diff --git a/PDP18B/pdp18b_dr15.c b/PDP18B/pdp18b_dr15.c
index d0a0b0f8..d00d8407 100644
--- a/PDP18B/pdp18b_dr15.c
+++ b/PDP18B/pdp18b_dr15.c
@@ -1,6 +1,6 @@
 /* pdp18b_dr15.c: DR15C simulator
 
-   Copyright (c) 2016, Robert M Supnik
+   Copyright (c) 2016-2020, Robert M Supnik
 
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
@@ -25,6 +25,8 @@
 
    dr           PDP-15 DR15C interface for UC15 system
 
+   04-Jul 20    RMS     Zero out shared state on first allocation
+
    The DR15C provides control communications with the DR11Cs in the UC15.
    Its state consists of an 18b register (the Task Control Block Pointer),
    a one bit flag (TCBP acknowledge, not wired to interrupt), four interrupt
@@ -280,6 +282,9 @@ if (uc15_shmem == NULL) {                               /* allocate shared state
     if (r != SCPE_OK)
         return r;
     uc15_shstate = (int32 *) basead;
+    for (i = 0; i < UC15_STATE_SIZE; i++) {             /* zero out shared state region */
+        UC15_SHARED_WR (i, 0);
+        }
     }
 if (pdp15_shmem == NULL) {                              /* allocate shared memory */
     r = sim_shmem_open ("PDP15MainMemory", MAXMEMSIZE * sizeof (int32), &pdp15_shmem, &basead);