What's the difference between an Intel Core i3, i5 and i7?

Intel Core i3, Core i5, and Core i7 CPUs have been around for over a year now, but some buyers still get stumped whenever they attempt to build their own systems and are forced to choose among the three. With the more recent Sandy Bridge architecture now on store shelves, we expect the latest wave of buyers to ask the same kind of questions.

Core i3, Core i5, Core i7 — the difference in a nutshell

If you want a plain and simple answer, then generally speaking, Core i7s are better than Core i5s, which are in turn better than Core i3s. Nope, Core i7 does not have seven cores nor does Core i3 have three cores. The numbers are simply indicative of their relative processing powers.

Their relative levels of processing power are also signified by their Intel Processor Star Ratings, which are based on a collection of criteria involving their number of cores, clockspeed (in GHz), size of cache, as well as some new Intel technologies like Turbo Boost and Hyper-Threading.

Core i3s are rated with three stars, i5s have four stars, and i7s have five. If you’re wondering why the ratings start with three, well they actually don’t. The entry-level Intel CPUs — Celeron and Pentium — get one and two stars respectively.

Note: Core processors can be grouped in terms of their target devices, i.e., those for laptops and those for desktops. Each has its own specific characteristics/specs. To avoid confusion, we’ll focus on the desktop variants. Note also that we’ll be focusing on the 2nd Generation (Sandy Bridge) Core CPUs.

Number of cores

The more cores there are, the more tasks (known as threads) can be served at the same time. The lowest number of cores can be found in Core i3 CPUs, i.e., which have only two cores. Currently, all Core i3s are dual-core processors.

Currently all Core i5 processors, except for the i5-661, are quad cores in Australia. The Core i5-661 is only a dual-core processor with a clockspeed of 3.33 GHz. Remember that all Core i3s are also dual cores. Furthermore, the i3-560 is also 3.33GHz, yet a lot cheaper. Sounds like it might be a better buy than the i5. What gives?

At this point, I’d like to grab the opportunity to illustrate how a number of factors affect the overall processing power of a CPU and determine whether it should be considered an i3, an i5, or an i7.

Even if the i5-661 normally runs at the same clockspeed as Core i3-560, and even if they all have the same number of cores, the i5-661 benefits from a technology known as Turbo Boost.

Intel Turbo Boost

The Intel Turbo Boost Technology allows a processor to dynamically increase its clockspeed whenever the need arises. The maximum amount that Turbo Boost can raise clockspeed at any given time is dependent on the number of active cores, the estimated current consumption, the estimated power consumption, and the processor temperature.

For the Core i5-661, its maximum allowable processor frequency is 3.6 GHz. Because none of the Core i3 CPUs have Turbo Boost, the i5-661 can outrun them when it needs to. Because all Core i5 processors are equipped with the latest version of this technology — Turbo Boost 2.0 — all of them can outrun any Core i3.

Cache size

Whenever the CPU finds that it keeps on using the same data over and over, it stores that data in its cache. Cache is just like RAM, only faster — because it’s built into the CPU itself. Both RAM and cache serve as holding areas for frequently used data. Without them, the CPU would have to keep on reading from the hard disk drive, which would take a lot more time.

Basically, RAM minimises interaction with the hard disk, while cache minimises interaction with the RAM. Obviously, with a larger cache, more data can be accessed quickly. All Core i3 processors have 3MB of cache. All Core i5s, except again for the 661 (only 4MB), have 6MB of cache. Finally, all Core i7 CPUs have 8MB of cache. This is clearly one reason why an i7 outperforms an i5 — and why an i5 outperforms an i3.

Hyper-Threading

Strictly speaking, only one thread can be served by one core at a time. So if a CPU is a dual core, then supposedly only two threads can be served simultaneously. However, Intel has introduced a technology called Hyper-Threading. This enables a single core to serve multiple threads.

For instance, a Core i3, which is only a dual core, can actually serve two threads per core. In other words, a total of four threads can run simultaneously. Thus, even if Core i5 processors are quad cores, since they don’t support Hyper-Threading (again, except the i5-661) the number of threads they can serve at the same time is just about equal to those of their Core i3 counterparts.

This is one of the many reasons why Core i7 processors are the creme de la creme. Not only are they quad cores, they also support Hyper-Threading. Thus, a total of eight threads can run on them at the same time. Combine that with 8MB of cache and Intel Turbo Boost Technology, which all of them have, and you’ll see what sets the Core i7 apart from its siblings.

The upshot is that if you do a lot of things at the same time on your PC, then it might be worth forking out a bit more for an i5 or i7. However, if you use your PC to check emails, do some banking, read the news, and download a bit of music, you might be equally served by the cheaper i3.

At DCA Computers, we regularly hear across the sales counter, “I don’t mind paying for a computer that will last, which CPU should I buy?” The sales tech invariably responds “Well that depends on what you use your computer for.” If it’s the scenario described above, we pretty much tell our customers to save their money and buy an i3 or AMD dual core.

Another factor in this deliberation is that more and more programs are being released with multithread capability. That is they can use more than one CPU thread to execute a single command. So things happen more quickly. Some photo editors and video editing programs are multi-threaded, for example. However, the Internet browser you use to access Netbank or your email client is not, and is unlikely to be in the foreseeable future.