diff --git a/hdr/mcb.h b/hdr/mcb.h
index 0531b4c..6ff077c 100644
--- a/hdr/mcb.h
+++ b/hdr/mcb.h
@@ -45,6 +45,8 @@ static BYTE *mcb_hRcsId =
 #define FIRST_FIT_U     0x80
 #define BEST_FIT_U      0x81
 #define LAST_FIT_U      0x82
+#define FIT_U_MASK      0xc0
+#define FIT_MASK        0x3f
 
 #define MCB_NORMAL      0x4d
 #define MCB_LAST        0x5a
diff --git a/kernel/inthndlr.c b/kernel/inthndlr.c
index f4233ed..0a7f815 100644
--- a/kernel/inthndlr.c
+++ b/kernel/inthndlr.c
@@ -1210,25 +1210,10 @@ dispatch:
           break;
 
         case 0x01:
-          {
-            switch (lr.BL)
-            {
-              case LAST_FIT:
-              case LAST_FIT_U:
-              case LAST_FIT_UO:
-              case BEST_FIT:
-              case BEST_FIT_U:
-              case BEST_FIT_UO:
-              case FIRST_FIT:
-              case FIRST_FIT_U:
-              case FIRST_FIT_UO:
-                mem_access_mode = lr.BL;
-                break;
-
-              default:
-                goto error_invalid;
-            }
-          }
+          if ((lr.BL & FIT_U_MASK) > FIRST_FIT_U || /* 0xc0, 0x80 */
+              (lr.BL & FIT_MASK) > LAST_FIT)        /* 0x3f, 0x02 */
+            goto error_invalid;
+          mem_access_mode = lr.BL;
           break;
 
         case 0x02: