What does an atomic test-and-set instruction do?
Correct answer A. Returns the old value of a memory word and sets it to true as one indivisible operation
Explanation
test_and_set reads a boolean and writes true in a single hardware operation that cannot be interrupted or interleaved with another CPU's access. A lock built on it spins while the returned old value is true; a thread that gets back false has acquired the lock. Because the read and write are indivisible, two threads can never both see false and enter together.
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