A possible processor machine instruction for writing to a device register:

OUT  <device register address>, R2

This means to move the contents of register R2 to the device register whose address is specified.

Example. OUT 0x0B10, R2
1. The address 0x0B10 is placed in the MAR.
   (which will "flow" onto the address bus)
2. The contents of R2 are copied into the MDR
   (which will "flow" onto the data bus)
3. The IOW signal is asserted in the control bus

Main memory is connected to the busses, but ignores the data since the IOW signal is asserted instead of the MEMW signal.

The device controler "sees" items 1, 2, and 3 and responds by:

The device controller copies the data on the data bus into 
the device register with address 0x0B10.