diff --git a/doc/simh.doc b/doc/simh.doc
index a932ecc9..4eaac45a 100644
Binary files a/doc/simh.doc and b/doc/simh.doc differ
diff --git a/sim_tape.c b/sim_tape.c
index cff9e361..7b46c899 100644
--- a/sim_tape.c
+++ b/sim_tape.c
@@ -163,6 +163,8 @@ struct tape_context {
 #define AIO_CALLSETUP                                                   \
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;       \
                                                                         \
+if (ctx == NULL)                                                        \
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");                    \
 if ((!callback) || !ctx->asynch_io)
 
 #define AIO_CALL(op, _buf, _bc, _fc, _max, _vbc, _gaplen, _bpi, _obj, _callback)\
@@ -348,7 +350,9 @@ if (ctx) {
     }
 }
 #else
-#define AIO_CALLSETUP
+#define AIO_CALLSETUP                                                       \
+    if (uptr->tape_ctx == NULL)                                             \
+        return sim_messagef (SCPE_IERR, "Bad Attach\n");
 #define AIO_CALL(op, _buf, _fc, _bc, _max, _vbc, _gaplen, _bpi, _obj, _callback) \
     if (_callback)                                                    \
         (_callback) (uptr, r);
@@ -522,7 +526,7 @@ uint32 f = MT_GET_FMT (uptr);
 t_stat r;
 t_bool auto_format = FALSE;
 
-if ((uptr == NULL) || !(uptr->flags & UNIT_ATT))
+if ((ctx == NULL) || (uptr == NULL) || !(uptr->flags & UNIT_ATT))
     return SCPE_IERR;
 
 if (uptr->io_flush)
@@ -586,6 +590,8 @@ static void sim_tape_data_trace(UNIT *uptr, const uint8 *data, size_t len, const
 {
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 
+if (ctx == NULL)
+    return;
 if (sim_deb && (ctx->dptr->dctrl & reason))
     sim_data_trace(ctx->dptr, uptr, (detail ? data : NULL), "", len, txt, reason);
 }
@@ -692,6 +698,8 @@ MT_CLR_PNU (uptr);                                      /* clear the position-no
 
 if ((uptr->flags & UNIT_ATT) == 0)                      /* if the unit is not attached */
     return MTSE_UNATT;                                  /*   then quit with an error */
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 
 sim_fseek (uptr->fileref, uptr->pos, SEEK_SET);         /* set the initial tape position */
 
@@ -902,6 +910,8 @@ MT_CLR_PNU (uptr);                                      /* clear the position-no
 
 if ((uptr->flags & UNIT_ATT) == 0)                      /* if the unit is not attached */
     return MTSE_UNATT;                                  /*   then quit with an error */
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 
 if (sim_tape_bot (uptr))                                /* if the unit is positioned at the BOT */
     return MTSE_BOT;                                    /*   then reading backward is not possible */
@@ -1065,6 +1075,8 @@ t_mtrlnt i, tbc, rbc;
 t_addr opos;
 t_stat st;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_rdrecf(unit=%d, buf=%p, max=%d)\n", (int)(uptr-ctx->dptr->units), buf, max);
 
 opos = uptr->pos;                                       /* old position */
@@ -1129,6 +1141,8 @@ uint32 f = MT_GET_FMT (uptr);
 t_mtrlnt i, rbc, tbc;
 t_stat st;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_rdrecr(unit=%d, buf=%p, max=%d)\n", (int)(uptr-ctx->dptr->units), buf, max);
 
 if (MTSE_OK != (st = sim_tape_rdlntr (uptr, &tbc)))     /* read rec lnt */
@@ -1179,6 +1193,8 @@ struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 uint32 f = MT_GET_FMT (uptr);
 t_mtrlnt sbc;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_wrrecf(unit=%d, buf=%p, bc=%d)\n", (int)(uptr-ctx->dptr->units), buf, bc);
 
 sim_tape_data_trace(uptr, buf, bc, "Record Write", ctx->dptr->dctrl & MTSE_DBG_DAT, MTSE_DBG_STR);
@@ -1239,6 +1255,8 @@ struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 MT_CLR_PNU (uptr);
 if ((uptr->flags & UNIT_ATT) == 0)                      /* not attached? */
     return MTSE_UNATT;
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 if (sim_tape_wrp (uptr))                                /* write prot? */
     return MTSE_WRP;
 sim_fseek (uptr->fileref, uptr->pos, SEEK_SET);         /* set pos */
@@ -1258,6 +1276,8 @@ t_stat sim_tape_wrtmk (UNIT *uptr)
 {
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_wrtmk(unit=%d)\n", (int)(uptr-ctx->dptr->units));
 if (MT_GET_FMT (uptr) == MTUF_F_P7B) {                  /* P7B? */
     uint8 buf = P7B_EOF;                                /* eof mark */
@@ -1282,6 +1302,8 @@ t_stat sim_tape_wreom (UNIT *uptr)
 t_stat result;
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_wreom(unit=%d)\n", (int)(uptr-ctx->dptr->units));
 if (MT_GET_FMT (uptr) == MTUF_F_P7B)                    /* cant do P7B */
     return MTSE_FMT;
@@ -1310,6 +1332,8 @@ t_stat sim_tape_wreomrw (UNIT *uptr)
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 t_stat r;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_wreomrw(unit=%d)\n", (int)(uptr-ctx->dptr->units));
 if (MT_GET_FMT (uptr) == MTUF_F_P7B)                    /* cant do P7B */
     return MTSE_FMT;
@@ -1442,6 +1466,8 @@ const uint32 format = MT_GET_FMT (uptr);                /* tape format */
 const uint32 meta_size = sizeof (t_mtrlnt);             /* bytes per metadatum */
 const uint32 min_rec_size = 2 + sizeof (t_mtrlnt) * 2;  /* smallest data record size */
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_wrgap(unit=%d, gaplen=%u)\n", (int)(uptr-ctx->dptr->units), gaplen);
 
