The signal system is designed to support independent simultaneous events,
so several signals can occur at the same time.  Each task has 32
independent signals, 16 of which are pre-allocated for use by the
operating system.  The signals in use by a particular task are represented
as bits in a 32-bit field in its task structure (<exec/tasks.h>).  two
other 32-bit fields in the Task structure indicate which signals the task
is waiting for, and which signals have been received.

Signals are task relative.  A task can only allocate its own signals, and
may only wait on its own signals.  In addition, a task may assign its own
significance to a particular signal.  Signals are not broadcast to all
tasks; they are directed only to individual tasks. A signal has meaning to
the task that defined it and to those tasks that have been informed of its
meaning.

For example, signal bit 12 may indicate a timeout event to one task, but
to another task it may indicate a message arrival event. You can never
wait on a signal that you did not directly or indirectly allocate
yourself, and any other task that wishes to signal you must use a signal
that you allocated.

 signal allocation    waiting for a signal    generating a signal