Ram of different size compatabillity issues?

Hello, I have a gaming desktop that I'm looking to upgrade with more ram. When i first built it, i got a 4gb 1600 mhz Cas 9, Kingston hyperX Ram stick (KHX16C9B1R). I was wondering if i could get a 8gb Kingston ram stick with the same speed and Cas latency, to upgrade my computer with. Now if i put both the 4gb stick and 8gb stick to get a total of 12Gb of ram, would i #1 be able to operate without any performance reduction of the ram? and #2 would i be able to run dual channel with the 2 ram sticks?



Will the computer work?  Yes.  Will you be able to use dual channel?  No.

Dual channel works by filling both sticks equally at the same time.  So say the computer needed to put 2gb of data into ram, it would put split it so 1gb went into 1 stick, and 1gb went into the other.  It then reads the data back the same way, reading both sticks at the same time.  This doubles the amount of 'bandwidth', twice as much ram can be written to and read from at the same time.

Now if you had a 4gb stick and an 8gb stick and the computer needed to put 9gb of data into ram, for it to run in dual channel it would need to split that evenly between the two sticks, 4.5gb into each stick.  Obviously the 4gb stick can't hold 4.5gb of data, so this can't work.

The computer would instead run both sticks in single channel giving you your full 12gb capacity, but filling up whichever stick is in the first slot first, then using the other.


The only way you can run mismatched memory sizes in dual channel is if you have two of each.  You would put a 4gb stick in slot 1 and 3, and an 8gb stick in slots 2 and 4.  That way the computer could fill two 4gb sticks at once, and when they're full start working on the 8gb pair, or vice versa.