UNIX v7 calling sched()

[Jim Balter]

In article <1991Apr12.172939.6348 at ncsu.edu> miler at osl.csc.ncsu.edu (George Miler) writes:
>  main ()
>  {
>    ......
>    if (newproc())      <====   true, create /etc/init process
>    {
>      copy (/etc/init)
>      return;           <====   exit main, starts copied process
>    }
>    sched ();           <====   never reached if did /etc/init
>  }

Do you understand how

main ()
{
  if (fork() == 0)
  {
    childstuff();
    return;
  }
  parentstuff();
}

works?  newproc() is just the kernel level equivalent of fork.
It returns twice, into two different processes, with a 0 return to the parent
and a non-zero return to the child (fork is the other way around).

Generally, newproc will create a new process that is pretty much a copy of the
current process, and set the pc of the new process to point to some code that
will do a return 1.  Since the stack is copied, that return will return to
the caller of newproc.  So the flow of control looks something like

<proc 0>
main
	newproc
		creates new process, returns 0
	sched
		look for schedulable processes
		find proc 1, start it running		

<proc 1>
[main]
	[newproc]
		return 1
	exec init
	at some point, init sleeps or forks to create a new process,
	 which causes sched to be resumed

<proc 0>
[main]
	[sched]
		look for schedulable processes, start one running

<proc n>
.
.
.


Basically, you need to understand the concept of concurrent processes and
how they function under UNIX.  Since you are in a class, why not get your
instructor or a fellow student to help you?