Event Flags

Event flags are used when a task needs to synchronize with the occurrence of multiple events. The task can be synchronized when any of the events have occurred. This is called disjunctive synchronization (logical OR). A task can also be synchronized when all events have occurred. This is called conjunctive synchronization (logical AND). Disjunctive and conjunctive synchronization are shown in Figure 2.15.

Common events can be used to signal multiple tasks, as shown in Figure 2.16. Events are typically grouped. Depending on the kernel, a group consists of 8, 16, or 32 events, each reprensnted by a bit. (mostly 32 bits, though). Tasks and ISRs can set or clear any event in a group. A task is resumed when all the events it requires are satisfied. The evaluation of which task will be resumed is performed when a new set of events occurs (i.e., during a SET operation).

Kernels supporting event flags offer services to SET event flags, CLEAR event flags, and WAIT for event flags (conjunctively or disjunctively). |iC/OS-II does not currently support event flags.

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