diff --git a/vvp/Makefile.in b/vvp/Makefile.in
index 0dd437501..3958a871b 100644
--- a/vvp/Makefile.in
+++ b/vvp/Makefile.in
@@ -16,7 +16,7 @@
 #    59 Temple Place - Suite 330
 #    Boston, MA 02111-1307, USA
 #
-#ident "$Id: Makefile.in,v 1.23 2001/06/12 03:53:11 steve Exp $"
+#ident "$Id: Makefile.in,v 1.24 2001/06/21 22:54:12 steve Exp $"
 #
 #
 SHELL = /bin/sh
@@ -58,8 +58,8 @@ distclean: clean
 check: all
 	./vvp -M../vpi $(srcdir)/examples/hello.vvp | grep 'Hello, World.'
 
-V = vpi_modules.o vpi_const.o vpi_iter.o vpi_mcd.o vpi_priv.o \
-vpi_scope.o vpi_signal.o vpi_tasks.o vpi_time.o vpi_memory.o \
+V = vpi_modules.o vpi_callback.o vpi_const.o vpi_iter.o vpi_mcd.o \
+vpi_priv.o vpi_scope.o vpi_signal.o vpi_tasks.o vpi_time.o vpi_memory.o \
 vpi_vthr_vector.o vvp_vpi.o
 
 O = main.o parse.o parse_misc.o lexor.o arith.o bufif.o compile.o debug.o \
diff --git a/vvp/functor.cc b/vvp/functor.cc
index b15ae0802..b26425da6 100644
--- a/vvp/functor.cc
+++ b/vvp/functor.cc
@@ -17,13 +17,14 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: functor.cc,v 1.22 2001/05/31 04:12:43 steve Exp $"
+#ident "$Id: functor.cc,v 1.23 2001/06/21 22:54:12 steve Exp $"
 #endif
 
 # include  "functor.h"
 # include  "udp.h"
 # include  "schedule.h"
 # include  "vthread.h"
+# include  "vpi_priv.h"
 # include  "debug.h"
 # include  <assert.h>
 
@@ -290,6 +291,11 @@ void functor_set(vvp_ipoint_t ptr, unsigned bit, unsigned str, bool push)
 	    break;
       }
 
+      if (fp->callback) {
+	    fp->callback = 0;
+	    vpip_trip_functor_callbacks(ptr);
+      }
+
 #if defined(WITH_DEBUG)
       if (fp->breakpoint)
 	    breakpoint();
@@ -359,6 +365,9 @@ const unsigned char ft_var[16] = {
 
 /*
  * $Log: functor.cc,v $
+ * Revision 1.23  2001/06/21 22:54:12  steve
+ *  Support cbValueChange callbacks.
+ *
  * Revision 1.22  2001/05/31 04:12:43  steve
  *  Make the bufif0 and bufif1 gates strength aware,
  *  and accurately propagate strengths of outputs.
diff --git a/vvp/functor.h b/vvp/functor.h
index 9d8c6a345..38b5791a6 100644
--- a/vvp/functor.h
+++ b/vvp/functor.h
@@ -19,7 +19,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: functor.h,v 1.25 2001/06/19 03:01:10 steve Exp $"
+#ident "$Id: functor.h,v 1.26 2001/06/21 22:54:12 steve Exp $"
 #endif
 
 # include  "pointers.h"
@@ -140,7 +140,8 @@ struct functor_s {
       unsigned odrive1    : 3;
 	/* Strength form of the output value. */
       unsigned ostr       : 8;
-
+	/* set this flag if there might be a waiting callback. */
+      unsigned callback   : 1;
 #if defined(WITH_DEBUG)
 	/* True if this functor triggers a breakpoint. */
       unsigned breakpoint : 1;
@@ -294,6 +295,9 @@ extern const unsigned char ft_var[];
 
