SXM

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Stream X-Machine

The Stream X-machine (SXM) is a model of computation introduced by Gilbert Laycock in his 1993 PhD thesis, The Theory and Practice of Specification Based Software Testing. Based on Samuel Eilenberg's X-machine, an extended finite state machine for processing data of the type X, the Stream X-Machine is a kind of X-machine for processing a memory data type Mem with associated input and output streams In* and Out*, that is, where X = Out* × Mem × In*. The transitions of a Stream X-Machine are labelled by functions of the form φ: Mem × In → Out × Mem, that is, which compute an output value and update the memory, from the current memory and an input value.

Although the general X-machine had been identified in the 1980s as a potentially useful formal model for specifying software systems, it was not until the emergence of the Stream X-Machine that this idea could be fully exploited. Florentin Ipate and Mike Holcombe went on to develop a theory of complete functional testing, in which complex software systems with hundreds of thousands of states and millions of transitions could be decomposed into separate SXMs that could be tested exhaustively, with a guaranteed proof of correct integration.

SXM (transactional memory)

SXM is a software transactional memory system under development at Microsoft Research. The SXM API is exposed via managed code and is used to synchronize access to shared memory without using locks. It achieves mutual exclusion using database-like atomicity transactions. SXM uses the Reflection and dynamic code generation capabilities of .NET Framework to emit CIL code to enable atomic transactions on shared data structures directly at runtime. SXM has been written in C#.

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