operating system loops variable stack question

cout << "blah blah blah";
for (int i=0; i < n; i++)
{
 cout << '#' << endl;;
}
cout << "blah blah blah";

is the integer i on the stack frame? if the integer can only be used in the loop, how does the operating system keep track of what variables can only be used in the 开发者_开发知识库loop and not in the entire function?

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The operating system doesn't have anything to do with it - it's the compiler's job.

Any decent system will keep i in a cpu register

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The variable i is almost certainly placed in a register in this case.

It is the compiler, not the operating system, which enforces the rule that the variable is accessible only within the loop. (It is a compile time rule, not a run time rule.)