Q&A: The limits of memory

Q I have heard that whenever bigger memory modules become available for a particular computer, you can install them into your Mac. Is it not true that the maximum usable memory is determined by the hardware, and is thus the amount stated by Apple? How much can I install in my MacBook Pro Early 2008, as Apple and suppliers state that it only supports 2 x 2 GB modules for a total of 4 GB: could I install 2 x 4 GB for a total of 8 GB?

A You are correct that it is the hardware that ultimately determines the maximum amount of memory that can be addressed. However that is not always the same as the maximum amount of memory that Apple states when it first releases a product, and even years afterwards you can sometimes squeeze more in than you thought.

For instance, the Mac Pro 8-core, when released, was claimed by Apple to support a maximum of 16 GB of memory, in 8 x 2 GB DIMMs.

At that time, 4 GB DIMMs were not available for it, but they are now, and lo and behold, you can upgrade it to hold 8 x 4 GB DIMMs, for a total of 32 GB. Of course if someone came out with ‘compatible’ 4 TB DIMMs, as the hardware could not address 32 TB, the hardware maximum is the ultimate maximum.

However there are Macs that have never been able to realise their hardware maximum because suitable memory modules have never been manufactured.

The maximum memory that can be installed in any Mac is the lower of:
[number of memory slots] x [largest memory module available to meet the slot specification], and
[maximum amount of memory addressable by the hardware of that Mac].

Your MacBook Pro is a good example: when Apple released it, the maximum memory was stated as 4 GB, based on 2 x 2 GB PC2-5300 DDR2 SO-DIMM modules.

However it can actually address 6 GB, so some Mac memory vendors now offer an upgrade including one 4 GB and one 2 GB module, although 2 x 4 GB SO-DIMMs will not work. Mainstream memory vendors, including Apple, will still tell you that it cannot use more than 4 GB, though.

Comments A good reference guide to memory capacity of every Mac model is of course MacTracker, from here.

Updated from the original, which was first published in MacUser volume 28 issue 19, 2012.