diff --git a/include/sirit/sirit.h b/include/sirit/sirit.h
index 996b8db..892158e 100644
--- a/include/sirit/sirit.h
+++ b/include/sirit/sirit.h
@@ -292,6 +292,15 @@ class Module {
     /// Integer addition of Operand 1 and Operand 2.
     Id OpIAdd(Id result_type, Id operand_1, Id operand_2);
 
+    // Extensions
+
+    /// Execute an instruction in an imported set of extended instructions.
+    Id OpExtInst(Id result_type, Id set, std::uint32_t instruction,
+                 const std::vector<Id>& operands);
+
+    /// Result is x if x >= 0; otherwise result is -x.
+    Id OpFAbs(Id result_type, Id x);
+
   private:
     Id AddCode(std::unique_ptr<Op> op);
 
@@ -301,27 +310,23 @@ class Module {
 
     Id AddAnnotation(std::unique_ptr<Op> op);
 
+    Id GetGLSLstd450();
+
     const std::uint32_t version;
 
     std::uint32_t bound{1};
 
     std::set<spv::Capability> capabilities;
-
-    std::set<std::string> extensions;
-
+    std::unique_ptr<Op> glsl_std_450;
     std::set<std::unique_ptr<Op>> ext_inst_import;
 
     spv::AddressingModel addressing_model{spv::AddressingModel::Logical};
     spv::MemoryModel memory_model{spv::MemoryModel::GLSL450};
 
     std::vector<std::unique_ptr<Op>> entry_points;
-
     std::vector<std::unique_ptr<Op>> execution_mode;
-
     std::vector<std::unique_ptr<Op>> debug;
-
     std::vector<std::unique_ptr<Op>> annotations;
-
     std::vector<std::unique_ptr<Op>> declarations;
 
     std::vector<Id> global_variables;
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index b3d717e..eacfdbb 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -24,6 +24,7 @@ add_library(sirit
     insts/conversion.cpp
     insts/bit.cpp
     insts/arithmetic.cpp
+    insts/extension.cpp
 )
 target_include_directories(sirit
                            PUBLIC ../include
diff --git a/src/insts/extension.cpp b/src/insts/extension.cpp
new file mode 100644
index 0000000..ad842b1
--- /dev/null
+++ b/src/insts/extension.cpp
@@ -0,0 +1,28 @@
+/* This file is part of the sirit project.
+ * Copyright (c) 2018 ReinUsesLisp
+ * This software may be used and distributed according to the terms of the GNU
+ * Lesser General Public License version 2.1 or any later version.
+ */
+
+#include "common_types.h"
+#include "op.h"
+#include "sirit/sirit.h"
+#include <memory>
+#include <spirv/unified1/GLSL.std.450.h>
+
+namespace Sirit {
+
+Id Module::OpExtInst(Id result_type, Id set, u32 instruction,
+                     const std::vector<Id>& operands) {
+    auto op{std::make_unique<Op>(spv::Op::OpExtInst, bound++, result_type)};
+    op->Add(set);
+    op->Add(instruction);
+    op->Add(operands);
+    return AddCode(std::move(op));
+}
+
+Id Module::OpFAbs(Id result_type, Id x) {
+    return OpExtInst(result_type, GetGLSLstd450(), GLSLstd450FAbs, {x});
+}
+
+} // namespace Sirit
\ No newline at end of file
diff --git a/src/sirit.cpp b/src/sirit.cpp
index 56b25b8..6726985 100644
--- a/src/sirit.cpp
+++ b/src/sirit.cpp
@@ -43,7 +43,9 @@ std::vector<u8> Module::Assemble() const {
     for (const auto capability : capabilities) {
         WriteEnum(stream, spv::Op::OpCapability, capability);
     }
-    // TODO write extensions
+    if (glsl_std_450) {
+        glsl_std_450->Write(stream);
+    }
     // TODO write ext inst imports
 
     Op memory_model_ref{spv::Op::OpMemoryModel};
@@ -123,4 +125,11 @@ Id Module::AddAnnotation(std::unique_ptr<Op> op) {
     return id;
 }
 
+Id Module::GetGLSLstd450() {
+    if (!glsl_std_450) {
+        glsl_std_450 = std::make_unique<Op>(spv::Op::OpExtInstImport, bound++);
+    }
+    return glsl_std_450.get();
+}
+
 } // namespace Sirit