How are the uni-directional data buses in the ARM7TDMI used?

The signal BUSEN indicates the data bus configuration:

• When BUSEN is HIGH, the two uni-directional data busses DIN[31:0] and DOUT[31:0] are used for transfer of data between processor and memory, and the bi-directional data bus must be left unconnected. Bus keepers are not necessary and should not be added.
• When BUSEN is LOW, the bi-directional data bus D[31:0] is used for transfer of data between processor and memory. Any data presented on DIN[31:0] is ignored (but should be tied off), and DOUT[31:0] is driven to the value 0x00000000. Depending on the design of the memory system, bus keepers may be required on D[31:0].