diff --git a/instrs.txt b/instrs.txt
index 45a3512..a141f2f 100644
--- a/instrs.txt
+++ b/instrs.txt
@@ -464,7 +464,7 @@ F3.0fbd               RM    GP     GP     -      -      LZCNT USE66
 0fc0                  MR    GP     GP     -      -      XADD SIZE_8 LOCK
 0fc1                  MR    GP     GP     -      -      XADD LOCK
 NP.0fc3/m             MR    MEM    GP     -      -      MOVNTI
-NP.0fc7/1m            M     MEMZ   -      -      -      CMPXCHGD LOCK INSTR_WIDTH
+0fc7/1m               M     MEMZ   -      -      -      CMPXCHGD LOCK IGN66 INSTR_WIDTH
 0fc8+                 O     GP     -      -      -      BSWAP
 0fff                  RM    GP     GP     -      -      UD0
 #
diff --git a/tests/test_decode.c b/tests/test_decode.c
index b0a4181..0c574b4 100644
--- a/tests/test_decode.c
+++ b/tests/test_decode.c
@@ -83,7 +83,13 @@ main(int argc, char** argv)
     TEST64("\x66\x90", "nop");
     TEST32("\x0f\xc7\x0f", "cmpxchg8b qword ptr [edi]");
     TEST64("\x0f\xc7\x0f", "cmpxchg8b qword ptr [rdi]");
+    TEST32("\x66\x0f\xc7\x0f", "cmpxchg8b qword ptr [edi]"); // 66h is ignored
+    TEST64("\x66\x0f\xc7\x0f", "cmpxchg8b qword ptr [rdi]"); // 66h is ignored
     TEST64("\x48\x0f\xc7\x0f", "cmpxchg16b xmmword ptr [rdi]");
+    TEST32("\xf2\x0f\xc7\x0f", "cmpxchg8b qword ptr [edi]");
+    TEST64("\xf2\x0f\xc7\x0f", "cmpxchg8b qword ptr [rdi]");
+    TEST32("\xf3\x0f\xc7\x0f", "cmpxchg8b qword ptr [edi]");
+    TEST64("\xf3\x0f\xc7\x0f", "cmpxchg8b qword ptr [rdi]");
     TEST("\x66", "PARTIAL");
     TEST("\xf0", "PARTIAL");
     TEST("\x0f", "PARTIAL");