1 (lock primitive) An indivisible operation that allows a process to ensure that it alone has access to a particular resource. On a single-processor system the indivisible nature of the operation can be guaranteed by turning off interrupts during the action, ensuring that no process switch can occur. On a multiprocessing system it is essential to have available a test-and-set instruction that, in a single uninterruptible sequence, can test whether a register's contents are zero, and if they are will make the contents nonzero. The same effect can be achieved by an exchange instruction. See also unlock, semaphore.
2 See locks and keys.
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