 
Summary: Beyond Atomic Registers: Bounded WaitFree
Implementations of Nontrivial Objects \Lambda
James H. Anderson y Bojan GroŸselj z
Department of Computer Science
The University of Maryland at College Park
College Park, Maryland 20742
January 1991
Revised September 1991, June 1992
Abstract
We define a class of operations called pseudo readmodifywrite (PRMW) operations, and show that non
trivial shared data objects with such operations can be implemented in a bounded, waitfree manner from
atomic registers. A PRMW operation is similar to a ``true'' readmodifywrite (RMW) operation in that it
modifies the value of a shared variable based upon the original value of that variable. However, unlike an
RMW operation, a PRMW operation does not return the value of the variable that it modifies. We consider
a class of shared data objects that can either be read, written, or modified by an associative, commutative
PRMW operation, and show that any object in this class can be implemented without waiting from atomic
registers. The implementations that we present are polynomial in both space and time and thus are an
improvement over previously published ones, all of which have unbounded space complexity.
Keywords: assertional reasoning, atomicity, atomic register, composite register, concurrency, counter, lin
earizability, readmodifywrite, shared variable, snapshot, UNITY
