diff --git a/memory.c b/memory.c
index 5f2c9ef..7dd7cac 100644
--- a/memory.c
+++ b/memory.c
@@ -14,6 +14,7 @@
 #include "memory.h"
 #include "exec-memory.h"
 #include "ioport.h"
+#include "bitops.h"
 #include <assert.h>
 
 typedef struct AddrRange AddrRange;
@@ -506,6 +507,10 @@
         return -1U; /* FIXME: better signalling */
     }
 
+    if (!mr->ops->read) {
+        return mr->ops->old_mmio.read[bitops_ffsl(size)](mr->opaque, addr);
+    }
+
     /* FIXME: support unaligned access */
 
     access_size_min = mr->ops->impl.min_access_size;
@@ -542,6 +547,11 @@
         return; /* FIXME: better signalling */
     }
 
+    if (!mr->ops->write) {
+        mr->ops->old_mmio.write[bitops_ffsl(size)](mr->opaque, addr, data);
+        return;
+    }
+
     /* FIXME: support unaligned access */
 
     access_size_min = mr->ops->impl.min_access_size;