diff --git a/swtp6800/common/mp-b2.c b/swtp6800/common/mp-b2.c
index c5253ec9..bcab28bd 100644
--- a/swtp6800/common/mp-b2.c
+++ b/swtp6800/common/mp-b2.c
@@ -214,7 +214,10 @@ int32 MB_get_mbyte(int32 addr)
             } else
                 return 0xFF;
         case 0x8000:
-            val = (dev_table[addr - 0x8000].routine(0, 0)) & 0xFF;
+            if (addr < 0x8020)
+                val = (dev_table[addr - 0x8000].routine(0, 0)) & 0xFF;
+            else
+                val = 0xFF;
             if (MB_dev.dctrl & DEBUG_read)
                 printf("MB_get_mbyte: I/O addr=%04X val=%02X\n", addr, val);
             return val;
@@ -290,7 +293,8 @@ void MB_put_mbyte(int32 addr, int32 val)
                 return;
             }
         case 0x8000:
-            dev_table[addr - 0x8000].routine(1, val);
+            if (addr < 0x8020)
+                dev_table[addr - 0x8000].routine(1, val);
             return;
         case 0xA000:
         case 0xB000:
@@ -320,3 +324,4 @@ void MB_put_mword(int32 addr, int32 val)
 }
 
 /* end of mp-b2.c */
+