text: /*
    (c) copyright 1988 by the Vrije Universiteit, Amsterdam, The Netherlands.
    See the copyright notice in the ACK home directory, in the file "Copyright".
  */
  
  /*
    Module:	SYSTEM
    Author:	Ceriel J.H. Jacobs
    Version:	$Id$
  */
  
  /*
      An implementation of the Modula-2 NEWPROCESS and TRANSFER facilities
      using the topsize, topsave, and topload facilities.
      For each coroutine, a proc structure is built. For the main routine,
      a static space is declared to save its stack. For the other coroutines,
      the user specifies this space.
  */
  
  #include <unistd.h>
  #include "libm2.h"
  #include <m2_traps.h>
  
  #define MAXMAIN 2048
  
  static struct proc mainproc[MAXMAIN / sizeof(struct proc) + 1];
  
  static struct proc* curproc = 0; /* current coroutine */
  extern char* MainLB; /* stack break of main routine */
  
  void _SYSTEM__NEWPROCESS(
      int (*p)(void) /* coroutine procedure */,
      struct proc* a /* pointer to area for saved stack-frame */,
      unsigned int n /* size of this area */,
      struct proc** p1 /* where to leave coroutine descriptor,
                     in this implementation the address of
                     the area for saved stack-frame(s) */
      )
  {
  	/*	This procedure creates a new coroutine, but does not
  	    transfer control to it. The routine "topsize" will compute the
  	    stack break, which will be the local base of this routine.
  	    Notice that we can do this because we do not need the stack
  	    above this point for this coroutine. In Modula-2, coroutines
  	    must be level 0 procedures without parameters.
  	*/
  	char* brk = 0;
  	unsigned sz = topsize(&brk);
  
  	if (sz + sizeof(struct proc) > n)
  	{
  		/* not enough space */
  		TRP(M2_TOOLARGE);
  	}
  	a->size = n;
  	a->proc = p;
  	a->brk = brk;
  	*p1 = a;
  	if (topsave(brk, a + 1))
  		/* stack frame saved; now just return */
  		;
  	else
  	{
  		/* We get here through the first transfer to the coroutine
  		   created above.
  		   This also means that curproc is now set to this coroutine.
  		   We cannot trust the parameters anymore.
  		   Just call the coroutine procedure.
  		*/
  		(*(curproc->proc))();
  		_cleanup();
  		_exit(0);
  	}
  }
  
  void _SYSTEM__TRANSFER(struct proc** a, struct proc** b)
  {
  	/*	transfer from one coroutine to another, saving the current
  	    descriptor in the space indicated by "a", and transfering to
  	    the coroutine in descriptor "b".
  	*/
  	unsigned size;
  
  	if (!curproc)
  	{
  		/* the current coroutine is the main process;
  		   initialize a coroutine descriptor for it ...
  		*/
  		mainproc[0].brk = MainLB;
  		mainproc[0].size = sizeof(mainproc);
  		curproc = &mainproc[0];
  	}
  	*a = curproc; /* save current descriptor in "a" */
  	if (*b == curproc)
  	{
  		/* transfer to itself is a no-op */
  		return;
  	}
  	size = topsize(&(curproc->brk));
  	if (size + sizeof(struct proc) > curproc->size)
  	{
  		TRP(M2_TOOLARGE);
  	}
  	if (topsave(curproc->brk, curproc + 1))
  	{
  		/* stack top saved. Now restore context of target
  		   coroutine
  		*/
  		curproc = *b;
  		topload(curproc + 1);
  		/* we never get here ... */
  	}
  	/* but we do get here, when a transfer is done to the coroutine in "a".
  	*/
  }
  