diff --git a/PDP10/pdp10_defs.h b/PDP10/pdp10_defs.h
index 643f4eb5..e82b8210 100644
--- a/PDP10/pdp10_defs.h
+++ b/PDP10/pdp10_defs.h
@@ -613,6 +613,7 @@ struct pdp_dib {
     int32               vloc;                           /* locator */
     int32               vec;                            /* value */
     int32               (*ack[VEC_DEVMAX])(void);       /* ack routines */
+    uint32              ulnt;                           /* IO length per unit */
 };
 
 typedef struct pdp_dib DIB;
diff --git a/PDP10/pdp10_ksio.c b/PDP10/pdp10_ksio.c
index a61b3a7a..554d1aab 100644
--- a/PDP10/pdp10_ksio.c
+++ b/PDP10/pdp10_ksio.c
@@ -1156,7 +1156,8 @@ for (i = 0; dib_tab[i] != NULL; i++) {                  /* print table */
                             (dib_tab[i]->vloc<=19)? 5: 4);
     else
         fprintf (st, "   ");
-    fprintf (st, " %2u %s\n", (dptr? dptr->numunits: 1), dptr? sim_dname (dptr): "CPU");
+    fprintf (st, " %2u %s\n", (dib_tab[i]->ulnt? dib_tab[i]->lnt/dib_tab[i]->ulnt:
+                               (dptr? dptr->numunits: 1)), dptr? sim_dname (dptr): "CPU");
     }
 return SCPE_OK;
 }
diff --git a/PDP10/pdp10_rp.c b/PDP10/pdp10_rp.c
index e65bd9a3..c307a6c2 100644
--- a/PDP10/pdp10_rp.c
+++ b/PDP10/pdp10_rp.c
@@ -387,7 +387,7 @@ t_stat rp_set_size (UNIT *uptr, int32 val, char *cptr, void *desc);
 
 DIB rp_dib = {
     IOBA_RP, IOLN_RP, &rp_rd, &rp_wr,
-    1, IVCL (RP), VEC_RP, { &rp_inta }
+    1, IVCL (RP), VEC_RP, { &rp_inta }, IOLN_RP
     };
 
 UNIT rp_unit[] = {
diff --git a/PDP10/pdp10_tu.c b/PDP10/pdp10_tu.c
index eb0cd890..f37c3ca8 100644
--- a/PDP10/pdp10_tu.c
+++ b/PDP10/pdp10_tu.c
@@ -359,7 +359,7 @@ t_stat tu_map_err (UNIT *uptr, t_stat st, t_bool qdt);
 
 DIB tu_dib = {
     IOBA_TU, IOLN_TU, &tu_rd, &tu_wr,
-    1, IVCL (TU), VEC_TU, { &tu_inta }
+    1, IVCL (TU), VEC_TU, { &tu_inta }, IOLN_TU,
     };
 
 UNIT tu_unit[] = {
diff --git a/PDP11/pdp11_dc.c b/PDP11/pdp11_dc.c
index e6b180fd..3e13b342 100644
--- a/PDP11/pdp11_dc.c
+++ b/PDP11/pdp11_dc.c
@@ -154,7 +154,7 @@ char *dcx_description (DEVICE *dptr);
 
 DIB dci_dib = {
     IOBA_AUTO, IOLN_DC * DCX_LINES, &dcx_rd, &dcx_wr,
-    2, IVCL (DCI), VEC_AUTO, { &dci_iack, &dco_iack }
+    2, IVCL (DCI), VEC_AUTO, { &dci_iack, &dco_iack }, IOLN_DC,
     };
 
 UNIT dci_unit = { UDATA (&dci_svc, 0, 0), KBD_POLL_WAIT };
diff --git a/PDP11/pdp11_defs.h b/PDP11/pdp11_defs.h
index 3f4c4157..d05a4d4f 100644
--- a/PDP11/pdp11_defs.h
+++ b/PDP11/pdp11_defs.h
@@ -513,6 +513,7 @@ struct pdp_dib {
     int32               vloc;                           /* locator */
     int32               vec;                            /* value */
     int32               (*ack[VEC_DEVMAX])(void);       /* ack routines */
+    uint32             ulnt;                            /* IO length per-unit */
     };
 
 typedef struct pdp_dib DIB;
diff --git a/PDP11/pdp11_dl.c b/PDP11/pdp11_dl.c
index 8570e274..f13a0813 100644
--- a/PDP11/pdp11_dl.c
+++ b/PDP11/pdp11_dl.c
@@ -118,7 +118,7 @@ void dlx_reset_ln (int32 ln);
 
