An unpublished algorithm of Haldar and Vidyasankar implements an atomic variable of an arbitrary type T for one writer and one reader by means of 4 unsafe variables of type T, three two-valued safe variables, and one three-valued regular variable. We present this algorithm, and prove its correctness by means of a refinement towards a known specification of an atomic variable. The refinement is a composition of refinement functions and a forward simulation. The correctness proof requires many nontrivial invariants. In its construction, we relied on the proof assistant PVS for the administration of invariants and proofs and the preservation of consistency. (c) 2008 Elsevier B.V. All rights reserved.