diff --git a/alethzero/Main.ui b/alethzero/Main.ui
index a01ac7856..f5b52c309 100644
--- a/alethzero/Main.ui
+++ b/alethzero/Main.ui
@@ -185,6 +185,7 @@
     <addaction name="killBlockchain"/>
     <addaction name="inject"/>
     <addaction name="forceMining"/>
+    <addaction name="enableOptimizer"/>
    </widget>
    <addaction name="menu_File"/>
    <addaction name="menu_Network"/>
@@ -563,7 +564,7 @@
        </widget>
        <widget class="QTextEdit" name="code">
         <property name="focusPolicy">
-         <enum>Qt::NoFocus</enum>
+         <enum>Qt::ClickFocus</enum>
         </property>
         <property name="frameShape">
          <enum>QFrame::NoFrame</enum>
@@ -571,6 +572,9 @@
         <property name="lineWidth">
          <number>0</number>
         </property>
+        <property name="readOnly">
+         <bool>true</bool>
+        </property>
        </widget>
       </widget>
      </item>
@@ -1664,6 +1668,14 @@ font-size: 14pt</string>
     <string>&amp;Refresh</string>
    </property>
   </action>
+  <action name="enableOptimizer">
+   <property name="checkable">
+    <bool>true</bool>
+   </property>
+   <property name="text">
+    <string>&amp;Enable LLL &amp;Optimizer</string>
+   </property>
+  </action>
  </widget>
  <layoutdefault spacing="6" margin="11"/>
  <customwidgets>
diff --git a/alethzero/MainWin.cpp b/alethzero/MainWin.cpp
index 6c18d39bd..a4fc22a43 100644
--- a/alethzero/MainWin.cpp
+++ b/alethzero/MainWin.cpp
@@ -332,6 +332,12 @@ void Main::on_forceMining_triggered()
 	m_client->setForceMining(ui->forceMining->isChecked());
 }
 
+void Main::on_enableOptimizer_triggered()
+{
+	m_enableOptimizer = ui->enableOptimizer->isChecked();
+	on_data_textChanged();
+}
+
 void Main::load(QString _s)
 {
 	QFile fin(_s);
@@ -523,6 +529,7 @@ void Main::writeSettings()
 	s.setValue("paranoia", ui->paranoia->isChecked());
 	s.setValue("showAll", ui->showAll->isChecked());
 	s.setValue("showAllAccounts", ui->showAllAccounts->isChecked());
+	s.setValue("enableOptimizer", m_enableOptimizer);
 	s.setValue("clientName", ui->clientName->text());
 	s.setValue("idealPeers", ui->idealPeers->value());
 	s.setValue("port", ui->port->value());
@@ -570,6 +577,8 @@ void Main::readSettings(bool _skipGeometry)
 	ui->paranoia->setChecked(s.value("paranoia", false).toBool());
 	ui->showAll->setChecked(s.value("showAll", false).toBool());
 	ui->showAllAccounts->setChecked(s.value("showAllAccounts", false).toBool());
+	m_enableOptimizer = s.value("enableOptimizer", true).toBool();
+	ui->enableOptimizer->setChecked(m_enableOptimizer);
 	ui->clientName->setText(s.value("clientName", "").toString());
 	ui->idealPeers->setValue(s.value("idealPeers", ui->idealPeers->value()).toInt());
 	ui->port->setValue(s.value("port", ui->port->value()).toInt());
@@ -1302,9 +1311,7 @@ void Main::on_data_textChanged()
 		}
 		else
 		{
-			auto asmcode = eth::compileLLLToAsm(src, false);
-			auto asmcodeopt = eth::compileLLLToAsm(ui->data->toPlainText().toStdString(), true);
-			m_data = eth::compileLLL(ui->data->toPlainText().toStdString(), true, &errors);
+			m_data = eth::compileLLL(src, m_enableOptimizer, &errors);
 			if (errors.size())
 			{
 				try
@@ -1319,7 +1326,15 @@ void Main::on_data_textChanged()
 				}
 			}
 			else
-				lll = "<h4>Opt</h4><pre>" + QString::fromStdString(asmcodeopt).toHtmlEscaped() + "</pre><h4>Pre</h4><pre>" + QString::fromStdString(asmcode).toHtmlEscaped() + "</pre>";
+			{
+				auto asmcode = eth::compileLLLToAsm(src, false);
+				lll = "<h4>Pre</h4><pre>" + QString::fromStdString(asmcode).toHtmlEscaped() + "</pre>";
+				if (m_enableOptimizer)
+				{
+					asmcode = eth::compileLLLToAsm(src, true);
+					lll = "<h4>Opt</h4><pre>" + QString::fromStdString(asmcode).toHtmlEscaped() + "</pre>" + lll;
+				}
+			}
 		}
 		QString errs;
 		if (errors.size())
diff --git a/alethzero/MainWin.h b/alethzero/MainWin.h
index f54a64053..46cb1843a 100644
--- a/alethzero/MainWin.h
+++ b/alethzero/MainWin.h
@@ -138,6 +138,7 @@ private slots:
 	void on_debugDumpState_triggered(int _add = 1);
 	void on_debugDumpStatePre_triggered();
 	void on_refresh_triggered();
+	void on_enableOptimizer_triggered();
 
