diff --git a/BUGS b/BUGS
new file mode 100644
index 0000000..93b54e4
--- /dev/null
+++ b/BUGS
@@ -0,0 +1,4 @@
+Failures on Fedora x86_64, reported by postmodern
+http://rafb.net/p/4L3z3Y69.html
+http://rafb.net/p/zp9cth25.html
+http://rafb.net/p/NYeXnd29.html
diff --git a/vm/config.h b/vm/config.h
index 55874bb..8da3dde 100644
--- a/vm/config.h
+++ b/vm/config.h
@@ -13,4 +13,14 @@
 /* enable CallSite optimization */
 #define TR_CALL_SITE
 
-#define MAX_INT (INT_MAX-2)  /* maximum value of an int (-2 for safety) */
\ No newline at end of file
+#define MAX_INT (INT_MAX-2)  /* maximum value of an int (-2 for safety) */
+
+/* TR_BITSINT defines the number of bits in an int. */
+#if INT_MAX-20 < 32760
+#define TR_BITSINT  16
+#elif INT_MAX > 2147483640L
+/* int has at least 32 bits */
+#define TR_BITSINT  32
+#else
+#error "you must define TR_BITSINT with number of bits in an integer"
+#endif
diff --git a/vm/opcode.h b/vm/opcode.h
index bafa474..2525246 100644
--- a/vm/opcode.h
+++ b/vm/opcode.h
@@ -15,8 +15,13 @@
 #define POS_B     (POS_C + SIZE_C)
 #define POS_Bx    POS_C
 
+#if SIZE_Bx < TR_BITSINT-1
 #define MAXARG_Bx   ((1<<SIZE_Bx)-1)
 #define MAXARG_sBx  (MAXARG_Bx>>1)         /* `sBx' is signed */
+#else
+#define MAXARG_Bx   MAX_INT
+#define MAXARG_sBx  MAX_INT
+#endif
 
 /* creates a mask with `n' 1 bits at position `p' */
 #define MASK1(n,p)  ((~((~(TrInst)0)<<n))<<p)