diff --git a/scp.c b/scp.c
index e29feac3..84e17597 100644
--- a/scp.c
+++ b/scp.c
@@ -337,6 +337,7 @@ int sim_aio_update_queue (void)
 {
 int migrated = 0;
 
+AIO_ILOCK;
 if (AIO_QUEUE_VAL != QUEUE_LIST_END) {  /* List !Empty */
     UNIT *q, *uptr;
     int32 a_event_time;
@@ -356,18 +357,22 @@ if (AIO_QUEUE_VAL != QUEUE_LIST_END) {  /* List !Empty */
             }
         else
             a_event_time = uptr->a_event_time;
+        AIO_IUNLOCK;
         uptr->a_activate_call (uptr, a_event_time);
         if (uptr->a_check_completion) {
             sim_debug (SIM_DBG_AIO_QUEUE, sim_dflt_dev, "Calling Completion Check for asynch event on %s\n", sim_uname(uptr));
             uptr->a_check_completion (uptr);
             }
+        AIO_ILOCK;
         }
     }
+AIO_IUNLOCK;
 return migrated;
 }
 
 void sim_aio_activate (ACTIVATE_API caller, UNIT *uptr, int32 event_time)
 {
+AIO_ILOCK;
 sim_debug (SIM_DBG_AIO_QUEUE, sim_dflt_dev, "Queueing Asynch event for %s after %d instructions\n", sim_uname(uptr), event_time);
 if (uptr->a_next) {
     uptr->a_activate_call = sim_activate_abs;
@@ -381,6 +386,7 @@ else {
         uptr->a_next = q;                               /* Mark as on list */
         } while (q != AIO_QUEUE_SET(uptr, q));
     }
+AIO_IUNLOCK;
 sim_asynch_check = 0;                             /* try to force check */
 if (sim_idle_wait) {
     sim_debug (TIMER_DBG_IDLE, &sim_timer_dev, "waking due to event on %s after %d instructions\n", sim_uname(uptr), event_time);
diff --git a/sim_defs.h b/sim_defs.h
index a95989a5..ed6c29d3 100644
--- a/sim_defs.h
+++ b/sim_defs.h
@@ -1064,6 +1064,8 @@ extern int32 sim_asynch_inst_latency;
 #else
 #error "Implementation of function InterlockedCompareExchangePointer() is needed to build with USE_AIO_INTRINSICS"
 #endif
+#define AIO_ILOCK AIO_LOCK
+#define AIO_IUNLOCK AIO_UNLOCK
 #define AIO_QUEUE_VAL (UNIT *)(InterlockedCompareExchangePointer((void * volatile *)&sim_asynch_queue, (void *)sim_asynch_queue, NULL))
 #define AIO_QUEUE_SET(val, queue) (UNIT *)(InterlockedCompareExchangePointer((void * volatile *)&sim_asynch_queue, (void *)val, queue))
 #define AIO_UPDATE_QUEUE sim_aio_update_queue ()
@@ -1103,33 +1105,11 @@ extern int32 sim_asynch_inst_latency;
       pthread_mutex_destroy(&sim_tmxr_poll_lock);                 \
       pthread_cond_destroy(&sim_tmxr_poll_cond);                  \
       } while (0)
-#define AIO_UPDATE_QUEUE                                                         \
-    do {                                                                         \
-      UNIT *uptr;                                                                \
-      AIO_LOCK;                                                                  \
-      while (sim_asynch_queue != QUEUE_LIST_END) { /* List !Empty */             \
-        int32 a_event_time;                                                      \
-        uptr = sim_asynch_queue;                                                 \
-        sim_debug (SIM_DBG_AIO_QUEUE, sim_dflt_dev, "Migrating Asynch event for %s after %d instructions\n", sim_uname(uptr), uptr->a_event_time);\
-        sim_asynch_queue = uptr->a_next;                                         \
-        uptr->a_next = NULL;            /* hygiene */                            \
-        if (uptr->a_activate_call != &sim_activate_notbefore) {                  \
-          a_event_time = uptr->a_event_time-((sim_asynch_inst_latency+1)/2);     \
-          if (a_event_time < 0)                                                  \
-            a_event_time = 0;                                                    \
-          }                                                                      \
-        else                                                                     \
-          a_event_time = uptr->a_event_time;                                     \
-        AIO_UNLOCK;                                                              \
-        uptr->a_activate_call (uptr, a_event_time);                              \
-        if (uptr->a_check_completion) {                                          \
-          sim_debug (SIM_DBG_AIO_QUEUE, sim_dflt_dev, "Calling Completion Check for asynch event on %s\n", sim_uname(uptr));\
-          uptr->a_check_completion (uptr);                                       \
-          }                                                                      \
-        AIO_LOCK;                                                                \
-        }                                                                        \
-      AIO_UNLOCK;                                                                \
-      } while (0)
+#define AIO_ILOCK AIO_LOCK
+#define AIO_IUNLOCK AIO_UNLOCK
+#define AIO_QUEUE_VAL sim_asynch_queue
+#define AIO_QUEUE_SET(val, queue) (sim_asynch_queue = val)
+#define AIO_UPDATE_QUEUE sim_aio_update_queue ()
 #define AIO_ACTIVATE(caller, uptr, event_time)                         \
     if (!pthread_equal ( pthread_self(), sim_asynch_main_threadid )) { \
       sim_debug (SIM_DBG_AIO_QUEUE, sim_dflt_dev, "Queueing Asynch event for %s after %d instructions\n", sim_uname(uptr), event_time);\