI frequency, uncore is a multiple of Bclk. Uncore can be set independently of those other frequencies, subject to certain stability limitations. The uncore must be at least 2:1 of the DRAM speed otherwise you will not get a stable overclock, in fact your PC will not even boot if the ratio is not honored. Increasing the uncore:dram ratio above 2:1 yields significant performance gains. However, when the ratio reaches 3:1 it is not possible to maintain full stability.[/tab]
[tab]Multiplier and Turbo – As mentioned above, the multiplier is the second factor in how CPU core speed is determined. Now, not all CPU’s have the same multiplier, it is dependent on where the CPU is positioned in the price/performance curve of Intel’s range of CPUs. Most of these come with a Turbo multiplier which is available if you enable the Speedstep option under the CPU settings. Care should be taken when using the turbo as you may not be able to see the resultant frequency in the BIOS. For instance, if your default multiplier of your CPU is 20 (i7-920) and you set your baseclock to 200 and you boot up with turbo enabled, you will leave the bios at 20 x 200 = 4 GHz, as soon as you enter your Operating system your turbo kicks in so you end up with 21 x 200 = 4.2 GHz. Now if you also have C-State enabled, one CPU core will actually have access to a 22 multiplier which enable that core to run at 22 x 200 = 4.4 GHz. You set your voltages expecting to run at 4 GHz and you cannot understand why you crashed X enter Ubuntu, well, that is the reason, so take care when using turbo and C-State and adjust voltage to accommodate for the higher multipliers.[/tab]
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Important Voltages when Overclocking
There a few important voltages which you will need to manipulate while overclocking, below are the main ones. Every motherboard BIOS differ but all of them have the voltages as set out below.
[tabs tab1=”VCore” tab2=”QPI voltage/CPU Vtt” tab3=”VDIMM/DRAM” tab4=”IOH Core” tab5=”ICH Core”]
[tab]V-Core – Directly related to the CPU frequency. As you increase the CPU frequency you would need incrementally increase the v-core as well.[/tab]
[tab]QPI voltage/CPU Vtt – Increase in this voltage is necessary from the default as you increase your RAM speed, tighten the timings or increase QPI frequency. It also helps to stabilize your overclock at higher base clocks.[/tab]
[tab]VDIMM/DRAM – This is directly related to your RAM memory modules and increase will assist in stabilizing increase in Ram speeds. Care should be taken not to increase this voltage more that 0.5 volts above your Vtt as you could cause permanent damage to your CPU.[/tab]
[tab]IOH Core Voltage– This voltage aids when increasing base clocks above say 200. In most cases leaving it at auto works best.[/tab]
[tab]ICH Core Voltage– This voltage feeds the chip that regulates the communication from the peripherals to the CPU via the DMI. It is best to set this at auto.[/tab]
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Now that we have covered all the basics let us jump to what this article is all about…overclocking
Utappia 