# 16.2. Background¶

## 16.2.1. Signal Manager Definitions¶

The signal manager allows a task to optionally define an asynchronous signal routine (ASR). An ASR is to a task what an ISR is to an application’s set of tasks. When the processor is interrupted, the execution of an application is also interrupted and an ISR is given control. Similarly, when a signal is sent to a task, that task’s execution path will be “interrupted” by the ASR. Sending a signal to a task has no effect on the receiving task’s current execution state.

A signal flag is used by a task (or ISR) to inform another task of the occurrence of a significant situation. Thirty-two signal flags are associated with each task. A collection of one or more signals is referred to as a signal set. The data type rtems_signal_set is used to manipulate signal sets.

A signal set is posted when it is directed (or sent) to a task. A pending signal is a signal that has been sent to a task with a valid ASR, but has not been processed by that task’s ASR.

## 16.2.2. A Comparison of ASRs and ISRs¶

The format of an ASR is similar to that of an ISR with the following exceptions:

• ISRs are scheduled by the processor hardware. ASRs are scheduled by RTEMS.

• ISRs do not execute in the context of a task and may invoke only a subset of directives. ASRs execute in the context of a task and may execute any directive.

• When an ISR is invoked, it is passed the vector number as its argument. When an ASR is invoked, it is passed the signal set as its argument.

• An ASR has a task mode which can be different from that of the task. An ISR does not execute as a task and, as a result, does not have a task mode.

## 16.2.3. Building a Signal Set¶

A signal set is built by a bitwise OR of the desired signals. The set of valid signals is RTEMS_SIGNAL_0 through RTEMS_SIGNAL_31. If a signal is not explicitly specified in the signal set, then it is not present. Signal values are specifically designed to be mutually exclusive, therefore bitwise OR and addition operations are equivalent as long as each signal appears exactly once in the component list.

This example demonstrates the signal parameter used when sending the signal set consisting of RTEMS_SIGNAL_6, RTEMS_SIGNAL_15, and RTEMS_SIGNAL_31. The signal parameter provided to the rtems_signal_send directive should be RTEMS_SIGNAL_6 | RTEMS_SIGNAL_15 | RTEMS_SIGNAL_31.

## 16.2.4. Building an ASR Mode¶

In general, an ASR’s mode is built by a bitwise OR of the desired mode components. The set of valid mode components is the same as those allowed with the task_create and task_mode directives. A complete list of mode options is provided in the following table:

 RTEMS_PREEMPT is masked by RTEMS_PREEMPT_MASK and enables preemption RTEMS_NO_PREEMPT is masked by RTEMS_PREEMPT_MASK and disables preemption RTEMS_NO_TIMESLICE is masked by RTEMS_TIMESLICE_MASK and disables timeslicing RTEMS_TIMESLICE is masked by RTEMS_TIMESLICE_MASK and enables timeslicing RTEMS_ASR is masked by RTEMS_ASR_MASK and enables ASR processing RTEMS_NO_ASR is masked by RTEMS_ASR_MASK and disables ASR processing RTEMS_INTERRUPT_LEVEL(0) is masked by RTEMS_INTERRUPT_MASK and enables all interrupts RTEMS_INTERRUPT_LEVEL(n) is masked by RTEMS_INTERRUPT_MASK and sets interrupts level n

Mode values are specifically designed to be mutually exclusive, therefore bitwise OR and addition operations are equivalent as long as each mode appears exactly once in the component list. A mode component listed as a default is not required to appear in the mode list, although it is a good programming practice to specify default components. If all defaults are desired, the mode DEFAULT_MODES should be specified on this call.

This example demonstrates the mode parameter used with the rtems_signal_catch to establish an ASR which executes at interrupt level three and is non-preemptible. The mode should be set to RTEMS_INTERRUPT_LEVEL(3) | RTEMS_NO_PREEMPT to indicate the desired processor mode and interrupt level.