8.6. Internal exclusive monitor

The processor L1 memory system has an internal exclusive monitor. This is a two state, open and exclusive, state machine that manages load/store exclusive (LDREXB, LDREXH, LDREX, LDREXD, STREXB, STREXH, STREX and STREXD) accesses and clear exclusive (CLREX) instructions. You can use these instructions, operating in the L1 memory system, to construct semaphores and ensure synchronization between different processes. By adding an external exclusive monitor, you can also use these instructions in the L2 memory system to construct semaphores and ensure synchronization between different processors. See the ARM Architecture Reference Manual for more information about how these instructions work.

When a load-exclusive access is performed, the internal exclusive monitor moves to the exclusive state. It moves back to the open state when a store exclusive access or clear exclusive instruction is performed. The internal exclusive monitor holds exclusivity state for an individual Cortex-R5 CPU only. It does not record the address of the memory that a load-exclusive access was performed to and it does not observe accesses from the other CPU in a twin-CPU group. Any store exclusive access performed when the state is open fails. If the state is exclusive, the access passes if it is to non-shared memory but, if it is to shared memory, the access must be performed as an exclusive using the L2 memory interface. Whether the shared store-exclusive access passes or fails depends on the state of an external exclusive monitor that can track accesses made by other processors in the system.

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