How Modern Overclocking Works

Question

Forgive me if I misunderstand some basic EE principles -- I'm a CS guy. After some googling, nobody really explains how the chip runs faster. I understand voltage must increase per this related article.

Do we actually increase the frequency at which the chip operates?

A CPU runs at a certain frequency, which is programmed into it's registers. This frequency can be modified at any time to account for drifting, which is used to account for distance between the crystal and the CPU. (This is going from memory from years ago -- chances are every assumption here is inaccurate.)

So, the original frequency is defined by the crystal which, by the nature of a crystal, oscillates at a static frequency band. This in turn we program into the CPU, which will go on to do X number of calculations per millisecond.

At this point, overclockers manipulate the drift that is programmed into the CPU? My gut tells me that the drift registers can't increase the frequency enough to be relevant. So where does the increase in frequency come from?

Something that has just occurred to me is that just by applying more voltage, the 'bits' actually move around a faster..but then that wouldn't mean an increase in frequency, right?

Answer 1

Do we actually increase the frequency at which the chip operates?

Yes, we do!

Modern CPUs have a unit called PLL - Phase Locked Loop - which make the multiple GHz out of relatively cheap crystals running at something like 33.3 or 100 MHz. These units are programmable in a wide range of output frequencies. That is used to slow down the core(s) when there is less work to do to save power - or to overclock them.

You can increase the clock frequency further when the voltage is higher - but at the price of massive additional generated heat. And the silicon will "wear out" faster, as bad things like Electromigration will increase too.

Answer 2

Direct answer:

Yes we increase the frequency of not only the CPU but also Bus, RAMs and Graphics chips.

Elaboration:

As TurboJ mentioned, the unit inside the CPU that is responsible for frequency multiplication is the PLL. You supply the CPU with a reference/base/bus clock frequency that is then internally multiplied by a value that is automatically or manually selected.

As a practical example, I use the famous program "CPU-Z" to show the detailed specs of my CPU:

I applied no load to the CPU, I only opened CPU-Z. Notice the "Multiplier" is now x8, which is multiplied by the base clock "Bus Speed" to generate the core frequency ~ 798MHz. Also take care that the Core voltage is 0.725V in this case.

Next, I applied stress tests on the CPU to show you its behavior under load:

Notice the multiplier jumped automatically to x28, increasing the frequency to ~ 2.8GHz. Also the Core voltage is now 1.131V which increases the power dissipation and thus the operating temperature of the CPU. (It reached 90C in this test!)

This is done automatically because intel wants to reduce the power consumption of its CPUs when no load is applied, and it really helps save the battery life of my laptop.

As an overclocker, you have two ways to increase the frequency of the CPU:

1. Increasing the multiplier manually from the BIOS or specialized software. For most commercial desktop CPUs, this multiplier has a maximum value that you cannot exceed. (Except for Intel Extreme CPUs and AMD Black Edition ones, which have an open multiplier)

2. Increasing the reference/base/bus frequency.