1. How is that I can install more memory on my Apple machine than Apple supports?
A. When manufacturers release a machine, the manuals are written with the available information at that time – for example, a machine may be specified with two 4 GB DDR3 modules for 8 GB total. If larger memory modules were not available at that time, they may not have been tested or included in the manuals or specifications.
Sometimes, after new, larger modules are introduced, they might be tested and the machine found to support them, such as two 8 GB modules for 16 GB total. But manufacturers seldom go back to correct the documentation or official specifications of the machine, they are on to a new line of machines by then, so the documentation would still say 8 GB maximum
Check with the memory manufacturer or Canadaram to see if they have tested and certified their modules as compatible with your specific model of machine.
Be aware that sometimes, a machine needs updated firmware, or a newer operating system, before it is able to recognize a larger memory module.
Another thing to realize though, is that memory modules can be built in many different ways; for example in the number of memory chips on them, the number of ranks, and the way the rows and columns of memory are organized. So if your machine does support a larger module, it may require a specific build of chip to make it work.
One rule of thumb is that if the machine takes a 4 GB module with 8 memory chips on it, it may well need to have 16 memory chips on the 8 GB module. It’s about the density of the memory modules; the memory controller of the machine can count only up to so many memory locations per chip. If you install a high-density 8 GB module with 4 or 8 chips on it, the machine may not be able to address the memory. When in doubt, try a dual rank, low density module.
2. What if I ran into issues?
A. It’s not a given that a machine will support larger memory modules even if they physically fit. If the BIOS or memory controller doesn’t support it, the machine may crash at boot up, or it may boot but only show half of the memory. Occasionally a machine will seem to work fine until you load enough programs to start accessing memory over its limit, and then it will crash. So test your memory before you start using the machine for real. You can use a program like Rember (Mac) https://www.kelleycomputing.net/rember/ or MemTest86 (Win) https://www.memtest86.com/ to thoroughly test the memory.
3. Does it mean that I can use DDR4 when the machine has DDR3
A. That’s a no. The different memory standards DDR2, DDR3, DDR4 have different pinouts and are different electronically, so you cannot ‘upgrade’ a given machine from DDR3 to DDR4.
Once upon a time, there were a few motherboards made that straddled the introduction of new memory standards, and put both the old and the new memory slots on them (two DDR2 and two DDR3 slots, for example) But these motherboards cannot use both standards at once – it’s one or the other.