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Transactional memory (TM) has been hailed as a paradigm for simplifying concurrent programming. While several consistency conditions have been suggested for TM, they fall short of formalizing the intuitive semantics of atomic blocks, the interface through which a TM is used in a programming language.To close this gap, we formalize the intuitive expectations of a programmer as observational refinement between TM implementations: a concrete TM observationally refines an abstract one if every user-observable behavior of a program using the former can be reproduced if the program uses the latter. This allows the programmer to reason about the behavior of a program using the intuitive semantics formalized by the abstract TM; the observational refinement relation implies that the conclusions will carry over to the case when the program uses the concrete TM. We show that, for a particular programming language and notions of observable behavior, a variant of the well-known consistency condition of opacity is sufficient for observational refinement, and its restriction to complete histories is furthermore necessary.Our results suggest a new approach to evaluating and comparing TM consistency conditions. They can also reduce the effort of proving that a TM implements its programming language interface correctly, by only requiring its developer to show that it satisfies the corresponding consistency condition.

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