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The purpose of Tightly Coupled Memory (TCM) is to provide low-latency memory that the processor can use without the unpredictability that is a feature of caches.
You can use TCM to hold time-critical routines, such as interrupt handling routines or real-time tasks where the indeterminacy of a cache is highly undesirable. In addition, you can use it to hold ordinary variables, data types whose locality properties are not well suited to caching, and critical data structures such as interrupt stacks.
TCM is used as part of the physical memory map of the system, and does not have to be backed by a level of external memory with the same physical addresses. In such regions, no external writes occur in the event of a write to memory locations contained in the TCM.
For more information, see the Technical Reference Manual for your processor.