Apple’s 8 GB MacBook Pro ambiguity

There is a minor stir occurring among owners of Apple’s new 15″ MacBook Pro Unibody machines concerning memory capabilities. The primary problem is that the models that can handle 8 GB of memory appear to be ambiguous by Apple’s own standard, and sorting it out is somewhat confusing. There’s also a lot of misinformation and missing information floating around. This is an attempt to clear things up.

The models that are affected actually go back to about July 2007, the so-called Classic MBPs. These MBPs were built on the mostly Santa Rosa platform^, which appears to be able to support 8 GB of RAM by Intel’s specifications. The models of laptops since then include:

  • July 2007 (Merom on SR)
  • Early 2008 (Penryn)
  • Late 2008 (first Unibody, Penryn) – with 2.4 GHz and 2.53 GHz processors
  • Early 2009 (also Penryn) – with 2.66 GHz and 2.93 GHz processors (the 2.4 GHz was not updated but is still available)

All of these models in the 15″ variety are claimed by Apple to support only 4 GB of RAM. This has been shown on a number of occasions to be wrong, already. For instance, all of these computers can support at least 6 GB of RAM. When installed, 8 GB of RAM is reported in each one of these machines, though until known, mysterious and considerable slow downs occur in at least the first three when addressing greater than 4 GB.

For a long time, it has been known that all of these models will successfully support 6 GB of RAM without problems, despite Apple’s passive claim that they all support up to 4 GB. However, recently Apple started quietly introducing 8 GB memory kits for only the Early 2009 15″ MBPs, and only the higher end ones (2.66 GHz and 2.93 GHz). While it was initially thought that these were mere processor refreshes, there’s something curious about why all of a sudden Apple would go beyond their published specifications and offer 8 GB of RAM for a model that only is supposed to support 4 GB.

It turns out that the answer lies in power distribution, according to the Apple engineer with whom I spoke. Despite the chipsets, there was something about the board design by Apple that did not provide enough power to the memory to allow for 2 simultaneous 4 GB DIMMs. However, when the most recent 17″ model was introduced, this was changed in the higher end 15″ models, which now support 8 GB of memory because this power distribution has been updated to account for two 4 GB DIMMs. Does this make sense? Assuming this is true, let’s see how the previous observations hold up.

Two 4 GB DIMMs are installed on older machines, but after 4 GB, the machine slows down terribly. If it’s a problem with supplying adequate power to the second DIMM, this makes perfect sense.

One 4 GB DIMM is installed alongside a 2 GB DIMM and works fine. This also makes sense, assuming that the power requirement for a 1×4 and 1×2 is less than a 2×4 and supported by the chipsets involved.

This is a huge hit for the people who purchased 2.53 GHz and 2.4 GHz machines, because this (if completely true) suggests that there is no possibility for these machines to ever support greater than 6 GB of memory. It’s hardware, not software.

The Ethical Issue
Granted, people who purchased these laptops knew that the published specification from Apple was 4 GB. However, past, pragmatic experience has taught us to pay attention to the unpublished specifications. Famously, certain laptops (and not others) secretly had a 802.11n wireless card in them, despite being advertised as G. A firmware upgrade for a nominal fee was later introduced that enabled these n cards for owners. With these laptops, it has been known for a long time that 6 GB was stable in laptops that specified 4 GB. The OS reported 8 GB, even when it didn’t work. So clearly something was amiss.

Nevertheless, people did purchase these laptops under the stated 4 GB maximum capacity that Apple specified. However, the combined ambiguity and major differences in hardware that are so easy to get confused about may make some Apple customers who use these machines in HPC environments less likely to purchase in the future, since subtle differences only existed in the unpublished and not the published specifications. How can one be sure of what one is getting? It’s been obvious since the beginning that the only “safe” option was the assume that 4 GB was the limit. In that case, Apple laptops are not even an option for those who need more.

^ I think there was some minor technical detail like the wireless cards were not Intel and thus the platform was not technically completely SR.

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