 /*
  * $Log: functor.h,v $
+ * Revision 1.26  2001/06/21 22:54:12  steve
+ *  Support cbValueChange callbacks.
+ *
  * Revision 1.25  2001/06/19 03:01:10  steve
  *  Add structural EEQ gates (Stephan Boettcher)
  *
diff --git a/vvp/vpi_callback.cc b/vvp/vpi_callback.cc
new file mode 100644
index 000000000..dbef2eed6
--- /dev/null
+++ b/vvp/vpi_callback.cc
@@ -0,0 +1,147 @@
+/*
+ * Copyright (c) 2001 Stephen Williams (steve@icarus.com)
+ *
+ *    This source code is free software; you can redistribute it
+ *    and/or modify it in source code form 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.
+ *
+ *    This program 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 this program; if not, write to the Free Software
+ *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
+ */
+#if !defined(WINNT)
+#ident "$Id: vpi_callback.cc,v 1.1 2001/06/21 22:54:12 steve Exp $"
+#endif
+
+/*
+ * Callbacks are objects that carry a function to be called when some
+ * event in the simulation occurs. The VPI code create a __vpiCallback
+ * object, and that object is put in some location that the simulation
+ * can look when the event in question is tripped.
+ */
+
+# include  <vpi_user.h>
+# include  "vpi_priv.h"
+# include  "schedule.h"
+# include  <stdio.h>
+# include  <assert.h>
+
+
+const struct __vpirt callback_rt = {
+      vpiCallback,
+
+      0,
+      0,
+      0,
+      0,
+
+      0,
+      0,
+      0
+};
+
+/*
+ * A value change callback is tripped when a bit of a signal
+ * changes. This function creates that value change callback and
+ * attaches it to the relevent vpiSignal object. Also flag the
+ * functors associated with the signal so that they know to trip.
+ */
+static struct __vpiCallback* make_value_change(p_cb_data data)
+{
+      struct __vpiCallback*obj = new __vpiCallback;
+      obj->base.vpi_type = &callback_rt;
+      obj->cb_data = *data;
+
+      assert((data->obj->vpi_type->type_code == vpiReg)
+	     || (data->obj->vpi_type->type_code == vpiNet));
+      struct __vpiSignal*sig = reinterpret_cast<__vpiSignal*>(data->obj);
+
+	/* Attach the callback to the signal who's value I'm waiting for. */
+      obj->next = sig->callbacks;
+      sig->callbacks = obj;
+
+      unsigned wid = (sig->msb >= sig->lsb)
+	    ? sig->msb - sig->lsb + 1
+	    : sig->lsb - sig->msb + 1;
+
+	/* Make sure the functors are tickled to trigger a callback. */
+      for (unsigned idx = 0 ;  idx < wid ;  idx += 1) {
+	    vvp_ipoint_t ptr = ipoint_index(sig->bits, idx);
+	    functor_t fun = functor_index(ptr);
+	    fun->callback |= 1;
+      }
+
+      return obj;
+}
+
+vpiHandle vpi_register_cb(p_cb_data data)
+{
+      struct __vpiCallback*obj = 0;
+
+      switch (data->reason) {
+
+	  case cbValueChange:
+	    obj = make_value_change(data);
+	    break;
+
+	  default:
+	    fprintf(stderr, "vpi error: vpi_register_cb invalid or "
+		    "unsupported callback reason: %d\n",
+		    data->reason);
+	    break;
+      }
+
+      return obj? &obj->base : 0;
+}
+
+
+int vpi_remove_cb(vpiHandle ref)
+{
+      assert(ref->vpi_type->type_code == vpiCallback);
+
+      struct __vpiCallback*obj = reinterpret_cast<__vpiCallback*>(ref);
+
+      fprintf(stderr, "vpi error: vpi_remove_cb not supported\n");
+
+      return 0;
+}
+
+void vpip_trip_functor_callbacks(vvp_ipoint_t ptr)
+{
+      struct __vpiSignal*sig = vpip_sig_from_ptr(ptr);
+      assert(sig);
+
+      struct __vpiCallback*callbacks = sig->callbacks;
+      sig->callbacks = 0;
+
+      while (callbacks) {
+	    struct __vpiCallback*cur = callbacks;
+	    callbacks = cur->next;
+
+	    cur->cb_data.time->type = vpiSimTime;
+	    cur->cb_data.time->low = schedule_simtime();
+	    cur->cb_data.time->high = 0;
+	    (cur->cb_data.cb_rtn)(&cur->cb_data);
+	    delete cur;
+      }
+}
+
+void vpip_trip_monitor_callbacks(void)
+{
+}
+
+
+/*
+ * $Log: vpi_callback.cc,v $
+ * Revision 1.1  2001/06/21 22:54:12  steve
+ *  Support cbValueChange callbacks.
+ *
+ */
+
diff --git a/vvp/vpi_priv.cc b/vvp/vpi_priv.cc
index dfe5e1e47..4f5260fe4 100644
--- a/vvp/vpi_priv.cc
+++ b/vvp/vpi_priv.cc
@@ -17,7 +17,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: vpi_priv.cc,v 1.5 2001/06/12 03:53:11 steve Exp $"
+#ident "$Id: vpi_priv.cc,v 1.6 2001/06/21 22:54:12 steve Exp $"
 #endif
 
