1. A data processing apparatus for processing digital data in accordance with a plurality of predetermined tasks of preassigned priority values and identified by a respective plurality of task request signals in order to service a respective plurality of devices connected to said apparatus, each device capable of generating the respective one of said task request signals when requiring service by said apparatus, said apparatus comprising: first memory means for storing a plurality of executable instructions each containing the address of a successor instruction; means for addressing said first memory means to provide for the execution of selected instructions, said addressing means including second memory means organized to contain next addresses of said first memory means in respective locations thereof, each of said locations being dedicated to one of said predetermined tasks; means for executing said selected instructions; means coupled to said addressing means for selecting said locations in said second memory means whereby a particular next address is selected for addressing said first memory means; means coupled to said first memory means for loading the address of a successor instruction contained in an addressed instruction into the dedicated location in said second memory means such that the successor instruction may be subsequently selected for execution; and priority determining means responsive to said task request signals for providing a control signal to said selecting means enabling said selecting means to select the location in said second memory means dedicated to the task with the highest current priority value.