 DIB dli_dib = {
     IOBA_AUTO, IOLN_DL * DLX_LINES, &dlx_rd, &dlx_wr,
-    2, IVCL (DLI), VEC_AUTO, { &dli_iack, &dlo_iack }
+    2, IVCL (DLI), VEC_AUTO, { &dli_iack, &dlo_iack }, IOLN_DL,
     };
 
 UNIT dli_unit = { UDATA (&dli_svc, 0, 0), KBD_POLL_WAIT };
diff --git a/PDP11/pdp11_dz.c b/PDP11/pdp11_dz.c
index c1aeeb3f..a23e8a05 100644
--- a/PDP11/pdp11_dz.c
+++ b/PDP11/pdp11_dz.c
@@ -293,7 +293,8 @@ char *dz_description (DEVICE *dptr);
 
 DIB dz_dib = {
     IOBA_AUTO, IOLN_DZ * DZ_MUXES, &dz_rd, &dz_wr,
-    2, IVCL (DZRX), VEC_AUTO, { &dz_rxinta, &dz_txinta }
+    2, IVCL (DZRX), VEC_AUTO, { &dz_rxinta, &dz_txinta },
+    IOLN_DZ,
     };
 
 UNIT dz_unit = { UDATA (&dz_svc, UNIT_IDLE|UNIT_ATTABLE|DZ_8B_DFLT, 0) };
diff --git a/PDP11/pdp11_hk.c b/PDP11/pdp11_hk.c
index e997e86b..433d476f 100644
--- a/PDP11/pdp11_hk.c
+++ b/PDP11/pdp11_hk.c
@@ -581,7 +581,7 @@ char *hk_description (DEVICE *dptr);
 
 DIB hk_dib = {
     IOBA_AUTO, IOLN_HK, &hk_rd, &hk_wr,
-    1, IVCL (HK), VEC_AUTO, { NULL }
+    1, IVCL (HK), VEC_AUTO, { NULL }, IOLN_HK,
     };
 
 UNIT hk_unit[] = {
diff --git a/PDP11/pdp11_io_lib.c b/PDP11/pdp11_io_lib.c
index b9395ca2..5b505443 100644
--- a/PDP11/pdp11_io_lib.c
+++ b/PDP11/pdp11_io_lib.c
@@ -327,7 +327,8 @@ for (i = 0, dibp = NULL; i < (IOPAGESIZE >> 1); i++) {  /* loop thru entries */
         l = strlen (dptr? sim_dname (dptr): "CPU");
         if (l>maxname)
             maxname = (int32)l;
-        j = (dptr? dptr->numunits: 1);
+        j = (dibp->ulnt? dibp->lnt/dibp->ulnt:
+             (dptr? dptr->numunits: 1));
         if (j > maxdev)
             maxdev = j;
         }                                               /* end if */
@@ -407,7 +408,9 @@ for (i = 0, dibp = NULL; i < (IOPAGESIZE >> 1); i++) {  /* loop thru entries */
             fprintf (st, " %2u", dibp->vloc/32);
         else
             fprintf (st, "   ");
-        fprintf (st, " %*u %s\n", maxdev, (dptr? dptr->numunits: 1), dptr? sim_dname (dptr): "CPU");
+        fprintf (st, " %*u %s\n", maxdev,  (dibp->ulnt? dibp->lnt/dibp->ulnt:
+                                            (dptr? dptr->numunits: 1)),
+                 dptr? sim_dname (dptr): "CPU");
         }                                               /* end if */
     }                                                   /* end for i */
 return SCPE_OK;
diff --git a/PDP11/pdp11_kg.c b/PDP11/pdp11_kg.c
index 121b766f..0a16800e 100644
--- a/PDP11/pdp11_kg.c
+++ b/PDP11/pdp11_kg.c
@@ -201,7 +201,7 @@ static DIB kg_dib = {
     (IOLN_KG + 2) * KG_UNITS,
     &kg_rd,
     &kg_wr,
-    0, 0, 0, { NULL }
+    0, 0, 0, { NULL }, IOLN_KG+2
 };
 
