no, not really from getting cached. the volatile keyword prevents the compiler from optimising away the variable and subsequent code whose access is controlled by that variable based on what value the variable has during static analysis/compilation. Consider a variable that is of type bool, and is set to 'true'. The compiler will see that and go "ok, I can just delete any code that would only be reached if this were false, because this is always going to be true". With volatile you tell the compiler "we don't know what this value will be at runtime, so don't be overly aggressive with your optimisation here, just leave it as is. the variable can change at any time during runtime, it's volatile, it's subject to change without notice"

Global and external are the same thing in C. Great care is needed when sharing variables between threads, as there must be atomic access. Better to use RTOS inter-task communication facilities.