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7.4.1. Local Monitor

The L1 memory system of the Cortex-A9 processor has a local monitor. This is a 2-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 to construct semaphores, ensuring synchronization between different processes running on the processor, and also between different processors that are using the same coherent memory locations for the semaphore.


A store exclusive can generate an MMU fault or cause the processor to take a data watchpoint exception regardless of the state of the local monitor. See Table 10.8.

See the ARM Architecture Reference Manual for more information about these instructions.

Treatment of intervening STR operations

In cases where there is an intervening STR operation in an LDREX/STREX code sequence, the intermediate STR does not produce any effect on the internal exclusive monitor. The local monitor is in the Exclusive Access state after the LDREX, remains in the Exclusive Access state after the STR, and returns to the Open Access state only after the STREX.

LDREX/STREX operations using different sizes

In cases where the LDREX and STREX operations are of different sizes a check is performed to ensure that the tagged address bytes match or are within the size range of the store operation.

The granularity of the tagged address for an LDREX instruction is eight words, aligned on an 8-word boundary. This size is implementation-defined, and as such, software must not rely on this granularity remaining constant on other ARM cores.

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