NAME
        ObtainSemaphore -- gain exclusive access to a semaphore

   SYNOPSIS
        ObtainSemaphore(signalSemaphore)
                        A0

        void ObtainSemaphore(struct signalsemaphore *);

   FUNCTION
        signal semaphores are used to gain exclusive access to an object.
        ObtainSemaphore is the call used to gain this access.  If another
        user currently has the semaphore locked the call will block until
        the object is available.

        If the current task already has locked the semaphore and attempts to
        lock it again the call will still succeed.  A "nesting count" is
        incremented each time the current owning task of the semaphore calls
        ObtainSemaphore().  This counter is decremented each time
        releasesemaphore() is called.  when the counter returns to zero the
        semaphore is actually released, and the next waiting task is called.

        A queue of waiting tasks is maintained on the stacks of the waiting
        tasks.  Each will be called in turn as soon as the current task
        releases the semaphore.

        signal semaphores are different than procure()/vacate() semaphores.
        The former requires less CPU time, especially if the semaphore is
        not currently locked.  They require very little set up and user
        thought.  The latter flavor of semaphore make no assumptions about
        how they are used -- they are completely general.  Unfortunately
        they are not as efficient as signal semaphores, and require the
        locker to have done some setup before doing the call.

   INPUT
       signalSemaphore -- an initialized signal semaphore structure

   NOTE
        This function preserves all registers (see BUGS).

   BUGS
        Until V37, this function could destroy A0.

   SEE ALSO
        obtainsemaphoreshared(), initsemaphore(), releasesemaphore(),
        attemptsemaphore(), obtainsemaphorelist(), procure(), vacate()