Asynchronous System Trap

Asynchronous System Trap (AST) refers to a mechanism used in several computer operating systems designed by the former Digital Equipment Corporation (DEC) of Maynard, Massachusetts. Various events within these systems can be optionally signalled back to the user processes via the AST mechanism. These ASTs act like subroutine calls but they are delivered asynchronously, that is, without any regard to the context of the main thread. Because of this, care must be taken: to ensure that any code that is shared between the main thread and the AST must be designed to be reentrant, and 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 critical sections. ASTs are most commonly encountered as a result of issuing QIO calls to the kernel. 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. The following operating systems implement ASTs: RSX-11 (including all of the variants) RSTS/E OpenVMS ASTs are roughly analogous to Unix signals. The important differences are: There are no "signal codes" assigned to ASTs: instead of assigning a handler to a signal code and raising that code, the AST is specified directly by its address. This allows any number of ASTs to be pending at once (subject to process quotas). ASTs never abort any system call in progress.