Question: Will my memory run in dual channel mode if I install a 2GB DDR2 SODIMM and a 1GB DDR2 SODIMM?

Read our answer below:

Most DDR2 memory based laptops support dual channel mode but some (i.e. early DDR2 memory based laptops) may only support what's known as synchronous dual channel mode. Synchronous is when two identical SODIMM memory modules are installed, for example two 1GB 200pin DDR2 SODIMM memory modules. Note that their capacity is the same, 1GB in each SODIMM memory slot enabling a total of 2GB system memory. A laptop supporting synchronous dual channel mode will run such configuration with the benefits of dual channel mode enabled for the full capacity of installed memory (i.e. 2GB).

Fortunately, most laptops that use DDR2 memory support both synchronous and asynchronous dual channel mode. Consequently, regardless whether you install identical capacity DDR2 SODIMM memory modules or otherwise (for example a 2GB DDR2 SODIMM and a 1GB DDR2 SODIMM) your laptop will run with dual channel mode enabled. There is however one caveat that is worthy of attention. Always try and install identical capacity SODIMM memory modules. Why? In asynchronous dual channel mode only part of the memory is run in a true dual channel manner. This is best shown by an example. For instance, if you install a 2GB DDR2 SODIMM and a 1GB DDR2 SODIMM then only the first 2GB of memory from both the first and second SODIMM memory module will run in dual channel mode. The remaining 1GB will simply operate in single channel mode thus yield a lower memory bandwidth.

The above recommendation also applies to DDR3 memory based laptops. You should always strive to install identical capacity SODIMM memory modules to enable maximum memory bandwidth thus overall system performance. Only install non-identical capacity SODIMM memory modules in dual channel compliant laptops if you're willing to sacrifice less memory bandwidth and/or are financially constrained to do so.

Finally, remember that not all DDR2 and DDR3 memory based types of laptop support dual channel mode, regardless whether synchronous or asynchronous. This may be true for select sub-notebooks and especially netbooks. A general yet by no means absolute method to determine this is by checking if they only feature a single SODIMM memory slot.

Learn more about upgrading both single channel and dual channel laptops in part 3 of the Laptop Memory Upgrades Guide.

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