 # include  "vpi_priv.h"
@@ -137,20 +137,6 @@ extern "C" void vpi_vprintf(const char*fmt, va_list ap)
       vprintf(fmt, ap);
 }
 
-/* STUBS */
-
-vpiHandle vpi_register_cb(p_cb_data data)
-{
-      fprintf(stderr, "vpi Sorry: vpi_register_cb not supported\n");
-      return 0;
-}
-
-
-int vpi_remove_cb(vpiHandle ref)
-{
-      assert(0);
-      return 0;
-}
 
 extern "C" void vpi_sim_vcontrol(int operation, va_list ap)
 {
@@ -166,6 +152,9 @@ extern "C" void vpi_sim_vcontrol(int operation, va_list ap)
 
 /*
  * $Log: vpi_priv.cc,v $
+ * Revision 1.6  2001/06/21 22:54:12  steve
+ *  Support cbValueChange callbacks.
+ *
  * Revision 1.5  2001/06/12 03:53:11  steve
  *  Change the VPI call process so that loaded .vpi modules
  *  use a function table instead of implicit binding.
diff --git a/vvp/vpi_priv.h b/vvp/vpi_priv.h
index d60b38a43..be8b22d03 100644
--- a/vvp/vpi_priv.h
+++ b/vvp/vpi_priv.h
@@ -19,7 +19,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: vpi_priv.h,v 1.16 2001/05/20 00:46:12 steve Exp $"
+#ident "$Id: vpi_priv.h,v 1.17 2001/06/21 22:54:12 steve Exp $"
 #endif
 
 # include  "vpi_user.h"
@@ -109,11 +109,31 @@ struct __vpiSignal {
       unsigned signed_flag  : 1;
 	/* The represented value is here. */
       vvp_ipoint_t bits;
+	/* Call these items on a callback event. */
+      struct __vpiCallback*callbacks;
+	/* Keep in a binary tree, ordered by bits member. */
+      struct __vpiSignal* by_bits[2];
 };
 extern vpiHandle vpip_make_reg(char*name, int msb, int lsb, bool signed_flag,
 			       vvp_ipoint_t base);
 extern vpiHandle vpip_make_net(char*name, int msb, int lsb, bool signed_flag,
 			       vvp_ipoint_t base);
+extern struct __vpiSignal*vpip_sig_from_ptr(vvp_ipoint_t ptr);
+
+/*
+ * Callback handles are created when the VPI function registers a
+ * callback. The handle is stored by the run time, and it triggered
+ * when the run-time thing that it is waiting for happens.
+ */
+struct __vpiCallback {
+      struct __vpiHandle base;
+
+      struct t_cb_data cb_data;
+
+      struct __vpiCallback*next;
+};
+
+extern void vpip_trip_functor_callbacks(vvp_ipoint_t ptr);
 
 /*
  * Memory is an array of bits that is accessible in N-bit chunks, with
@@ -231,6 +251,9 @@ vpiHandle vpip_sim_time(void);
 