 static UNIT kg_unit[] = {
diff --git a/PDP11/pdp11_rc.c b/PDP11/pdp11_rc.c
index c1103334..ce6df647 100644
--- a/PDP11/pdp11_rc.c
+++ b/PDP11/pdp11_rc.c
@@ -187,7 +187,7 @@ static DIB rc_dib = {
     IOLN_RC,
     &rc_rd,
     &rc_wr,
-    1, IVCL (RC), VEC_AUTO, { NULL }
+    1, IVCL (RC), VEC_AUTO, { NULL }, IOLN_RC,
 };
 
 static UNIT rc_unit = {
diff --git a/PDP11/pdp11_rf.c b/PDP11/pdp11_rf.c
index e11c7195..1e75a9c6 100644
--- a/PDP11/pdp11_rf.c
+++ b/PDP11/pdp11_rf.c
@@ -145,7 +145,7 @@ uint32 update_rfcs (uint32 newcs, uint32 newdae);
 
 DIB rf_dib = {
     IOBA_AUTO, IOLN_RF, &rf_rd, &rf_wr,
-    1, IVCL (RF), VEC_AUTO, {NULL}
+    1, IVCL (RF), VEC_AUTO, {NULL}, IOLN_RF,
     };
 
 
diff --git a/PDP11/pdp11_rh.c b/PDP11/pdp11_rh.c
index fc268f17..2141392e 100644
--- a/PDP11/pdp11_rh.c
+++ b/PDP11/pdp11_rh.c
@@ -207,7 +207,7 @@ static int32 mba_mapofs[(MBA_OFSMASK + 1) >> 1] = {
 
 DIB mba0_dib = {
     IOBA_AUTO, IOLN_RP, &mba_rd, &mba_wr,
-    1, IVCL (RP), VEC_AUTO, { &mba0_inta }
+    1, IVCL (RP), VEC_AUTO, { &mba0_inta }, IOLN_RP,
     };
 
 UNIT mba0_unit = { UDATA (NULL, 0, 0) };
diff --git a/PDP11/pdp11_rk.c b/PDP11/pdp11_rk.c
index 4c71319a..ebc1a378 100644
--- a/PDP11/pdp11_rk.c
+++ b/PDP11/pdp11_rk.c
@@ -216,7 +216,7 @@ t_stat rk_boot (int32 unitno, DEVICE *dptr);
 
 DIB rk_dib = {
     IOBA_AUTO, IOLN_RK, &rk_rd, &rk_wr,
-    1, IVCL (RK), VEC_AUTO, { &rk_inta }
+    1, IVCL (RK), VEC_AUTO, { &rk_inta }, IOLN_RK,
     };
 
 UNIT rk_unit[] = {
diff --git a/PDP11/pdp11_rl.c b/PDP11/pdp11_rl.c
index 99bcd817..625a51ca 100644
--- a/PDP11/pdp11_rl.c
+++ b/PDP11/pdp11_rl.c
@@ -275,7 +275,7 @@ char *rl_description (DEVICE *dptr);
 
 static DIB rl_dib = {
     IOBA_AUTO, IOLN_RL, &rl_rd, &rl_wr,
-    1, IVCL (RL), VEC_AUTO, { NULL } };
+    1, IVCL (RL), VEC_AUTO, { NULL }, IOLN_RL };
 
 static UNIT rl_unit[] = {
     { UDATA (&rl_svc, UNIT_FIX+UNIT_ATTABLE+UNIT_DISABLE+
diff --git a/PDP11/pdp11_rq.c b/PDP11/pdp11_rq.c
index e00ce750..04bfc00f 100644
--- a/PDP11/pdp11_rq.c
+++ b/PDP11/pdp11_rq.c
@@ -857,7 +857,7 @@ MSC rq_ctx = { 0 };
 
 DIB rq_dib = {
     IOBA_AUTO, IOLN_RQ, &rq_rd, &rq_wr,
-    1, IVCL (RQ), 0, { &rq_inta }
+    1, IVCL (RQ), 0, { &rq_inta }, IOLN_RQ
     };
 
