This part of the MIG Design Assistant guidesyou to information on masking data with the user interface.
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The write data mask inputs (pX_wr_mask) to the user interface can be used to offset the starting address byte location.
For example, to begin writing at byte address 0x01 when using a 32-bit (4-byte) user interface, the byte address presented to the command port of the user interface should be 0x00, but the least significant mask bit should be set to 1 such that only bytes at address 0x01 and higher are actually written.
For timing diagrams and more information, see the Spartan-6 FPGA Memory Controller User Guide(UG388); under "MCB Operation" -> "Addressing":