1. In a coprocessing data processing system, a method for selectively changing the flow of the instruction stream of a first data processor from a second data processor, comprising the steps of: receiving a predetermined instruction for execution by the first data processor; providing to the second data processor from the first data processor an indication that the predetermined instruction has been received for execution by the first data processor; receiving from the second data processor a response to the indication provided by the first data processor, the response indicating how the flow of the instruction stream of the first data processor is to be changed; and changing the flow of the instruction stream of the first data processor in accordance with the response received from the second data processor wherein the flow of the instruction stream of the first data processor is controlled by the value of a program counter; wherein the response received by the second data processor indicates a selected value to be loaded into the program counter; and wherein the step of changing the flow of the instruction stream of the first data processor is further characterized as loading into the program counter of the first data processor the value indicated by the response received from the second data processor.