DDR2 RAMs have these control signals
RAS, CAS – address strobes
UDQ, LDQ – byte strobes
WE – write enable
UDM, LDM – write mask
Why do we need UDM and LDM? Can't you write a byte by asserting WE and only one DQ?
Here's a link to a typical data sheet. It tells me what the write mask does, but not why I need it.