Processor communication tokens

The invention provides a method of transmitting messages over an interconnect between processors, each message comprising a header token specifying a destination processor and at least one of a data token and a control token. The method comprises: executing a first instruction on a first one of the processors to generate a data token comprising a byte of data and at least one additional bit to identify that token as a data token, and outputting the data token from the first processor onto the interconnect as part of one of the messages. The method also comprises executing a second instruction on said first processor to generate a control token comprising a byte of control information and at least one additional bit to identify that token as a control token, and outputting the control token from the first processor onto the interconnect as part of one of the messages.