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A process can obtain a lock on a resource by ''enqueueing'' a lock request. This is similar to the [[QIO]] technique that is used to perform I/O. The enqueue lock request can either complete synchronously, in which case the process waits until the lock is granted, or asynchronously, in which case an [[Asynchronous System Trap|AST]] occurs when the lock has been obtained.
It is also possible to establish a ''blocking AST'', which is triggered when a process has obtained a lock that is preventing access to the resource by another process. The original process can then optionally take action to allow the other access (e.g. by demoting or releasing the lock).
===Lock value block===
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