diff -r 34a337481b2e -r f38c93f2a771 src/arch/x86/isa_traits.hh
--- a/src/arch/x86/isa_traits.hh	Sun Aug 22 18:28:47 2010 -0700
+++ b/src/arch/x86/isa_traits.hh	Sun Aug 22 18:30:25 2010 -0700
@@ -76,6 +76,18 @@
 
     // Memory accesses can be unaligned
     const bool HasUnalignedMemAcc = true;
+
+    const ExtMachInst NoopMachInst = {
+        0x0,                            // No legacy prefixes.
+        0x0,                            // No rex prefix.
+        { 1, 0x0, 0x0, 0x90 },          // One opcode byte, 0x90.
+        0x0, 0x0,                       // No modrm or sib.
+        0, 0,                           // No immediate or displacement.
+        8, 8, 8,                        // All sizes are 8.
+        0,                              // Displacement size is 0.
+        SixtyFourBitMode                // Behave as if we're in 64 bit
+                                        // mode (this doesn't actually matter).
+    };
 };
 
 #endif // __ARCH_X86_ISATRAITS_HH__