 UNIT rq_unit[] = {
diff --git a/PDP11/pdp11_rx.c b/PDP11/pdp11_rx.c
index ef8ba659..ab6fbb56 100644
--- a/PDP11/pdp11_rx.c
+++ b/PDP11/pdp11_rx.c
@@ -144,7 +144,7 @@ void rx_done (int32 esr_flags, int32 new_ecode);
 
 DIB rx_dib = {
     IOBA_AUTO, IOLN_RX, &rx_rd, &rx_wr,
-    1, IVCL (RX), VEC_AUTO, { NULL }
+    1, IVCL (RX), VEC_AUTO, { NULL }, IOLN_RX,
     };
 
 UNIT rx_unit[] = {
diff --git a/PDP11/pdp11_ry.c b/PDP11/pdp11_ry.c
index b7f065c8..ec7f303c 100644
--- a/PDP11/pdp11_ry.c
+++ b/PDP11/pdp11_ry.c
@@ -180,7 +180,7 @@ char *ry_description (DEVICE *dptr);
 
 DIB ry_dib = {
     IOBA_AUTO, IOLN_RY, &ry_rd, &ry_wr,
-    1, IVCL (RY), VEC_AUTO, { NULL }
+    1, IVCL (RY), VEC_AUTO, { NULL }, IOLN_RY,
     };
 
 UNIT ry_unit[] = {
diff --git a/PDP11/pdp11_ta.c b/PDP11/pdp11_ta.c
index 012a98c4..a9c2c438 100644
--- a/PDP11/pdp11_ta.c
+++ b/PDP11/pdp11_ta.c
@@ -150,7 +150,7 @@ uint32 ta_crc (uint8 *buf, uint32 cnt);
 
 DIB ta_dib = {
     IOBA_AUTO, IOLN_TA, &ta_rd, &ta_wr,
-    1, IVCL (TA), VEC_AUTO, { NULL }
+    1, IVCL (TA), VEC_AUTO, { NULL }, IOLN_TA
     };
 
 UNIT ta_unit[] = {
diff --git a/PDP11/pdp11_tc.c b/PDP11/pdp11_tc.c
index d3b09511..4ac9d998 100644
--- a/PDP11/pdp11_tc.c
+++ b/PDP11/pdp11_tc.c
@@ -319,7 +319,7 @@ int32 dt_gethdr (UNIT *uptr, int32 blk, int32 relpos);
 
 DIB dt_dib = {
     IOBA_AUTO, IOLN_TC, &dt_rd, &dt_wr,
-    1, IVCL (DTA), VEC_AUTO, { NULL }
+    1, IVCL (DTA), VEC_AUTO, { NULL }, IOLN_TC,
     };
 
 UNIT dt_unit[] = {
diff --git a/PDP11/pdp11_tm.c b/PDP11/pdp11_tm.c
index d6d15b0d..1bccae54 100644
--- a/PDP11/pdp11_tm.c
+++ b/PDP11/pdp11_tm.c
@@ -191,7 +191,7 @@ t_stat tm_vlock (UNIT *uptr, int32 val, char *cptr, void *desc);
 
 DIB tm_dib = {
     IOBA_AUTO, IOLN_TM, &tm_rd, &tm_wr,
-    1, IVCL (TM), VEC_AUTO, { NULL }
+    1, IVCL (TM), VEC_AUTO, { NULL }, IOLN_TM,
     };
 
 UNIT tm_unit[] = {
diff --git a/PDP11/pdp11_tq.c b/PDP11/pdp11_tq.c
index 04c25eee..e0f30d9f 100644
--- a/PDP11/pdp11_tq.c
+++ b/PDP11/pdp11_tq.c
@@ -417,7 +417,7 @@ UNIT *tq_getucb (uint32 lu);
 
 DIB tq_dib = {
     IOBA_AUTO, IOLN_TQ, &tq_rd, &tq_wr,
-    1, IVCL (TQ), 0, { &tq_inta }
+    1, IVCL (TQ), 0, { &tq_inta }, IOLN_TQ,
     };
 
