The 21.5 inch version of the iMac has the RAM soldered to the motherboard. The only iMac currently on the market with user-serviceable RAM is the 27 inch model. If you use all four slots, your computer will run as if all the slots have DDR3 memory installed and will not make use of the performance of the DDR4 sticks. This is particularly important if you have one of the motherboards mentioned earlier where it has slots for DDR3 and DDR4 memory. Modern motherboards all come with a feature called “Interleaving” which will bring down the performance of all RAM to the slowest RAM on the board. Make sure the RAM you install is the same (same type, same capacity, same chip) across the pairs, or in the case of a triple channel motherboard, across all three slots. The RAM slots will be color coded to indicate which RAM pairs go together. If you have a quad channel motherboard, that is a motherboard capable of supporting four memory cards, make sure you install the RAM in corresponding pairs.
The RAM fits into long channels on the motherboard in sets of two, three or four. RAM slots on a motherboard come in sets, and it is important to make sure you install RAM in sequence corresponding to these sockets.
Some things to remember when installing desktop RAM:.
