I can’t remember a CPU launch being more highly anticipated than Intel’s upcoming Haswell, expected to be released this year. Much has been said about the microarchitecture – if you want to learn more about it, I would highly recommend reading this article at Real World Tech. In it, you will learn everything that has been made public of Haswell, including details on the scheduling engine, instruction set configuration, memory hierarchy, and more, all in great detail.
However, the most important aspect to overclockers is usually the system architecture itself – how the chip is engineered can determine the feasibility of tweaking a CPU to squeeze more performance out of it. It is fully expected that Intel will continue the policy of having an ‘unlocked’ SKU (currently K-Series), giving buyers the ability to overclock their CPU easily by increasing the CPU multiplier. However this has its limits – typically only the most expensive processors have been unlocked, although that has improved since the Nehalem days. Still, currently the only unlocked Ivy Bridge CPUs are the $220 Core i5 3570K and $320 Core i7 3770K. If you buy anything else, you essentially won’t be able to overclock it very much at all, due to the reasons below.
Why Base Clock Overclocking Matters
Starting with the Nehalem microarchitecture, Intel did away with the “Frontside Bus” and introduced the “Base Clock” upon which all components on the processor got its frequency. There were four individual frequencies within the CPU, all based on a main Base Clock speed (BCLK):
- CPU Frequency
- Memory Frequency
- QPI Frequency (interface with the rest of the motherboard)
- Uncore Frequency (everything else on the CPU, such as the L3 cache, and later the PCI-E controller, etc)
Each of these had its own multiplier, and thus you would not have been limited to having to buy a completely unlocked CPU to overclock. You could increase the CPU multiplier to the highest number available, then increase BCLK speed from its stock 133 MHz to increase the main CPU frequency. From there, you would step down the other multipliers to compensate. The Uncore in particular is very sensitive to overclocking, and that only got worse with Sandy Bridge and Ivy Bridge as more components were integrated into it, and one other major change was made:
Matters got worse with Sandy Bridge, as the CPU and Uncore frequencies became tied together completely. This was done for both performance and power consumption – having the L3 and CPU Core tied together meant that they would be able to sleep and wake faster, and Sandy Bridge saw the L3 cache running at a higher frequency.
This spelled doom for overclocking normal CPU SKUs however; by increasing the BLCK by more than a few MHz, the Uncore would become unstable (sometimes to the point where components could be irreparably damaged), thus limiting base clock overclocking completely. Although unlocked processors were made available at reasonable price levels, those who recall the Celeron 300a days were let down by no longer being able to buy an extreme budget CPU and overclocking it to perform like a higher end product. Not that this was feasible for most every day users, but for many it was a hobby, and in fact it is how we got our start back in 1999.
Bad News for Intel Means Good News for Us
Intel immediately discovered a design flaw in tying the L3 and CPU speeds together – if the GPU ever needed to access the L3 cache, the CPU speed would have to wake up as well. With Intel’s focus on power efficiency, this was a huge deal, which is why they will actually go back to individual clock speeds for the Uncore and CPU Core with Haswell.
This is pure speculation on my part, but it looks like the days of overclocking a locked CPU may be upon us again. By being able to manipulate the Uncore frequency separately from the CPU (and GPU) frequencies, there is a good chance that the lower end Haswell chips will become viable overclocking targets for enthusiasts once again. We may never see another 300a, but if we can have a $100 Haswell CPU running at similar frequencies to the $300 part, it will renew interest in overclocking for a lot of people.
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