 signals:
 	void poll();
@@ -221,6 +222,7 @@ private:
 	QMap<unsigned, unsigned> m_pcWarp;
 	QList<WorldState> m_history;
 	std::map<eth::u256, eth::bytes> m_codes;	// and pcWarps
+	bool m_enableOptimizer = true;
 
 	QNetworkAccessManager m_webCtrl;
 
diff --git a/libevm/VM.h b/libevm/VM.h
index 15f05fd26..f014e2da8 100644
--- a/libevm/VM.h
+++ b/libevm/VM.h
@@ -241,9 +241,9 @@ template <class Ext> eth::bytesConstRef eth::VM::go(Ext& _ext, OnOpFunc const& _
 		{
 			require(2);
 			auto base = m_stack.back();
-			unsigned expon = (unsigned)m_stack[m_stack.size() - 2];
+			auto expon = m_stack[m_stack.size() - 2];
 			m_stack.pop_back();
-			m_stack.back() = boost::multiprecision::pow(base, expon);
+			m_stack.back() = (u256)boost::multiprecision::powm((bigint)base, (bigint)expon, bigint(2) << 256);
 			break;
 		}
 		case Instruction::NEG:
diff --git a/liblll/Assembly.cpp b/liblll/Assembly.cpp
index 78552f3c5..491d3812e 100644
--- a/liblll/Assembly.cpp
+++ b/liblll/Assembly.cpp
@@ -227,14 +227,14 @@ Assembly& Assembly::optimise(bool _enable)
 	{
 		{ Instruction::SUB, [](u256 a, u256 b)->u256{return a - b;} },
 		{ Instruction::DIV, [](u256 a, u256 b)->u256{return a / b;} },
-        { Instruction::SDIV, [](u256 a, u256 b)->u256{return s2u(u2s(a) / u2s(b));} },
+		{ Instruction::SDIV, [](u256 a, u256 b)->u256{return s2u(u2s(a) / u2s(b));} },
 		{ Instruction::MOD, [](u256 a, u256 b)->u256{return a % b;} },
-        { Instruction::SMOD, [](u256 a, u256 b)->u256{return s2u(u2s(a) % u2s(b));} },
-		{ Instruction::EXP, [](u256 a, u256 b)->u256{return boost::multiprecision::pow(a, (unsigned)b);} },
+		{ Instruction::SMOD, [](u256 a, u256 b)->u256{return s2u(u2s(a) % u2s(b));} },
+		{ Instruction::EXP, [](u256 a, u256 b)->u256{return (u256)boost::multiprecision::powm((bigint)a, (bigint)b, bigint(2) << 256);} },
 		{ Instruction::LT, [](u256 a, u256 b)->u256{return a < b ? 1 : 0;} },
 		{ Instruction::GT, [](u256 a, u256 b)->u256{return a > b ? 1 : 0;} },
-        { Instruction::SLT, [](u256 a, u256 b)->u256{return u2s(a) < u2s(b) ? 1 : 0;} },
-        { Instruction::SGT, [](u256 a, u256 b)->u256{return u2s(a) > u2s(b) ? 1 : 0;} },
+		{ Instruction::SLT, [](u256 a, u256 b)->u256{return u2s(a) < u2s(b) ? 1 : 0;} },
+		{ Instruction::SGT, [](u256 a, u256 b)->u256{return u2s(a) > u2s(b) ? 1 : 0;} },
 		{ Instruction::EQ, [](u256 a, u256 b)->u256{return a == b ? 1 : 0;} },
 	};
 	map<Instruction, function<u256(u256, u256)>> c_associative =