/* -*-C-*-

$Id: trap.h,v 9.53 2007/01/05 21:19:25 cph Exp $

Copyright (C) 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994,
    1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
    2006, 2007 Massachusetts Institute of Technology

This file is part of MIT/GNU Scheme.

MIT/GNU Scheme is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at
your option) any later version.

MIT/GNU Scheme is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

You should have received a copy of the GNU General Public License
along with MIT/GNU Scheme; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301,
USA.

*/

#ifndef SCM_TRAP_H
#define SCM_TRAP_H

/* Kinds of traps:

   For efficiency, some traps are immediate, while some are
   pointer objects.  The type code is multiplexed, and the
   garbage collector handles it specially.

   The odd-numbered traps used to be "dangerous" versions of the
   even-numbered ones, but all that complexity has been flushed.  */

typedef unsigned long trap_kind_t;

/* The following are immediate traps: */
#define TRAP_UNASSIGNED				0
#define TRAP_UNBOUND				2
#define TRAP_EXPENSIVE				6
/* TRAP_MAX_IMMEDIATE is defined in const.h */

/* The following are non-immediate traps: */
#define TRAP_COMPILER_CACHED			14
#define TRAP_MACRO				15

/* Usages of the above traps:
   TRAP_UNASSIGNED can appear in a value cell or a cache.
   TRAP_UNBOUND can appear in the following locations:
     * The value cell of a global variable.  All symbols initially
       have their value cell set to UNBOUND_OBJECT.
     * A cache that is in the value cell of a global variable.  This
       is like the previous case except that some compiled code has
       referenced the unbound variable.
     * The value cell of a procedure's argument frame.  This is caused
       by calling unbind_variable on a procedure's argument.
     * A cache that is not stored in an environment.  This is caused
       by referring to an unbound variable in an environment that does
       not inherit from the global environment.
   TRAP_EXPENSIVE can only appear in a "clone" cache.  This causes
     assignments to this cache to trap out to the microcode, where the
     updating of the variable's associated UUO links can be performed.
   TRAP_COMPILER_CACHED can only appear in a value cell.  It is used
     to associate a cache with the variable.
   TRAP_MACRO can appear in a value cell or a cache.  It is used for
   storage of macro transformers in the environment structure.
*/

/* The following never appear in value cells.  */
/* NON_TRAP_KIND is returned by get_trap_kind when its argument is not
   a reference trap object.  */
#define NON_TRAP_KIND				32

/* The garbage collector knows that pointers of type CACHE_TYPE point
   to three words of storage, because these pointers are embedded in
   compiled-code linkage sections (TC_LINKAGE_SECTION) without types.
   */
#define CACHE_TYPE				TC_HUNK3
#define CACHE_REFERENCES_TYPE			TC_HUNK3

#if (SIZEOF_UNSIGNED_LONG == 4)	/* 32 bit objects */
#  if (TYPE_CODE_LENGTH == 8)
#    define UNASSIGNED_OBJECT	0x32000000
#    define UNBOUND_OBJECT	0x32000002
#    define EXPENSIVE_OBJECT	0x32000006
#  endif
#  if (TYPE_CODE_LENGTH == 6)
#    define UNASSIGNED_OBJECT	0xc8000000
#    define UNBOUND_OBJECT	0xc8000002
#    define EXPENSIVE_OBJECT	0xc8000006
#  endif
#  if (TC_REFERENCE_TRAP != 0x32)
#    include "error: trap.h and types.h are inconsistent"
#  endif
#endif

#ifndef UNASSIGNED_OBJECT	/* Safe version */
#  define UNASSIGNED_OBJECT (MAKE_OBJECT (TC_REFERENCE_TRAP, TRAP_UNASSIGNED))
#  define UNBOUND_OBJECT    (MAKE_OBJECT (TC_REFERENCE_TRAP, TRAP_UNBOUND))
#  define EXPENSIVE_OBJECT  (MAKE_OBJECT (TC_REFERENCE_TRAP, TRAP_EXPENSIVE))
#endif

#endif /* not SCM_TRAP_H */
