From my understanding, when dealing with an exchange you first transfer your currency to the exchange's address. The exchange then pools the currency of its participants.
When transactions then take place across the exchange, the exchange doesn't write to the underlying blockchain, but records all these transaction by changing their records of ownership in their own internal database.
When you withdraw your funds, you then retrieve your currency back to your own wallet.
Does this mean an exchange will only write to the blockchain when you transfer currency to or withdraw money from the exchange?