There are also two separate memories.
Memory 1 is the "trip" memory which resets after you have left the vehicle with the ignition switched off for two hours or more.
Memory 2 resets after 999,999 miles or something like that.
On cars with steering wheel mounted controls, pressing the "OK" button briefly will alternate between displaying memory 1 and memory 2. Holding the "OK" button down for two seconds will reset the currently selected memory.
I presume that stalk controlled cars have an equivalent to the "OK" button.