As I mentioned in a previous article here, Intel has been most forthcoming with processor specifications at its Web site. The level of detail is excellent, making it possible to actually discern the differences between its processor models. You can also compare any processor to any other; the table the site produces is comprehensive.

Unfortunately, Intel's processor marketing and the model followed by its customers, the manufacturers who build the PCs you buy, is less clear. I think the marketing model obfuscates the details of the processor, to the detriment of the consumer. This is the result of the "i" campaign, which depicts only the three families of the contemporary processor line - i3, i5, and i7. PC companies follow this model; an i5 processor is always an "upgrade" to an i3 processor, for example.

The problem is that there are currently several dozen processors in the three families and that the families overlap in performance. In the table below, I attempt to show the relative performance of the individual processor models in the hopes of shedding a tiny ray of light on the matter.

EMCP

Making an actual determination of performance is not easy. There are many factors, including the extent to which an operating system and its applications avail themselves of the processor's features. I have settled on just three features to examine - number of processor cores, whether Intel Hyper-Threading (HT) is supported (number of threads), and the clock rate. I use these to calculate an "Estimated Maximum Computing Power" (EMCP) number, which is my rough measure of the oomph delivered by each model. The table below is ordered in ascending EMCP and thus shows the overlap between models.

Here is a explanation for some of the columns:

• If a processor supports Intel HT, the number of threads will be double the number of cores.
• Clock rate given in GHz.
• TDP is "Thermal Design Power" and is a measure (watts) of how much cooling capability a given processor requires. This is a clue as to how noisy the PC with the processor might be, as more fans might be needed.
• Factor shows the EMCP multiplier. For example, the i3-330M has an EMCP twice that of the i5-520UM.
• Cost is also a multiplicative factor. The least expensive processor is the i3-530 and all other processor are factors of that. I used Intel's current list price for the chips as the basis for these factors, so this is something that could change rapidly.
• TBoost shows the maximum clock rate in GHz that can be achieved with Intel's Turbo Boost technology. If there is no number, Turbo Boost is not supported.
• IIG indicates whether the chip includes Intel Integrated Graphics.
• RAM shows the maximum amount of main memory supported by the CPU, in GB.
• Cache shows the amount of cache memory on the CPU, in MB.

How do I calculate EMCP? The obvious starting point is cores times clock, but this ignores the contribution of Intel HT. In past experiments, I determined that a Hyper-Threading-capable processor was equivalent to 1.6 cores. In other words, HT provides a boost of about 60%. It has been some time since I did my own studies, so I decided to use a more conservative factor of 50% for EMCP.

Here's how the calculation goes for the i5-520UM (the first model in the table):

• 2 cores x 1.06 clock = 2.12
• 4 total threads - 2 cores = 2 Hyper-Threads
• 2 Hyper-Threads x 1.06 clock x 50% HT factor = 1.06
• 2.12 + 1.06 = 3.18 EMCP (I round the results in the table to the nearest tenth)

I do not include Turbo Boost in the calculation. Because this feature is controlled entirely by the processor itself, and because it is affected by the chip's maximums for voltage and especially temperature, the user cannot control how long the boost will last. It will surely not be continuous; if it could, Intel would just rate the chip at that level.

And now for my results. I have color-coded the rows in the table below by family:

• i3 - yellow
• i5 - green
• i7 - faint green

Caveat Emptor

What should you buy? My usual advice to most consumers is the same advice Consumer Reports used to give - buy in the middle. The magazine used to be good at finding models with a balanced set of features, above average reliability, and a middle-of-the-road price.

The range of the EMCP factor is 1.0 to 9.1. However, the powerful, six-core i7-970 is an outlier, with an extreme EMCP. A more rational range is 1.0 to 6.0. The middle of the range is thus 3.5. The middle of the cost factor range, again excluding the i7-970, is 3.0. Using either of those scales to make a decision should hold you in good stead.

There is an interesting wrinkle here. Take a look a the five entries above with an EMCP factor of 3.0. Consider the raw numbers - cores, threads, and clock rate. If you have a lower clock rate and your software does not take good advantage of multiple threads, you end up with a slower program execution and your PC may not be able to hit my theoretical EMCP. So even though the i7-720QM with its four 1.60GHz cores has an EMCP of 9.6, it might run much slower than the 3.20GHz, dual-core i5-650 for some tasks.

For my money, the best value for a desktop computer is probably one of three i5 models - the 650, 660, or 670. The EMCP is 3.0 and the cost factor is 1.6, meaning that these processors contribute just 60% more cost than the least expensive processor. After that, you're in i7 territory with twice the cores and optional video, with attendant higher costs. For a laptop, consider the i5-540M.

Don't overlook the i3-530 and  i3-540. They are the least expensive processors in the table above but with the exception of TurboBoost they are very capable. Expect a desktop computer with either of these to be in the $500 ballpark.

By the way, the minimum system requirement for Windows 7 is a processor with at least a 1.0GHz clock rate. All processors on the list exceed that requirement. My recommendation is to double that minimum.

Conclusion

The moral of this story is to make sure you know which processor model you are buying, not just the family, and that you use something like the EMCP calculation to determine how much computing performance you are buying.

But don't let that be your only guide. There are some differences in these various models that may or may not make a difference to you, such as the amount of cache, the maximum RAM capacity, and Turbo Boost. When in doubt, dig in to the manufacturer's Web site to get precise information about the CPU and Chipset being used and explore Intel's Web site to get the exact specifications for those chips. Keep in mind that systems using processors without integrated graphics will be more expensive, as either a dedicated graphics chip will be provided or you will have to buy an optional graphics card.

Finally, three key features are present in all of the processor models listed above:

• 64-bit capability
• Hardware support for virtualization
• Data execution prevention (DEP, a security feature)

Note: The meaning of letter suffixes to model designations can be found at Intel's site. M means the processor is intended for mobile applications.

Updated 07 Aug 2010