Synchronization primitives. STMicroelectronics STM32H7S3L8, STM32H7R3L8, STM32H7S7A8, STM32H7R3V8, STM32H7R3R8, STM32H7S3Z8, STM32H7S3I8, STM32H7S3V8, STM32H7S7Z8, STM32H7R7I8

PM0253 The Cortex-M7 processor

Address A

A+1

A+2

A+3

Figure 9. Little-endian format

7

Memory

0

Register lsbyte

31

B3

24 23

B2

16 15

B1

8 7

B0

0

B0

B1

B2

B3 msbyte

MSv39644V1

The instruction set support for the Cortex ® -M7 processor includes pairs of synchronization primitives . These provide a non-blocking mechanism that a thread or process can use to obtain exclusive access to a memory location. Software can use them to perform a guaranteed read-modify-write memory update sequence, or for a semaphore mechanism.

A pair of synchronization primitives comprises:

A Load-Exclusive instruction

Used to read the value of a memory location, requesting exclusive access to that location.

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 was 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 an exclusive read-modify-write of a memory location, software must:

1.

Use a Load-Exclusive instruction to read the value of the location.

2. Modify the value, as required.

3. Use a Store-Exclusive instruction to attempt to write the new value back to the memory location.

4. Test 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 This indicates that the value returned at step 1 might be out of date.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 entire read-modify-write sequence.

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