 /*
  * $Log: vpi_priv.h,v $
+ * Revision 1.17  2001/06/21 22:54:12  steve
+ *  Support cbValueChange callbacks.
+ *
  * Revision 1.16  2001/05/20 00:46:12  steve
  *  Add support for system function calls.
  *
@@ -243,46 +266,5 @@ vpiHandle vpip_sim_time(void);
  * Revision 1.14  2001/05/08 23:59:33  steve
  *  Add ivl and vvp.tgt support for memories in
  *  expressions and l-values. (Stephan Boettcher)
- *
- * Revision 1.13  2001/04/18 04:21:23  steve
- *  Put threads into scopes.
- *
- * Revision 1.12  2001/04/05 01:34:26  steve
- *  Add the .var/s and .net/s statements for VPI support.
- *
- * Revision 1.11  2001/04/04 17:43:19  steve
- *  support decimal strings from signals.
- *
- * Revision 1.10  2001/04/04 04:33:09  steve
- *  Take vector form as parameters to vpi_call.
- *
- * Revision 1.9  2001/04/02 00:24:31  steve
- *  Take numbers as system task parameters.
- *
- * Revision 1.8  2001/03/31 19:00:44  steve
- *  Add VPI support for the simulation time.
- *
- * Revision 1.7  2001/03/25 00:35:35  steve
- *  Add the .net statement.
- *
- * Revision 1.6  2001/03/23 02:40:23  steve
- *  Add the :module header statement.
- *
- * Revision 1.5  2001/03/21 05:13:03  steve
- *  Allow var objects as vpiHandle arguments to %vpi_call.
- *
- * Revision 1.4  2001/03/20 06:16:24  steve
- *  Add support for variable vectors.
- *
- * Revision 1.3  2001/03/18 04:35:18  steve
- *  Add support for string constants to VPI.
- *
- * Revision 1.2  2001/03/18 00:37:55  steve
- *  Add support for vpi scopes.
- *
- * Revision 1.1  2001/03/16 01:44:34  steve
- *  Add structures for VPI support, and all the %vpi_call
- *  instruction. Get linking of VPI modules to work.
- *
  */
 #endif
diff --git a/vvp/vpi_signal.cc b/vvp/vpi_signal.cc
index 447ee6b7f..48ba124a2 100644
--- a/vvp/vpi_signal.cc
+++ b/vvp/vpi_signal.cc
@@ -17,7 +17,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: vpi_signal.cc,v 1.15 2001/05/30 03:02:35 steve Exp $"
+#ident "$Id: vpi_signal.cc,v 1.16 2001/06/21 22:54:12 steve Exp $"
 #endif
 
 /*
@@ -366,6 +366,58 @@ static vpiHandle signal_put_value(vpiHandle ref, s_vpi_value*vp,
 }
 
 
+static struct __vpiSignal*by_bits_root = 0;
+static void by_bits_insert(struct __vpiSignal*sig)
+{
+      if (by_bits_root == 0) {
+	    by_bits_root = sig;
+	    return;
+      }
+
+      struct __vpiSignal*cur = by_bits_root;
+      for (;;) {
+	    if (cur->bits > sig->bits) {
+		  if (cur->by_bits[0] == 0) {
+			cur->by_bits[0] = sig;
+			break;
+		  }
+		  cur = cur->by_bits[0];
+
+	    } else {
+		  if (cur->by_bits[1] == 0) {
+			cur->by_bits[1] = sig;
+			break;
+		  }
+		  cur = cur->by_bits[1];
+	    }
+      }
+}
+
+struct __vpiSignal* vpip_sig_from_ptr(vvp_ipoint_t ptr)
+{
+      struct __vpiSignal*cur = by_bits_root;
+
+      while (cur) {
+	    if (ptr < cur->bits) {
+		  cur = cur->by_bits[0];
+		  continue;
+	    }
+
+	    unsigned wid = (cur->msb > cur->lsb)
+		  ? cur->msb - cur->lsb
+		  : cur->lsb - cur->msb;
+
+	    if (ptr > ipoint_index(cur->bits, wid)) {
+		  cur = cur->by_bits[1];
+		  continue;
+	    }
+
+	    return cur;
+      }
+
+      return cur;
+}
+
 static const struct __vpirt vpip_reg_rt = {
       vpiReg,
       signal_get,
@@ -401,9 +453,14 @@ vpiHandle vpip_make_reg(char*name, int msb, int lsb, bool signed_flag,
       obj->lsb = lsb;
       obj->signed_flag = signed_flag? 1 : 0;
       obj->bits = base;
+      obj->callbacks = 0;
+      obj->by_bits[0] = 0;
+      obj->by_bits[1] = 0;
 
       obj->scope = vpip_peek_current_scope();
 
+      by_bits_insert(obj);
+
       return &obj->base;
 }
 
@@ -423,15 +480,23 @@ vpiHandle vpip_make_net(char*name, int msb, int lsb, bool signed_flag,
       obj->lsb = lsb;
       obj->signed_flag = signed_flag? 1 : 0;
       obj->bits = base;
+      obj->callbacks = 0;
+      obj->by_bits[0] = 0;
+      obj->by_bits[1] = 0;
 
       obj->scope = vpip_peek_current_scope();
 
+      by_bits_insert(obj);
+
       return &obj->base;
 }
 
 
 /*
  * $Log: vpi_signal.cc,v $
+ * Revision 1.16  2001/06/21 22:54:12  steve
+ *  Support cbValueChange callbacks.
+ *
  * Revision 1.15  2001/05/30 03:02:35  steve
  *  Propagate strength-values instead of drive strengths.
  *