 UNIT tq_unit[] = {
diff --git a/PDP11/pdp11_ts.c b/PDP11/pdp11_ts.c
index f531f6ef..3fd3f716 100644
--- a/PDP11/pdp11_ts.c
+++ b/PDP11/pdp11_ts.c
@@ -311,7 +311,7 @@ char *ts_description (DEVICE *dptr);
 
 DIB ts_dib = {
     IOBA_AUTO, IOLN_TS, &ts_rd, &ts_wr,
-    1, IVCL (TS), VEC_AUTO, { NULL }
+    1, IVCL (TS), VEC_AUTO, { NULL }, IOLN_TS
     };
 
 UNIT ts_unit = { UDATA (&ts_svc, UNIT_ATTABLE + UNIT_ROABLE + UNIT_DISABLE, 0) };
diff --git a/PDP11/pdp11_vh.c b/PDP11/pdp11_vh.c
index 2bde9bbe..b3324c8c 100644
--- a/PDP11/pdp11_vh.c
+++ b/PDP11/pdp11_vh.c
@@ -361,7 +361,8 @@ static DIB vh_dib = {
     2,          /* # of vectors */
     IVCL (VHRX),
     VEC_FLOAT,
-    { &vh_rxinta, &vh_txinta }  /* int. ack. routines */
+    { &vh_rxinta, &vh_txinta },  /* int. ack. routines */
+    IOLN_VH,    /* IO space per device */
 };
 
 static UNIT vh_unit[VH_MUXES] = {
diff --git a/PDP11/pdp11_xq.c b/PDP11/pdp11_xq.c
index 36f83f6d..c51502e3 100644
--- a/PDP11/pdp11_xq.c
+++ b/PDP11/pdp11_xq.c
@@ -326,7 +326,7 @@ struct xq_device    xqb = {
 #define IOLN_XQ         020
 
 DIB xqa_dib = { IOBA_AUTO, IOLN_XQ, &xq_rd, &xq_wr,
-        1, IVCL (XQ), 0, { &xq_int } };
+                1, IVCL (XQ), 0, { &xq_int }, IOLN_XQ };
 
 UNIT xqa_unit[] = {
  { UDATA (&xq_svc, UNIT_IDLE|UNIT_ATTABLE|UNIT_DISABLE, 2047) },  /* receive timer */
diff --git a/PDP11/pdp11_xu.c b/PDP11/pdp11_xu.c
index f29cc82f..76fb3f1e 100644
--- a/PDP11/pdp11_xu.c
+++ b/PDP11/pdp11_xu.c
@@ -134,7 +134,7 @@ char *xu_description (DEVICE *dptr);
 #define IOLN_XU        010
 
 DIB xua_dib = { IOBA_AUTO, IOLN_XU, &xu_rd, &xu_wr,
-1, IVCL (XU), VEC_AUTO, {&xu_int} };
+1, IVCL (XU), VEC_AUTO, {&xu_int}, IOLN_XU };
 
 UNIT xua_unit[] = {
  { UDATA (&xu_svc, UNIT_IDLE|UNIT_ATTABLE|UNIT_DISABLE, 0) },     /* receive timer */
@@ -234,7 +234,7 @@ DEVICE xu_dev = {
 #define IOLN_XU         010
 
 DIB xub_dib = { IOBA_AUTO, IOLN_XU, &xu_rd, &xu_wr,
-        1, IVCL (XU), 0, { &xu_int } };
+        1, IVCL (XU), 0, { &xu_int }, IOLN_XU };
 
 UNIT xub_unit[] = {
  { UDATA (&xu_svc, UNIT_IDLE|UNIT_ATTABLE|UNIT_DISABLE, 0) }      /* receive timer */
diff --git a/VAX/vax610_defs.h b/VAX/vax610_defs.h
index 1dbd6f67..1c1654f5 100644
--- a/VAX/vax610_defs.h
+++ b/VAX/vax610_defs.h
@@ -196,6 +196,7 @@ typedef struct {
     int32               vloc;                           /* locator */
     int32               vec;                            /* value */
     int32               (*ack[VEC_DEVMAX])(void);       /* ack routine */
+    uint32              ulnt;                           /* IO length per unit */
     } DIB;
 
