The (G)MCH supports interleaved addressing in dual-channel memory configurations even when the two channels have unequal amounts of memory populated. This is called Intel(R) Flex Memory Technology.
Intel Flex Memory provides higher performance with different sized channel populations than Asymmetric mode (where no interleaving is used) by allowing some interleaving.
The memory channel addresses up to twice the size of the smaller SO-DIMM are interleaved on a 64-B boundary using address bit 6 (including any XOR-ing already using in interleaved mode). Above this, the rest of the address space is assigned to the remaining memory in the larger channel.