diff --git a/src/include/ngspice/cmconstants.h b/src/include/ngspice/cmconstants.h
index abfd45fa4..73563e14f 100644
--- a/src/include/ngspice/cmconstants.h
+++ b/src/include/ngspice/cmconstants.h
@@ -52,6 +52,7 @@ NON-STANDARD FEATURES
 
 #define ANALOG       MIF_ANALOG
 #define EVENT        MIF_EVENT_DRIVEN
+#define STEP_PENDING MIF_STEP_PENDING
 
 
 #endif
diff --git a/src/include/ngspice/cmproto.h b/src/include/ngspice/cmproto.h
index 7ab6ffbd5..17518160c 100644
--- a/src/include/ngspice/cmproto.h
+++ b/src/include/ngspice/cmproto.h
@@ -97,6 +97,7 @@ int cm_message_printf(const char *fmt, ...);
 double cm_netlist_get_c(void);
 double cm_netlist_get_l(void);
 
+void        cm_irreversible(unsigned int);
 const char *cm_get_node_name(const char *, unsigned int);
 bool        cm_probe_node(unsigned int, unsigned int, void *);
 bool        cm_schedule_output(unsigned int, unsigned int, double, void *);
diff --git a/src/include/ngspice/dllitf.h b/src/include/ngspice/dllitf.h
index 1d9295d81..eaeef3d77 100644
--- a/src/include/ngspice/dllitf.h
+++ b/src/include/ngspice/dllitf.h
@@ -59,6 +59,7 @@ struct coreInfo_t {
 	int       ((*dllitf_cm_message_send)(char *));
 	double    ((*dllitf_cm_netlist_get_c)(void));
 	double    ((*dllitf_cm_netlist_get_l)(void));
+        void      ((*dllitf_cm_irreversible)(unsigned int));
     const char *  ((*dllitf_cm_get_node_name)(const char *, unsigned int));
     bool          ((*dllitf_cm_probe_node)(unsigned int, unsigned int,
                                            void *));
diff --git a/src/include/ngspice/evtproto.h b/src/include/ngspice/evtproto.h
index 83066409b..ea2bd5f92 100644
--- a/src/include/ngspice/evtproto.h
+++ b/src/include/ngspice/evtproto.h
@@ -94,6 +94,8 @@ void EVTdequeue(CKTcircuit *ckt, double time);
 
 int EVTload(CKTcircuit *ckt, MIFinstance *inst);
 
+int EVTload_with_event(CKTcircuit *ckt, MIFinstance *inst, Mif_Call_Type_t type);
+
 void EVTprint(wordlist *wl);
 void EVTprintvcd(wordlist *wl);
 void EVTsave(wordlist *wl);
diff --git a/src/include/ngspice/mifdefs.h b/src/include/ngspice/mifdefs.h
index 801166f44..c3564e754 100644
--- a/src/include/ngspice/mifdefs.h
+++ b/src/include/ngspice/mifdefs.h
@@ -80,6 +80,7 @@ struct MIFinstance {
 
     Mif_Boolean_t       analog;           /* true if this inst is analog or hybrid type */
     Mif_Boolean_t       event_driven;     /* true if this inst is event-driven or hybrid type */
+    unsigned int        irreversible;     /* non-zero for special treatment */
 
     int                 inst_index;       /* Index into inst_table in evt struct in ckt */
     Mif_Callback_t      callback;         /* instance callback function */
diff --git a/src/include/ngspice/miftypes.h b/src/include/ngspice/miftypes.h
index aa8c6f4b5..532b884e1 100644
--- a/src/include/ngspice/miftypes.h
+++ b/src/include/ngspice/miftypes.h
@@ -75,6 +75,7 @@ typedef enum {
 typedef enum {
     MIF_ANALOG,         /* Analog call */
     MIF_EVENT_DRIVEN,   /* Event-driven call */
+    MIF_STEP_PENDING,   /* Special event call for irreversible Code Models */
 } Mif_Call_Type_t;
 
 
diff --git a/src/xspice/cm/cm.c b/src/xspice/cm/cm.c
index 61ba08819..8a3a283e6 100644
--- a/src/xspice/cm/cm.c
+++ b/src/xspice/cm/cm.c
@@ -38,6 +38,7 @@ INTERFACES
     cm_get_path()
     cm_get_circuit()
 
