Mixable versus Non-Mixable Apple Computer Memory
Apple RAM for Mac Pro 4,1 (when upgraded to Lion and above) and Mac Pro 5,1 can use 8GB and 16GB modules. These modules are a great Mac Memory upgrade for your Mac Pro, packing a large RAM upgrade into one slot, allowing you to boost your machine's memory incredibly high - but they present a unique challenge smaller sized modules do not have.
16GB DIMMs currently only come in a non-mixable (sometimes called registered) variety - at this time, there is no manufacturer producing mixable (or unbuffered) 16GB DIMMs. This means 16GB DIMMs can only be installed in your Mac Pro with other 16GB DIMMs that are identical (i.e. purchased at the same time.) Even 16GB DIMMs from the same manufacturer but purchased at different times will frequently not work together.
8GB modules are slightly more challenging because they are currently manufactured both in mixable (unbuffered) and non-mixable (registered) varieties.
Currently, Ramjet only carries 8GB mixable modules. Our 8GB DIMMs can be mixed with any other size DIMM - 4GB, 2GB, or 1GB. Our 8GB DIMMs can also be mixed with other 8GB DIMMs provided those DIMMs are also mixable. If you have previously bought 8GB DIMMs from another manufacturer, there is a possibility that they are non-mixable - therefore those non-mixable DIMMs will not allow you to mix in any other DIMMs. Non-mixable 8GB DIMMs, just like the 16GB DIMMs, will not work with any other size, and even from the same manufacturer but purchased at different times frequently will not work together. Non-mixable 8GB DIMMs will not recognize a mixable 8GB DIMM as compatible.
Ramjet has in the past carried non-mixable 8GB DIMMs, but at this time, the only 8GB DIMMs we carry are mixable.
It is important to note this only applies to the Mac Pro. 8GB DIMMs for the MacBook Pro, Mac Mini, or iMac do not present the same challenge of mixable versus non-mixable.