I have successfully replaced 2 * 4GB DDR3 1333 MHz RAM (Samsung) with 2 * 8GB DDR3 1600 MHz RAM (CnMemory). Everything runs fine and System Report is displaying the RAM is running at 1600 MHz.

Now I wonder how accurate this report is. I have learned the RAM clock rate is determined by the Front Side Bus (FSB), so I would have expected the new RAM to still work at 1333 MHz instead of now 1600 MHz.

So does System Report report the accurate clock rate of the FSB or does it report the advertised maximum clock rate from the DDR3 RAM somehow?

(I should have done a memory benchmark before the switch, but I really didn't expect these better results).

  • Can you add some details about your specific Mac model and also include the relevant part (with the RAM specs) of the system report?
    – nohillside
    Commented Mar 18, 2013 at 15:43
  • For what it's worth, it's a MacBook Pro 8,2 (early 2011) with Mac OS X 10.8.3. As said System Report is reporting the RAM as 2 * 8 GB DDR3 1600 MHz. Commented Mar 18, 2013 at 15:46
  • 1
    My guess is that it is simply reading the RAM specs. Your model doesn't officially support the 1600MHz speed. As a side note, your Mac does not have a Front-Side Bus. Getting rid of that processor bottleneck was one of the great enhancements of Intel's i5 & i7 series.
    – bispymusic
    Commented Mar 18, 2013 at 15:54
  • Well, when I run About This Mac and click on the Memory tab, then I get the following text: "Your Mac contains 2 memory slots, each of which accepts 1600 MHz DDR3 memory module." So it seems to fit, although the support page at support.apple.com/kb/HT1270?viewlocale=en_US#link3 says it should only accept 2 * 4 GB DDR3 1333 MHz. Commented Mar 18, 2013 at 16:47

2 Answers 2


I have the same machine (MacBookPro8,2). A quick web search seems to indicate that my 2.2 GHz model has an Intel Core i7 2720QM CPU.

According to Intel's specifications for this CPU family, the supported memory types are actually DDR3-1066/1333/1600. (Incidentally, the memory limit is also 32 GB, but this may require more than two slots; I'm not sure.)

Technically, this CPU doesn't have a traditional "front side bus", which is a connection between the CPU and an external memory controller. Its memory controller is integrated with the CPU, meaning that the supported RAM configuration isn't further subject to the specifications of an external memory controller.

So, it does seem like this system plausibly supports 1600 MHz RAM, after all! I'll be sure to keep that in mind when I upgrade to 16 GB. :)

Interestingly, when I open the same dialog in "About This Mac", my system reports that it supports 1333 MHz memory. It seems like it might just report the current memory speed.


This depends mostly on the capabilities of the FSB and the RAM itself. If, for example, you install a 1600 MHz stick next to a 1333 MHz chip, most FSBs will time at 1333 for both, if it is even capable of doing so. Most FSBs also have a maximum clock speed as you indicated in the comments.

The FSB is not likely fudging the numbers. There is a chance the 'limit' imposed by KB HT1270 is a soft one. Though my 2008 MBP had a limit of 2x2GB I ran it for years with 2x4GB sticks. I couldn't say for certain regarding your specific model, but I'm inclined to disregard this list.

Don't go crazy though, there are some hard limits on the size that will be accommodated, which you've likely reached with your 16GB.

Additionally, in my experience, transitioning from 1333 to 1600 typically doesn't produce noticeable speed increases, though the doubling of the capacity would. If you're noticing this under moderate load with no paging, it's likely the effects are psychosomatic. :)

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