1. In a cyclically operating data processing system which provides for the concurrent performance of a plurality of tasks in a staged manner over a plurality of cycles, wherein a task may include one or more subroutines for use in performing its respective task, subroutine control means comprising: a multi-level return address means, one for each task, for storing return addresses for the respective task during performance thereof; and return address control means commonly used by said plurality of tasks and including first, second and third means which are concurrently operable during a cycle; said first means being operable during a cycle in response to a subroutine entry indication and a task identification indication provided by said system when a task enters a subroutine for determining a return address for the indicated subroutine, said second means being operable during a cycle for storing a return address determined by said first means during a previous cycle in the multi-level return address storage means of the respective task; and said third means being operable during a cycle in response to a subroutine entry return indication and a task identification indication provided by said system when a task arrives at the end of a routine for accessing a particular return address stored in the respective storage means of the identified task during a previous cycle for application to said system; said return address control means operating during a cycle to permit the determining by said first means of a return address for a first task to be performed concurrently with the storing by said second means of a return address for a second task in its respective return address storage means, and also concurrently with the accessing by said third means of a return address for a third task from its respective return address storage means for transfer to said system, said data processing system choosing the particular tasks for which said first, second and third means operate over a plurality of cycles so as to permit the tasks to time-share use of said first, second and third means for subroutine control using a staging which conforms to the staging of tasks by said system.