Generally speaking, an atomic action is one which cannot be divided; it is guaranteed to either finish itself completely, or else return to the state before it began.
In the context of operating systems programming, an atomic action is one which cannot be interrupted, either because it is performed by a single CPU instruction that locks the memory bus, or because it blocks the interrupt mechanism while the operation is running. Atomic actions are important in issues of process synchronization and Mutual Exclusion.
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