 MT_CLR_PNU (uptr);
@@ -1609,6 +1635,8 @@ t_stat sim_tape_sprecf (UNIT *uptr, t_mtrlnt *bc)
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 t_stat st;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_sprecf(unit=%d)\n", (int)(uptr-ctx->dptr->units));
 
 st = sim_tape_rdlntf (uptr, bc);                        /* get record length */
@@ -1650,6 +1678,8 @@ struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 t_stat st;
 t_mtrlnt tbc;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_sprecsf(unit=%d, count=%d)\n", (int)(uptr-ctx->dptr->units), count);
 
 *skipped = 0;
@@ -1695,6 +1725,8 @@ t_stat sim_tape_sprecr (UNIT *uptr, t_mtrlnt *bc)
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 t_stat st;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_sprecr(unit=%d)\n", (int)(uptr-ctx->dptr->units));
 
 if (MT_TST_PNU (uptr)) {
@@ -1742,6 +1774,8 @@ struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 t_stat st;
 t_mtrlnt tbc;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_sprecsr(unit=%d, count=%d)\n", (int)(uptr-ctx->dptr->units), count);
 
 *skipped = 0;
@@ -1791,6 +1825,8 @@ t_stat st;
 t_bool last_tapemark = FALSE;
 uint32 filerecsskipped;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_spfilebyrecf(unit=%d, count=%d, check_leot=%d)\n", (int)(uptr-ctx->dptr->units), count, check_leot);
 
 if (check_leot) {
@@ -1859,6 +1895,8 @@ t_stat sim_tape_spfilef (UNIT *uptr, uint32 count, uint32 *skipped)
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 uint32 totalrecsskipped;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_spfilef(unit=%d, count=%d)\n", (int)(uptr-ctx->dptr->units), count);
 
 return sim_tape_spfilebyrecf (uptr, count, skipped, &totalrecsskipped, FALSE);
@@ -1900,6 +1938,8 @@ struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 t_stat st;
 uint32 filerecsskipped;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_spfilebyrecr(unit=%d, count=%d)\n", (int)(uptr-ctx->dptr->units), count);
 
 *skipped = 0;
@@ -1953,6 +1993,8 @@ t_stat sim_tape_spfiler (UNIT *uptr, uint32 count, uint32 *skipped)
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 uint32 totalrecsskipped;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_spfiler(unit=%d, count=%d)\n", (int)(uptr-ctx->dptr->units), count);
 
 return sim_tape_spfilebyrecr (uptr, count, skipped, &totalrecsskipped);
@@ -1974,6 +2016,8 @@ t_stat sim_tape_rewind (UNIT *uptr)
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 
 if (uptr->flags & UNIT_ATT) {
+    if (ctx == NULL)                                    /* if not properly attached? */
+        return sim_messagef (SCPE_IERR, "Bad Attach\n");/*   that's a problem */
     sim_debug (ctx->dbit, ctx->dptr, "sim_tape_rewind(unit=%d)\n", (int)(uptr-ctx->dptr->units));
     }
 uptr->pos = 0;
@@ -1997,6 +2041,8 @@ t_stat sim_tape_position (UNIT *uptr, uint32 flags, uint32 recs, uint32 *recsski
 struct tape_context *ctx = (struct tape_context *)uptr->tape_ctx;
 t_stat r = MTSE_OK;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_position(unit=%d, flags=0x%X, recs=%d, files=%d)\n", (int)(uptr-ctx->dptr->units), flags, recs, files);
 
 *recsskipped = *filesskipped = *objectsskipped = 0;
@@ -2061,6 +2107,8 @@ MT_CLR_PNU (uptr);
 if (!(uptr->flags & UNIT_ATT))                          /* attached? */
     return SCPE_OK;
 
+if (ctx == NULL)                                        /* if not properly attached? */
+    return sim_messagef (SCPE_IERR, "Bad Attach\n");    /*   that's a problem */
 sim_debug (ctx->dbit, ctx->dptr, "sim_tape_reset(unit=%d)\n", (int)(uptr-ctx->dptr->units));
 
 _sim_tape_io_flush(uptr);