With only a single dimm installed the mode would be set in single channel. The manual will
explain how to set each mode as a rule. Usually the blue not black colored slots are advised
for dual channel mode when two dimms are installed. With three slots taken, both blue and
a black, you would probably still see the dual channel mode enabled. A1 and B1 are usually
the recommended as those are the blue in color. If the board supports 1gb dimms using one
1gb dimm will see the single channel mode. Most would obviously prefer two 1gb dimms to
see the gain with the dual channel mode active.