====== Exclusive Monitor ======


The ''Cortex-M4'' processor implements a **local** exclusive monitor. For more information about
semaphores and the local exclusive monitor see the [[https://developer.arm.com/documentation/ddi0403/latest/|ARMv7M ARM Architecture Reference
Manual]].

Instructions:
  * LDREX - Load-Exclusive
  * STREX - Store-Exclusive
  * CLREX - Clear-Exclusive

Exclusive instructions must be used in **pairs**:
  * LDREX:
    * Tags memory for exclusive access.
    * Always successfully reads a value from memory address x.
  * STREX:
    * Clears tagged memory for exclusive access. 
    * Success only memory was tagger == if no other processor or process performed a more recent store of address x.
    * returns status bit that indicates whether the memory write succeeded.

So LDEX/STREX must be placed in loop, to repeat LDREX/modify/STREX if STREX fails.