I don’t remember where I first heard about it, but I think I was reading up on the newly release MacBook Pros when someone commented that the older ones could actually hold up to 6GB of RAM. That was news to me. I knew that Apple specified a maximum of 4GB, so I was intrigued. More RAM is better after all.
A quick Google search turned up a couple interesting forum topics that did indeed indicate that 6GB worked as long as you had a Santa Rosa chipset or newer (June 2007 or newer MacBook Pro).
That was enough for me, so last week I ordered myself a 4GB stick from NewEgg ($160). It arrived today.
Installation
Since my MacBook Pro already had 4GB of RAM, that means I needed to pull out one of my 2GB sticks and swap it with the new 4GB stick. This took just a few minutes. Rebooting into OS X saw that it recognized the 6GB right away. I was able to create a 4GB Windows VM with VMware Fusion and it worked like a charm, with no slowdown due to swapping. Of course Windows XP could only see 3GB of the 4GB that was allocated.
Why does it Work?
There seems to be some confusion as to why this works. Before the Santa Rosa MacBook Pros, the most RAM a MacBook Pro could use was 3GB. This was because those older laptops could only address up to 4GB of memory and the last 1GB or so was reserved for addressing video RAM and other stuff.
With Santa Rosa, the last 3 revisions of the MacBook Pro (June 2007, Feb 2008, Oct 2008) can all address up to 8GB of memory. Unfortunately this doesn’t mean that you can plug 8GB of RAM into these laptops. Just like the older laptops could only use 3GB of RAM out of 4GB of memory space, these laptops also have about 1GB that cannot be used for RAM.
So theoretically the MacBook Pro could use the remaining 7GB for RAM. Unfortunately that would require a 4GB and a 3GB RAM stick, but the latter size is not manufactured. So we’re stuck with a maximum of 6GB (2GB + 4GB).
Ideally, we would plug in 8GB of RAM and any of the RAM that could not be addressed (because it was overlaid by reserved memory addresses) would just be ignored, leaving us with about 7GB of usable RAM. But testing by others seem to indicate that this does not work as expected. OS X seems to get confused and as a result starts to run slowly. I don’t know if this is merely an OS X problem or a hardware issue, so for now it looks like 6GB is a high as you can go.
Of course, the Intel Core 2 Duo CPU in the MacBook Pro is a full 64-bit chip. It can address up to 16 million terabytes. But that’s probably at least a couple years away 
And I wonder how much that would cost?
Update 10/29/2008: People have asked me to provide updates as to how this is going. So far, it’s been completely reliable. I’ve used the MacBook Pro on battery, plugged in and running external monitors. There’s been no noticeable change in temperature. Everything is working great!
5 Responses to “Upgrading a MacBook Pro to 6GB RAM”
As Johnny Carson used to say “I did not know that!”
Useful thing to know. I’ll have to try this.
“It can address up to 16 million terabytes. But that’s probably at least a couple years away. And I wonder how much that would cost?”
A lot less than it would cost today, I would guess. In the early 80′s I had an Apple ][+. I added 16K (yes K) of memory to it. Best price I could find anywhere: $125 (Apple’s price was, as you might expect, out of the question for me!). So 16G would cost $125M in the early 80′s at that rate while it costs well under a grand today! Bottom line: I was willing to spend $125 to get a “reasonable” amount of memory back then and I am still willing to do that today.
Just a caveat…. I bought the same Newegg memory and have not been able to get my Macbook pro it up to 6 gb. The newegg memory will work by itself but if you add 2g, the computer locks ups and won’t boot. (Macbook pro 2.6ghz/4mb cache – late 2007). You can’t return the memory for a refund (I did a warranty return, it didn’t work either). So be prepared to eat the cost if you try this.
That’s unfortunate. So far, my 6GB has been working great.
One thing you might want to try is to swap the RAM chips into different sockets. I seem to remember that having an effect for some people.
Great blog entry – i also upgraded to 6GB (other world computing) and everything has been perfect. Paging is now a thing of the past…. for a while at least!
Thanks for sharing your experience