diff --git a/CMakeLists.txt b/CMakeLists.txt
index d43741d..17af31e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -77,6 +77,9 @@ add_library(lib${PROJECT_NAME} SHARED
         src/vm/vm_exceptions.hh
         src/vm/vm.cc
         src/vm/vm.hh
+        src/vm/expr.cc
+        src/vm/expr.hh
+        src/vm/location.hh
 )
 
 target_compile_options(lib${PROJECT_NAME} PRIVATE ${warnings})
@@ -85,8 +88,7 @@ target_compile_options(lib${PROJECT_NAME} PRIVATE ${warnings})
 # tests
 #
 
-add_executable(${PROJECT_NAME}-bytecode-test src/bytecode/tests.cc
-        src/vm/location.hh)
+add_executable(${PROJECT_NAME}-bytecode-test src/bytecode/tests.cc)
 target_link_libraries(${PROJECT_NAME}-bytecode-test lib${PROJECT_NAME} gtest_main)
 add_test(NAME tyche_bytecode_test COMMAND ${PROJECT_NAME}-bytecode-test)
 
diff --git a/TODO.md b/TODO.md
index a00258d..842b83a 100644
--- a/TODO.md
+++ b/TODO.md
@@ -25,9 +25,10 @@ After some additional development:
       - [x] Output bytecode format
   - [x] Value object
   - [x] Stack object
-  - [ ] External interface
-  - [ ] Code execution (except functions)
-  - [ ] Functions
+  - [x] External interface
+  - [x] Code execution (except functions)
+  - [x] Functions
+  - [ ] Print stack
 
 After some additional development:
 - [ ] Bytecode loader
diff --git a/src/vm/expr.cc b/src/vm/expr.cc
new file mode 100644
index 0000000..6475798
--- /dev/null
+++ b/src/vm/expr.cc
@@ -0,0 +1,18 @@
+#include "expr.hh"
+
+#include "vm_exceptions.hh"
+
+namespace tyche {
+
+Value binary_operation(Value const& a, Value const& b, BinaryOperationType op)
+{
+    // TODO - this is temporary code
+
+    if (a.type() == Type::Integer && b.type() == Type::Integer && op == BinaryOperationType::Sum) {
+        return Value::CreateInteger(a.as_integer() + b.as_integer());
+    } else {
+        throw VMInvalidOperation(op, a.type(), b.type());
+    }
+}
+
+}
\ No newline at end of file
diff --git a/src/vm/expr.hh b/src/vm/expr.hh
new file mode 100644
index 0000000..1641534
--- /dev/null
+++ b/src/vm/expr.hh
@@ -0,0 +1,12 @@
+#ifndef TYCHE_EXPR_HH
+#define TYCHE_EXPR_HH
+#include "value.hh"
+
+namespace tyche {
+
+enum class BinaryOperationType { Sum };
+
+Value binary_operation(Value const& a, Value const& b, BinaryOperationType op);
+
+}
+#endif //TYCHE_EXPR_HH
diff --git a/src/vm/vm.cc b/src/vm/vm.cc
index 18c4ed9..a6b0e5b 100644
--- a/src/vm/vm.cc
+++ b/src/vm/vm.cc
@@ -1,6 +1,7 @@
 #include "vm.hh"
 
 #include "vm_exceptions.hh"
+#include "expr.hh"
 
 namespace tyche {
 
@@ -21,7 +22,7 @@ void VM::call(size_t n_params)
     loc_.emplace(f.as_function_id(), 0);
     stack_.push_fp();
     run_until_return();
-    stack_.pop_fp();
+    // stack_.pop_fp();
     loc_.pop();
 }
 
@@ -54,6 +55,12 @@ void VM::step()
         case Instruction::PushInt32:
             push_integer(op.operator_);
             break;
+        case Instruction::Sum:
+            stack_.push(binary_operation(stack_.pop(), stack_.pop(), BinaryOperationType::Sum));
+            break;
+        case Instruction::Return:
+            stack_.pop_fp();
+            return;
         default:
             throw VMInvalidOpcode((uint8_t) op.instruction);
     }
diff --git a/src/vm/vm_exceptions.hh b/src/vm/vm_exceptions.hh
index d20eac5..49152dc 100644
--- a/src/vm/vm_exceptions.hh
+++ b/src/vm/vm_exceptions.hh
@@ -4,6 +4,8 @@
 #include <stdexcept>
 #include <string>
 
+#include "expr.hh"
+
 namespace tyche {
 
 class VMRuntimeError : public std::runtime_error
@@ -36,6 +38,12 @@ public:
     explicit VMInvalidOpcode(uint8_t opcode) : VMRuntimeError("Invalid opcode " + std::to_string(opcode)) {}
 };
 
+class VMInvalidOperation : public VMRuntimeError
+{
+public:
+    explicit VMInvalidOperation(BinaryOperationType op, Type type1, Type type2) : VMRuntimeError("Invalid binary operation") {}  // TODO - print types
+};
+
 }
 
 #endif //TYCHE_VM_EXCEPTIONS_HH