Restricted Stack Implementations

We introduce a new object, BH, and prove that a system with one BH object and single-writer Registers has the same computational power as a system with countably many commutative and overwriting objects. This provides a simple characterization of the class of objects that can be implemented from commutative and overwriting objects, and creates a potential tool for proving impossibility results. It has been conjectured that Stacks and Queues shared by three or more processes are not in this class. In this paper, we use a BH object to show that two different restricted versions of Stacks are in this class. Specifically, we give an implementation of a Stack that supports any number of poppers, but at most two pushers. We also implement a Stack (or Queue) shared by any number of processes, but, in which, all stored elements are the same.