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I have a question regarding my laptop's RAM.

I have 2 slots. One is empty; the other one has a Samsung SODIMM 204-pin 2GB DDR3 1333MHz SDRAM CL9, non-ECC, unbuffered module, running at 1066 (!) MHz, because the motherboard supports this maximum frequency, and in single channel mode. Is it OK for me to add, in the empty slot, a Samsung SODIMM 204-pin 2GB DDR3 1066 MHz SDRAM CL8 (!), non-ECC, unbuffered RAM module? Will they work fine with both running at 1066 MHz? And will it be activated in dual channel mode in this case?

Well, the different apps show me that the RAM module is running at 1066 MHz, the guide for my laptop says it supports 1066 MHz DDR3 Dual Channel. And also they show me that it runs in single channel mode. Tried to see the tech specs on an almost similar laptop, with 2 modules identical to this one, they run both at 1066 MHz (even if they are 1333 MHz both), and the Dual Channel mode is activated (Dual Channel Ungaged, supose it's ok).

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migrated from Aug 14 '11 at 4:44

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Both memory modules should work and operate at 1066 MHz. If properly supported, the modules will operate in dual-channel mode, but both modules will be limited to the slower one, which is unlikely to be noticeable. There should not be any significant problems in terms of system performance or stability. You can consider using Memtest86+ to check for problems after you install the new memory module.

Edit: You stated that your motherboard supports 1066 MHz in single channel mode only. You can still use the memory upgrade, but you will not obtain any performance improvements related to dual-channel operation.

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In over 95% of the cases you'll be able to use faster memory chips like 1333 on older computers without problems.

Still there is a small number of cases when this will not work. One good example is the Mac Mini from around 2010 that will became unstable with 1333 MHz memory but that will run just fine with original 1066 Mhz memory.

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