+    cm_irreversible()
     cm_get_node_name()
     cm_probe_node()
 
@@ -57,6 +58,7 @@ NON-STANDARD FEATURES
 #include "ngspice/enh.h"
 #include "ngspice/mif.h"
 #include "ngspice/cktdefs.h"
+#include "ngspice/cpextern.h"
 //#include "util.h"
 
 
@@ -730,6 +732,100 @@ CKTcircuit *cm_get_circuit(void)
     return(g_mif_info.ckt);
 }
 
+/* Set the "irreversible" flag on the current instance and shuffle it to the
+ * requested position among any other irreversibles in the hybrid_index array.
+ * Array entries are sorted so that non-zero values of instance->irreversible
+ * are decreasing: an instance with instance->irreversible == 1 is fully
+ * protected.
+ */
+
+static void duplicate(MIFinstance *instance)
+{
+    fprintf(cp_err,
+            "Warning: Duplicate value %d in cm_irreversible() "
+            "for instance %s.\n",
+            instance->irreversible, instance->gen.GENname);
+}
+
+void cm_irreversible(unsigned int place)
+{
+    MIFinstance      *instance;
+    Evt_Ckt_Data_t   *evt;
+    int               num_hybrids;
+    MIFinstance     **hybrids;
+    int               old_index, i;
+    unsigned int      value;
+
+    instance = g_mif_info.instance;
+    if (!g_mif_info.circuit.init) {
+        fprintf(cp_err,
+                "%s: Ignoring call to cm_irreversible(): not in INIT\n",
+                instance->gen.GENname);
+        return;
+    }
+    if (instance->irreversible || place == 0) {
+        if (instance->irreversible != place) {
+            fprintf(cp_err, "%s: Ignoring new value %d in cm_irreversible()\n",
+                   instance->gen.GENname, place);
+        }
+        return;
+    }
+    instance->irreversible = place;
+
+    evt = g_mif_info.ckt->evt;
+    num_hybrids = evt->counts.num_hybrids;
+    hybrids = evt->info.hybrids;
+
+    /* Already a hybrid? */
+
+    for (old_index = 0; old_index < num_hybrids; ++old_index) {
+        if (hybrids[old_index] == instance)
+            break;
+    }
+
+    if (old_index < num_hybrids) {
+        /* Existing hybrid, move down, shuffling other entries up. */
+
+        for (i = old_index + 1; i < num_hybrids; ++i) {
+            value = hybrids[i]->irreversible;
+            if (value == 0 || value > place) {
+                hybrids[i - 1] = hybrids[i];
+            } else if (value == place) {
+                duplicate(instance);
+                break;
+            } else {
+                break;
+            }
+        }
+        hybrids[i - 1] = instance;
+    } else {
+        /* Instance is not hybrid, add an entry. */
+
+        num_hybrids++;
+        hybrids = TREALLOC(MIFinstance *, hybrids, num_hybrids);
+        evt->counts.num_hybrids = num_hybrids;
+        evt->info.hybrids = hybrids;
+        if (hybrids == NULL) {
+            fprintf(cp_err, "Allocation failed in cm_irreversible()\n");
+            abort();
+        }
+
+        /* Shuffle entries down. */
+
+        for (i = num_hybrids - 2; i >= 0; --i) {
+            value = hybrids[i]->irreversible;
+            if (value != 0 && value < place) {
+                hybrids[i + 1] = hybrids[i];
+            } else if (value == place) {
+                duplicate(instance);
+            } else {
+                break;
+            }
+        }
+        hybrids[i + 1] = instance;
+    }
+}
+
 /* Get the name of a circuit node connected to a port. */
 
 const char *cm_get_node_name(const char *port_name, unsigned int index)
diff --git a/src/xspice/cm/cmexport.c b/src/xspice/cm/cmexport.c
index a644cb921..eb7846363 100644
--- a/src/xspice/cm/cmexport.c
+++ b/src/xspice/cm/cmexport.c
@@ -58,6 +58,7 @@ struct coreInfo_t  coreInfo =
   cm_message_send,
   cm_netlist_get_c,
   cm_netlist_get_l,
+  cm_irreversible,
   cm_get_node_name,
   cm_probe_node,
   cm_schedule_output,
diff --git a/src/xspice/evt/evtcall_hybrids.c b/src/xspice/evt/evtcall_hybrids.c
index 1427b746d..a45f2506e 100644
--- a/src/xspice/evt/evtcall_hybrids.c
+++ b/src/xspice/evt/evtcall_hybrids.c
@@ -20,10 +20,12 @@ MODIFICATIONS
 SUMMARY
 
