diff --git a/sigma/sigma_cpu.c b/sigma/sigma_cpu.c
old mode 100644
new mode 100755
index 179573bb..e7469bbb
--- a/sigma/sigma_cpu.c
+++ b/sigma/sigma_cpu.c
@@ -174,6 +174,7 @@ uint32 cpu_pdf = 0;                                     /* proc detected fault *
 uint32 cons_alarm = 0;                                  /* console alarm */
 uint32 cons_alarm_enb = 0;                              /* alarm enable */
 uint32 cons_pcf = 0;
+uint32 wait_state = 0;                                  /* wait state */
 uint32 rf_bmax = 4;                                     /* num reg blocks */
 uint32 exu_lim = 32;                                    /* nested EXU limit */
 uint32 stop_op = 0;                                     /* stop on ill op */
@@ -446,7 +447,8 @@ while (reason == 0) {                                   /* loop until stop */
 
     if (int_hireq < NO_INT) {                           /* interrupt req? */
         uint32 sav_hi, vec, wd, op;
-        
+
+        wait_state = 0;                                 /* exit wait state */
         sav_hi = int_hireq;                             /* save level */
         vec = io_ackn_int (int_hireq);                  /* get vector */
         if (vec == 0) {                                 /* illegal vector? */
@@ -474,6 +476,8 @@ while (reason == 0) {                                   /* loop until stop */
             }
         else reason = tr2;                              /* normal status code */
         }
+    else if (wait_state != 0)                           /* wait state? don't fetch */
+         continue;
     else {                                              /* normal instruction */
         if (sim_brk_summ &&
             sim_brk_test (PC, SWMASK ('E'))) {          /* breakpoint? */
@@ -1501,7 +1505,7 @@ switch (op) {
         if (!io_poss_int ())                            /* intr possible? */
             return STOP_WAITNOINT;                      /* machine is hung */
 // put idle here
-        int_hireq = io_eval_int ();                     /* re-eval intr */
+        wait_state = 1;                                 /* wait for intr */
         break;
 
     case OP_AIO:                                        /* acknowledge int */
@@ -2513,6 +2517,7 @@ cpu_new_PSD (1, PSW1_DFLT | (PSW1 & PSW1_PCMASK), PSW2_DFLT);
 cpu_pdf = 0;
 cons_alarm = 0;
 cons_pcf = 0;
+wait_state = 0;
 set_rf_display (R);
 if (M == NULL)
     M = (uint32 *) calloc (MAXMEMSIZE, sizeof (uint32));
diff --git a/sigma/sigma_io.c b/sigma/sigma_io.c
old mode 100644
new mode 100755
index be0b8ed6..1c075e7c
--- a/sigma/sigma_io.c
+++ b/sigma/sigma_io.c
@@ -1,4 +1,4 @@
-/* sigma_io.c: XDS Sigma IO simulator
+ /* sigma_io.c: XDS Sigma IO simulator
 
    Copyright (c) 2007-2022, Robert M Supnik
 
@@ -499,11 +499,12 @@ if ((rn != 0) && !(dvst & DVT_NOST)) {                  /* return status? */
     if (tdv)
         mrgst = (DVT_GETDVS (dvst) << 8) | (chan[ch].chf[dev] & 0xFF);
     else mrgst = ((DVT_GETDVS(dvst) << 8) & ~CHF_ALL) | (chan[ch].chf[dev] & CHF_ALL);
-    R[rn] = chan[ch].clc[dev];                          /* even reg */
-    if (!odd)                                           /* even pair? */
-        WritePW (0x20, R[rn]);                          /* write to 20 */
+    if ((rn & 1) == 0) {                                /* even reg? */
+        R[rn] = chan[ch].clc[dev];                      /* current addr to R */
+        WritePW (0x20, R[rn]);                          /* and loc 20 */
+        }
     R[rn|1] = (mrgst << 16) | chan[ch].bc[dev];         /* odd reg */
-    WritePW (0x20 + odd, R[rn|1]);                      /* write to 20/21 */
+    WritePW (0x21, R[rn|1]);                            /* write loc 21 */
     }
 return DVT_GETCC (dvst);
 }