2.5.2.7.2 A Store-Exclusive Instruction

Used to attempt to write to the same memory location, returning a status bit to a register. If this bit is:
  • 0: it indicates that the thread or process gained exclusive access to the memory, and the write succeeds.
  • 1: it indicates that the thread or process did not gain exclusive access to the memory, and no write is performed.

The pairs of Load-Exclusive and Store-Exclusive instructions are:

  • the word instructions LDREX and STREX
  • the halfword instructions LDREXH and STREXH
  • the byte instructions LDREXB and STREXB.

Software must use a Load-Exclusive instruction with the corresponding Store-Exclusive instruction.

To perform a guaranteed read-modify-write of a memory location, software must:

  1. Use a Load-Exclusive instruction to read the value of the location.
  2. Update the value, as required.
  3. Use a Store-Exclusive instruction to attempt to write the new value back to the memory location, and tests the returned status bit. If this bit is:
    • 0: The read-modify-write completed successfully.
    • 1: No write was performed. This indicates that the value returned at step 1 might be out of date. The software must retry the read-modify-write sequence.

Software can use the synchronization primitives to implement a semaphores as follows:

  1. Use a Load-Exclusive instruction to read from the semaphore address to check whether the semaphore is free.
  2. If the semaphore is free, use a Store-Exclusive to write the claim value to the semaphore address.
  3. If the returned status bit from step2 indicates that the Store-Exclusive succeeded then the software has claimed the semaphore. However, if the Store-Exclusive failed, another process might have claimed the semaphore after the software performed step 1.

The Cortex-M3 processor includes an exclusive access monitor, that tags the fact that the processor has executed a Load-Exclusive instruction. If the processor is part of a multiprocessor system, the system also globally tags the memory locations addressed by exclusive accesses by each processor.

The processor removes its exclusive access tag if:

  • It executes a CLREX instruction
  • It executes a Store-Exclusive instruction, regardless of whether the write succeeds.
  • An exception occurs. This means the processor can resolve semaphore conflicts between different threads.

In a multiprocessor implementation:

  • By executing a CLREX instruction removes only the local exclusive access tag for the processor
  • By executing a Store-Exclusive instruction, or an exception, removes the local exclusive access tags, and all global exclusive access tags for the processor.

For more information about the synchronization primitive instructions, see 2.6.4.8 LDREX and STREX and 2.6.4.9 CLREX.