If no threads are ready to run or active, or the threads in the system don't do much, then the kernel will wait for something to happen (i.e. an interrupt). It will execute the wfi instruction _WFI() in tx_thread_schedule.c that will cause the CPU core to wait, if you then stop the execution of the code, the debugger will show the execution of the code is at __ISB(), as this is the next instruction after the __WFI() instruction (which has been exectuted).
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