Syntax:
pragma Passive [(Semaphore | No)];
Syntax checked, but otherwise ignored by GNAT. This is recognized for
compatibility with DEC Ada 83 implementations, where it is used within a
task definition to request that a task be made passive. If the argument
Semaphore is present, or the argument is omitted, then DEC Ada 83
treats the pragma as an assertion that the containing task is passive
and that optimization of context switch with this task is permitted and
desired. If the argument No is present, the task must not be
optimized. GNAT does not attempt to optimize any tasks in this manner
(since protected objects are available in place of passive tasks).
For more information on the subject of passive tasks, see the section ’Passive Task Optimization’ in the GNAT Users Guide.