Creation and usage of mutually exclusive messages

1. A method of attempting a write to a target entity having first and second security fields to cause the target entity to perform an action, the method including the steps of:

initializing the first security field with a first initial value and the second security field with a second initial value;

sending a first message to the target entity that is an integrated circuit, the first message being configured to cause the entity to perform the action and to adjust the first and second initial values in the first and second security fields by respective first and second amounts resulting in respective first and second adjusted values;

sending a second message to the target entity, the second message being configured to cause the entity to adjust the first and second initial values in the first and second security fields by respective third and fourth amounts resulting in respective third and fourth adjusted values;

wherein the security fields have write restrictions associated with them which prevent values in the security fields being adjusted:

in accordance with the first message, if the initial values have been adjusted in accordance with the second message; and

in accordance with the second message, if the initial values have been adjusted in accordance with the first message, and

wherein the action is only performed when the initial values in the security fields have been adjusted in accordance with the first message, and

the first and second adjusted values are different than the third and fourth adjusted values, respectively.

2. A method according to claim 1 , wherein the write restrictions:

prevent the second write from being performed in the event that the first write was previously performed; and

prevent the first write from subsequently being performed in the event that the second write is performed.

3. A method according to claim 1 , wherein the second write is sent in response to a notification that the first write was not successfully performed.

4. A method according to claim 1 , wherein the second write is sent in response to a notification that the first write was not received.

5. A method according to claim 1 , wherein the second write is sent after a predetermined time has elapsed after sending the first write without receiving confirmation of the first write being received or successfully performed.

6. A method according to claim 1 , wherein the messages are sent by a second integrated circuit.

7. A method according to claim 6 , wherein the second integrated circuit is a printer controller.

8. A method according to claim 7 , wherein the first integrated circuit is installed in a cartridge that is releasably attachable to a printer in which the printer controller is installed, such that the messages can be sent via a communications link between the printer controller and cartridge.

9. A method according to claim 8 , wherein the performance of the action causes writing of ink remaining data in a data field of the first integrated circuit as an indicator of ink remaining in the cartridge.