From a9061b0332df4cbd64966b65e2b846505970fb27 Mon Sep 17 00:00:00 2001
From: Mark Pizzolato <mark@infocomm.com>
Date: Mon, 1 Oct 2018 13:43:43 -0700
Subject: [PATCH] PDP11: Fix regression where, depending on CPU model, the SR
 register disappears

This change fixes #623.
---
 PDP11/pdp11_io_lib.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/PDP11/pdp11_io_lib.c b/PDP11/pdp11_io_lib.c
index f7e31202..42b51635 100644
--- a/PDP11/pdp11_io_lib.c
+++ b/PDP11/pdp11_io_lib.c
@@ -414,8 +414,10 @@ for (i = 0; i < (int32) dibp->lnt; i = i + 2) {         /* create entries */
     if ((dibp->rd == NULL) && (dibp->wr == NULL) && (dibp->vnum == 0)) 
         iodibp[idx] = NULL;                         /* deregister DIB */
     else {
-        iodispR[idx] = dibp->rd;                    /* set rd dispatch */
-        iodispW[idx] = dibp->wr;                    /* set wr dispatch */
+        if (dibp->rd)
+            iodispR[idx] = dibp->rd;                /* set rd dispatch */
+        if (dibp->wr)
+            iodispW[idx] = dibp->wr;                /* set wr dispatch */
         iodibp[idx] = dibp;                         /* remember DIB */
         }
     }