I have had to play around with the process of swapping out ram sticks until you "trick the machine"to boot.
Then, if you add the other 8, it will recognize both, or some combo of this kind of trickery. But, if you put in an 8G RAM stick in the bottom slot and a 4G stick in the upper slot and then boot, it will recognize the two slots. This is a quick video explaining the characteristics of memory (RAM) that will function properly with your Mac (MacBook Pro. In a Mac Pro you have: bank 0 channel 1 channel 2 channel 3 bank 1 channel 1 With all slots filled it should interleave (0:1 and 0:2) and (0:3 and 1:1) then interleave those two pair. And slower memory is better than less memory. 3 Alarm sounds indicate that the machine cannot recognize the slot and 1 Alarm indicates that the machine cannot recognize the RAM. I agree you won't notice the differences unless memory bandwidth poses a limitation which it does not for most things. Here is the caveat- sometimes machines do not "want" to recognize the cards and either gave the 3 ALARM sound on start up, refused to boot, or the 1 Alarm sound at start up. I just installed 16GB of Ram, a new battery and a 1 TB SSD Hard Drive and the machine runs like a porche, even though the Genius at the Apple Store told me that my 2009, 2011, and now 2013 Macbook pros were a bunch of "Mini-vans."
The 2009 Mac Pro has up to 8 memory slots which can handle up to 1. How much RAM do I need for my Mac laptop Is 4GB of RAM enough in 2017 How much does a RAM upgrade cost Find out more about the random access memory requirements for different activities.
I want to affirm that you can run 16GB of RAM on a 2009 Macbook Pro 13 inch. Experience the true speed of your 2009 Mac Pro with OWC Memory upgrades from.