Processors implement a set of "machine" instructions.

A typical such machine instruction is the Store instruction, which stores the contents of some register to some memory location.

How do the system components do this?

Example. Store R2, 0x0B10  
(Store the contents of register R2 in memory location 0x0B10)
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 MEMW signal is asserted in the control bus

Main memory is connected to all three components of the bus.

Main memory "sees" items 1, 2, and 3 and responds by:

Main memory copies the data on the data bus into 
memory location 0x0B10.