249:. In fact, it is possible for a process to put itself into a "hibernate" state (with the $ HIBER system call), or to wait for an event flag by calling e.g. $ WAITFR, whereupon it does nothing but wait for ASTs to be delivered. When an AST is delivered (triggered by an IO completion, timer, or other event), the process is temporarily taken out of the wait to execute the AST. After the AST procedure completes, the call that put the process into hibernation or the event flag wait is made again; in essence, the reason for the wait is re-evaluated. The only way to get out of this loop (apart from process deletion) is to execute a $ WAKE or $ SETEF system call to satisfy the wait. This can be done by the process itself by invoking $ WAKE or $ SETEF within the AST, or (if a global event flag is used) $ SETEF within another process.
206:. Completion of the I/O can be signalled by the issuance of an AST to the calling process/task. Certain runtime errors could also be signalled using the AST mechanism. Within OpenVMS, Special Kernel-Mode ASTs are used as the standard mechanism for getting relatively convenient access to a process context (including getting the process paged into physical memory as may be needed). These types of ASTs are executed at the highest possible per-process priority the next time the scheduler makes that process current, and are used among other things for retrieving process-level information (in response to a $ GETJPI "getjob/process information" system call) and for performing process deletion.
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278:) it would check the second flag to see if it had become set during this time: if so, then ASTs really have become disabled, and $ SETAST should be called to re-enable them. In the most common case, no ASTs would have become pending during this time, so there would be no need to call $ SETAST at all.
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So for user mode only (the least privileged ring, normally used by ordinary user programs), a pair of bit flags was provided at a predefined user-writable memory location (in per-process "P1" space). The meanings of these two flags could be construed as "don't deliver any ASTs" and "ASTs have been
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needing protection from ASTs was only a few instructions long, then the overhead of making the $ SETAST calls could far outweigh the time to execute those instructions.
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any data that is shared must be safe against corruption if modified at any time by the AST. Otherwise, the data must be guarded by blocking ASTs during
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Internals and Data Structures : Scheduling and Process Control : Version 7.0, Ruth Goldenberg, Saro Saravanan, Denise Dumas,
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family of operating systems is a similar mechanism.
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