A master unit and a slave unit in a data processor are coupled together by a data bus. The master unit sends data transactions to the slave unit through the data bus and the slave unit executes the data transactions. The present invention comprises an apparatus and method for executing a data transaction either (1) by executing the data transaction “in order” with respect to other data transactions received by the slave unit, or (2) by executing the data transaction “out of order” with respect to other data transactions received by the slave unit. The master unit assigns a priority identifier to each data transaction. The slave unit reads the priority identifier to determine whether to execute the data transaction “in order” or “out of order” with respect to the other data transactions.