diff --git a/sim/simX/Makefile b/sim/simX/Makefile
index 29b53fc3..88e030ac 100644
--- a/sim/simX/Makefile
+++ b/sim/simX/Makefile
@@ -21,7 +21,9 @@ VPATH := $(sort $(dir $(SRCS)))
 
 # Debugigng
 ifdef DEBUG
-	CXXFLAGS += -g -O0 -DDEBUG_LEVEL=$(DEBUG)
+	# CXXFLAGS += -g -O0 -DDEBUG_LEVEL=$(DEBUG)
+	# simx64
+	CXXFLAGS += -g -O0 -DDEBUG_LEVEL=4
 else    
 	CXXFLAGS += -O2 -DNDEBUG
 endif
diff --git a/sim/simX/debug.h b/sim/simX/debug.h
index ad7fd16f..94a31804 100644
--- a/sim/simX/debug.h
+++ b/sim/simX/debug.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #ifndef DEBUG_LEVEL
-#define DEBUG_LEVEL 3
+#define DEBUG_LEVEL 4
 #endif
 
 #define DEBUG_HEADER << "DEBUG "
diff --git a/sim/simX/execute.cpp b/sim/simX/execute.cpp
index 47bf4e04..7b5ecc5b 100644
--- a/sim/simX/execute.cpp
+++ b/sim/simX/execute.cpp
@@ -16,7 +16,7 @@
 using namespace vortex;
 
 static bool HasDivergentThreads(const ThreadMask &thread_mask,                                
-                                const std::vector<std::vector<Word>> &reg_file,
+                                const std::vector<std::vector<DoubleWord>> &reg_file,
                                 unsigned reg) {
   bool cond;
   size_t thread_idx = 0;
diff --git a/sim/simX/types.h b/sim/simX/types.h
index ca732040..f5b9dd0f 100644
--- a/sim/simX/types.h
+++ b/sim/simX/types.h
@@ -9,6 +9,8 @@ namespace vortex {
 typedef uint8_t  Byte;
 typedef uint32_t Word;
 typedef int32_t  WordI;
+// simx64
+typedef uint64_t DoubleWord;
 
 typedef uint32_t Addr;
 typedef uint32_t Size;
diff --git a/sim/simX/warp.cpp b/sim/simX/warp.cpp
index a505fe5c..e3e47d97 100644
--- a/sim/simX/warp.cpp
+++ b/sim/simX/warp.cpp
@@ -13,7 +13,7 @@ using namespace vortex;
 Warp::Warp(Core *core, Word id)
     : id_(id)
     , core_(core) {
-  iRegFile_.resize(core_->arch().num_threads(), std::vector<Word>(core_->arch().num_regs(), 0));
+  iRegFile_.resize(core_->arch().num_threads(), std::vector<DoubleWord>(core_->arch().num_regs(), 0));
   fRegFile_.resize(core_->arch().num_threads(), std::vector<Word>(core_->arch().num_regs(), 0));
   vRegFile_.resize(core_->arch().num_regs(), std::vector<Byte>(core_->arch().vsize(), 0));
   this->clear();
diff --git a/sim/simX/warp.h b/sim/simX/warp.h
index 7473d858..757da048 100644
--- a/sim/simX/warp.h
+++ b/sim/simX/warp.h
@@ -98,7 +98,8 @@ private:
   Word PC_;
   ThreadMask tmask_;  
   
-  std::vector<std::vector<Word>> iRegFile_;
+  // simx64
+  std::vector<std::vector<DoubleWord>> iRegFile_;
   std::vector<std::vector<Word>> fRegFile_;
   std::vector<std::vector<Byte>> vRegFile_;
   std::stack<DomStackEntry> domStack_;