 /* Qbus I/O page layout - see pdp11_io_lib.c for address layout details */
diff --git a/VAX/vax630_defs.h b/VAX/vax630_defs.h
index 30214c6a..71226846 100644
--- a/VAX/vax630_defs.h
+++ b/VAX/vax630_defs.h
@@ -243,6 +243,7 @@ typedef struct {
     int32               vloc;                           /* locator */
     int32               vec;                            /* value */
     int32               (*ack[VEC_DEVMAX])(void);       /* ack routine */
+    uint32              ulnt;                           /* IO length per unit */
     } DIB;
 
 /* Qbus I/O page layout - see pdp11_io_lib.c for address layout details */
diff --git a/VAX/vax730_defs.h b/VAX/vax730_defs.h
index 9db8abff..728eff0e 100644
--- a/VAX/vax730_defs.h
+++ b/VAX/vax730_defs.h
@@ -237,6 +237,7 @@ typedef struct {
     int32               vloc;                           /* locator */
     int32               vec;                            /* value */
     int32               (*ack[VEC_DEVMAX])(void);       /* ack routine */
+    uint32              ulnt;                           /* IO length per unit */
     } DIB;
 
 /* Unibus I/O page layout - see pdp11_io_lib.c for address layout details */
diff --git a/VAX/vax750_defs.h b/VAX/vax750_defs.h
index 7342482d..efcc96a4 100644
--- a/VAX/vax750_defs.h
+++ b/VAX/vax750_defs.h
@@ -274,6 +274,7 @@ typedef struct {
     int32               vloc;                           /* locator */
     int32               vec;                            /* value */
     int32               (*ack[VEC_DEVMAX])(void);       /* ack routine */
+    uint32              ulnt;                           /* IO length per unit */
     } DIB;
 
 /* Unibus I/O page layout - see pdp11_io_lib.c for address layout details
diff --git a/VAX/vax780_defs.h b/VAX/vax780_defs.h
index a9a6153a..a16901ab 100644
--- a/VAX/vax780_defs.h
+++ b/VAX/vax780_defs.h
@@ -286,6 +286,7 @@ typedef struct {
     int32               vloc;                           /* locator */
     int32               vec;                            /* value */
     int32               (*ack[VEC_DEVMAX])(void);       /* ack routine */
+    uint32              ulnt;                           /* IO length per unit */
     } DIB;
 
 /* Unibus I/O page layout - see pdp11_io_lib.c for address layout details
diff --git a/VAX/vax860_defs.h b/VAX/vax860_defs.h
index 2d8b9fbb..af78c992 100644
--- a/VAX/vax860_defs.h
+++ b/VAX/vax860_defs.h
@@ -321,6 +321,7 @@ typedef struct {
     int32               vloc;                           /* locator */
     int32               vec;                            /* value */
     int32               (*ack[VEC_DEVMAX])(void);       /* ack routine */
+    uint32              ulnt;                           /* IO length per unit */
     } DIB;
 
 /* Unibus I/O page layout - XUB,RQB,RQC,RQD float based on number of DZ's
diff --git a/VAX/vaxmod_defs.h b/VAX/vaxmod_defs.h
index 9ab73332..cd53dff7 100644
--- a/VAX/vaxmod_defs.h
+++ b/VAX/vaxmod_defs.h
@@ -284,6 +284,7 @@ typedef struct {
     int32               vloc;                           /* locator */
     int32               vec;                            /* value */
     int32               (*ack[VEC_DEVMAX])(void);       /* ack routine */
+    uint32              ulnt;                           /* IO length per unit */
     } DIB;
 
 /* Qbus I/O page layout - see pdp11_io_lib.c for address layout details */
diff --git a/scp.c b/scp.c
index a1be25a4..00c5369a 100644
--- a/scp.c
+++ b/scp.c
@@ -3664,7 +3664,8 @@ if (uptr->flags & UNIT_ATT)                             /* already attached? */
             return r;
         }
     else
-        return SCPE_ALATT;                              /* Already attached */
+        if (!(uptr->dynflags & UNIT_ATTMULT))
+            return SCPE_ALATT;                          /* Already attached */
 sim_trim_endspc (cptr);                                 /* trim trailing spc */
 return scp_attach_unit (dptr, uptr, cptr);              /* attach */
 }