     This file contains function EVTcall_hybrids which calls all models
-    which have both analog and event-driven ports.  It is called following
+    which have both analog and event-driven ports or have declared
+    themselves to be irreversible (no back-out).  It is called following
     successful evaluation of an analog iteration attempt to allow
     events to be scheduled by the hybrid models.  The 'CALL_TYPE' is set
-    to 'EVENT_DRIVEN' when the model is called from this function.
+    to 'EVENT_DRIVEN' or 'STEP_PENDING' when the model is called
+    from this function.
 
 INTERFACES
 
@@ -71,6 +73,7 @@ void EVTcall_hybrids(
     hybrids = ckt->evt->info.hybrids;
 
     /* Call EVTload for all hybrids */
+
     for(i = 0; i < num_hybrids; i++)
-        EVTload(ckt, hybrids[i]);
+        EVTload_with_event(ckt, hybrids[i], MIF_STEP_PENDING);
 }
diff --git a/src/xspice/evt/evtload.c b/src/xspice/evt/evtload.c
index edce5e119..a0b75326a 100644
--- a/src/xspice/evt/evtload.c
+++ b/src/xspice/evt/evtload.c
@@ -85,6 +85,16 @@ int EVTload(
     CKTcircuit   *ckt,        /* The circuit structure */
     MIFinstance  *inst)       /* The instance to call */
 {
+    return EVTload_with_event(ckt, inst, MIF_EVENT_DRIVEN);
+}
+
+/* "Internal" version, also used by EVTcall_hybrids(). */
+
+int EVTload_with_event(
+    CKTcircuit      *ckt,     /* The circuit structure */
+    MIFinstance     *inst,    /* The instance to call */
+    Mif_Call_Type_t type)     /* Type of call (EVENT or STEP_PENDING). */
+{
 
     int                 i;
     int                 j;
@@ -104,8 +114,7 @@ int EVTload(
     /* Prepare the code model inputs */
     /* ***************************** */
 
-    /* Get pointer to instance data structure and other data */
-    /* needed for fast access */
+    /* Get pointer to data structure needed for fast access */
 
     node_data = ckt->evt->data.node;
 
@@ -124,7 +133,14 @@ int EVTload(
     else
         cm_data.circuit.time = 0.0;
 
-    cm_data.circuit.call_type = MIF_EVENT_DRIVEN;
+    /* Instances that have declared themselves as irreversible
+     * are expected to distinguish STEP_PENDING from ordinary events.
+     */
+
+    if (type == MIF_STEP_PENDING && inst->irreversible)
+        cm_data.circuit.call_type = MIF_STEP_PENDING;
+    else
+        cm_data.circuit.call_type = MIF_EVENT_DRIVEN;
     cm_data.circuit.temperature = ckt->CKTtemp - 273.15;
 
     /* Setup data needed by cm_... functions */
@@ -141,9 +157,11 @@ int EVTload(
 
 
     /* If after initialization and in transient analysis mode */
-    /* create a new state for the instance */
+    /* create a new state for the instance, */
+    /* except analog-only irreversibles. */
 
-    if((g_mif_info.circuit.anal_type == MIF_TRAN) && inst->initialized)
+    if((g_mif_info.circuit.anal_type == MIF_TRAN) && inst->initialized &&
+       inst->inst_index >= 0)
         EVTcreate_state(ckt, inst->inst_index);
 
     /* Loop through all connections on the instance and setup */
diff --git a/src/xspice/icm/dlmain.c b/src/xspice/icm/dlmain.c
index 87f6050f9..6c93d4c4b 100644
--- a/src/xspice/icm/dlmain.c
+++ b/src/xspice/icm/dlmain.c
@@ -340,6 +340,11 @@ double cm_netlist_get_l(void) {
 	return (coreitf->dllitf_cm_netlist_get_l)();
 }
 
+void  cm_irreversible(unsigned int place)
+{
+	(coreitf->dllitf_cm_irreversible)(place);
+}
+
 const char *cm_get_node_name(const char *port, unsigned int index) {
     return coreitf->dllitf_cm_get_node_name(port, index);
 }