Fans of Apple's most diminutive professional notebook were let down this week when the newest 13" MacBook Pro revision didn't get an upgrade to Intel's latest processor architecture. The update to Nehalem-based cores, as well as a die shrink to 32nm, gives these Core i3, Core i5, and Core i7 mobile processors clear performance advantages over the Core 2 Duo that Apple retained for the 13" MacBook Pro. So why didn't Apple give the 13" MacBook Pro some Arrandale love?

It turns out that there are several reasons that factored into Apple's decision, including cost, graphics performance, battery life—and the laws of physics.

First, consider the basic architecture of the previous MacBook Pro models, first launched in October of 2008. All of them were equipped with a Core 2 Duo processor, a NVIDIA GeForce 9400M controller with integrated graphics, and, in the case of the 15" and 17" models, a second discrete GPU. The 16-core integrated GPU in the 9400M far exceeded the capability of the integrated Intel GMA950 or X3100 graphics in the Intel controllers used in previous Mac models. The 9400M also had the benefit of being CUDA-compatible, which was more important down the line when Apple rolled support for the CUDA-like OpenCL into Mac OS X 10.6.

In designing the Arrandale mobile processor family, Intel moved the memory controller, PCIe bus controller, and integrated graphics onto a separate, on-package die right next to the CPU. That means that these processors still require an additional, though less-capable and power-hungry, controller chip to handle the traditional southbridge functions, like USB and FIreWire I/O. And if you want something besides the integrated Intel HD graphics—a major improvement over the older IGPs Intel offered, but still not quite as good as the 9400M at certain tasks—you'll also need a discrete GPU.

For the 15" and 17" MacBook Pros, Apple did just that. Those models come with an NVIDIA GeForce GT 330M mobile GPU in addition to either a Core i5 or Core i7 processor mated with an Intel HM55 controller. (If you want to know why the new MacBook Pros don't support USB 3.0, blame Intel—the Arrandale chipsets don't include USB 3.0 support.)

For the 13" MacBook Pro, Apple faced a tough choice. The 13" MacBook Pro is the smallest "professional" portable that Apple makes, so it's both light and affordable (relative to larger MacBook Pros). Still, users also have an expectation of performance—not just from the CPU, but also from the GPU.

Apple might have opted for the Core i5, but that likely would have bumped the cost higher than Apple was willing to go. Or, Apple could have opted for the less expensive Core i3 processors, which skip on the Turbo Boost feature included in the Core i5 and i7 processors. (Turbo Boost lets one of the two cores bump up the clock speed while shutting down the other core on an as-needed basis, great for certain kinds of processing tasks that don't work well or aren't optimized for multiple cores.) The Core i3 still retains hyper-threading, which lets each core run two threads simultaneously, so it can offer a speed boost for multicore-aware and multi-threaded tasks. So the Core i3 might seem like a good fit, but for reasons we'll outline in a moment, it wasn't.

The faster 2.4GHz and 2.66GHz Core 2 Duo that Apple is using is cheaper than both the i3 and i5 since it's an older architecture, and we are confident that Intel was willing to cut Apple a killer deal to clear out inventories. Apple claims that these older processors also offer as much at 90 percent of the performance of the comparable Core i3s, but of course this is highly dependent on the workload, and it comes at the cost of more power draw (the older parts are at 45nm and are clocked higher than the 32nm i3).

Graphics performance would have taken a significant step backwards if Apple opted to rely solely on the Intel HD IGP that comes with Core i3 or i5, so Apple would have also had to include a discrete GPU like that used on the 15" and 17" models. Besides the added cost, there's simply no room on the 13" MacBook Pro logic board to include an additional discrete GPU. Compare the photos below:

Top: The tiny logic board for the 13" MacBook Pro (2009). Bottom: The similar, but much larger, logic boards for the 15" and 17" MacBook Pros (2009). Highlighted are the Intel CPU (red), NVIDIA 9400M (green), and discete NVIDIA GPU (blue).

It doesn't take a computer or electrical engineer to see there's just no room on the 13" MacBook Pro logic board for a discrete GPU in addition to a Core i3 and HM55. If Apple wanted to modify the logic board to include one, it would have meant carving out space used by the battery, both for the GPU and possibly additional thermal routing to keep the machine from bursting into flames. Doing so would have been a double whammy on runtime—more power would be used by the GPU, and there would be less battery to power it. At best, Apple may have been able to keep the stated seven hour battery life of the previous model, but it certainly wouldn't have come close to the 10-hour battery life claim of the new one.

So, we already know that Apple didn't want to move backwards in graphics performance by relying solely on the Intel HD IGP, and didn't want to compromise on battery life by trying to cram in a discrete GPU. A solution like the NVIDIA 9400M, which combines a controller with an integrated GPU, was Apple's only other option given these constraints. Unfortunately, the licensing spat between Intel and NVIDIA prevents NVIDIA from making a solution like this for the Arrandale family of processors (or really, any Intel processor with an integrated memory controller). There's no telling what the end result will be of the lawsuits the companies filed against each other, and Apple can't afford to sit idly by and wait for them to settle their differences.

Fortunately for Apple, NVIDIA made several improvements to the graphics capabilities in its integrated controllers, including moving them down to a 40nm process and building an improved low-end core (GT216) to base them on. Apple had NVIDIA build a special 48-core variant of its GeForce 310M mated to an improved version of the controller in the 9400M. Called the GeForce 320M, it gives the new 13" MacBook Pro as much as an 80 percent boost in graphics performance when pushed to its limits (and NVIDIA says the 16-core 310M is 10x faster than the Intel HD IGP). At the same time, it still offers as much as a 40 percent power savings when performing more mundane computing tasks (this likely outweighs the power penalty for sticking with Core 2 Duo). In addition, it gives Mac OS X's Grand Central Dispatch three times the resources to tap for GPGPU processing.

Clearly, Apple wasn't actively trying to cripple the 13" MacBook Pro by leaving it "stuck" with yesterday's Core 2 Duo technology. Instead, an Apple spokesperson told Ars that the company wanted to maintain the 13" MacBook Pro's unique combination of performance, portability, and extended battery life, as well as its $1,200 starting price. The combination of a faster Core 2 Duo processor and the NVIDIA 320M allowed Apple to do that.

Of course, it's easy to find notebooks from other manufacturers that offer comparable performance, comparable portability, possibly comparable battery life, and as good or better pricing. However, you're unlikely to find a comparable notebook that offers all four, and (depending on your personal taste) with as much style as the 13" MacBook Pro.

As Steve Jobs recently explained in one of his increasingly frequent, succinct e-mails to customers: "We chose killer graphics plus 10 hour battery life over a very small CPU speed increase. Users will see far more performance boost from the speedy graphics."