Jon Masters has put together a summary of how atomic operations work on the ARM architecture for those who are not afraid of the grungy details. "To provide for atomic access to a given memory location, ARM processors implement a reservation engine model. A given memory location is first loaded using a special 'load exclusive' instruction that has the side-effect of setting up a reservation against that given address in the CPU-local reservation engine. When the modified value it is later written back into memory, using the corresponding 'store exclusive' processor instruction, the reservation engine verifies that it has an outstanding reservation against that given address, and furthermore confirms that no external agents have interfered with the memory commit. A register returns success or failure."