diff --git a/main.go b/main.go
index 54edfc7..25481dd 100644
--- a/main.go
+++ b/main.go
@@ -17,6 +17,8 @@ const (
 )
 
 const InstructionBytes = 32
+const GlobalMemoryEntries = 16
+const FunctionMemoryEntries = 16
 
 type Operand struct {
 	Type     OperandType
@@ -39,15 +41,14 @@ type State struct {
 	InstructionIndex int64
 
 	ComparisonResult bool
-	GlobalMemory     [16]uint64
+	GlobalMemory     [GlobalMemoryEntries]uint64
 	Stack            []*StackFrame
-	StackFrame       *StackFrame
 }
 
 type StackFrame struct {
 	PreviousFunctionIndex    int64
 	PreviousInstructionIndex int64
-	FunctionMemory           [16]uint64
+	FunctionMemory           [FunctionMemoryEntries]uint64
 }
 
 type OpHandler func(*State, *Instruction)
@@ -86,15 +87,26 @@ func NewState(functionByteCode [][]byte) *State {
 	}
 }
 
+func (state *State) StackFrame() *StackFrame {
+	return state.Stack[len(state.Stack) - 1]
+}
+
+func (state *State) Function() []byte {
+	return state.FunctionByteCode[state.FunctionIndex]
+}
+
 func (state *State) Execute() {
 	state.call(0)
 
 	for len(state.Stack) > 0 && state.Error == nil {
-		fnc := state.FunctionByteCode[state.FunctionIndex]
 		start := state.InstructionIndex * InstructionBytes
-		chunk := fnc[start : start+InstructionBytes]
+		chunk := state.Function()[start : start+InstructionBytes]
 		state.InstructionIndex += 1
 		state.ProcessInstruction(chunk)
+
+		if state.InstructionIndex >= int64(len(state.Function()) / InstructionBytes) {
+			state.ret()
+		}
 	}
 
 	if state.Error != nil {
@@ -128,10 +140,21 @@ func (state *State) ReadUnsigned(op *Operand) uint64 {
 	switch op.Type {
 	case Literal:
 		return op.Value
+
 	case FunctionMemoryIndex:
-		return state.StackFrame.FunctionMemory[op.Value]
+		if op.Value >= FunctionMemoryEntries {
+			state.Error = fmt.Errorf("Invalid function memory index: %016x", op.Value)
+			return 0
+		}
+		return state.StackFrame().FunctionMemory[op.Value]
+
 	case GlobalMemoryIndex:
+		if op.Value >= GlobalMemoryEntries {
+			state.Error = fmt.Errorf("Invalid global memory index: %016x", op.Value)
+			return 0
+		}
 		return state.GlobalMemory[op.Value]
+
 	default:
 		state.Error = fmt.Errorf("Unknown operand type: 0x%02x", op.Type)
 		return 0
@@ -146,10 +169,21 @@ func (state *State) WriteUnsigned(op *Operand, value uint64) {
 	switch op.Type {
 	case Literal:
 		state.Error = fmt.Errorf("Write to literal operand")
+
 	case FunctionMemoryIndex:
-		state.StackFrame.FunctionMemory[op.Value] = value
+		if op.Value >= FunctionMemoryEntries {
+			state.Error = fmt.Errorf("Invalid function memory index: %016x", op.Value)
+			return
+		}
+		state.StackFrame().FunctionMemory[op.Value] = value
+
 	case GlobalMemoryIndex:
+		if op.Value >= GlobalMemoryEntries {
+			state.Error = fmt.Errorf("Invalid global memory index: %016x", op.Value)
+			return
+		}
 		state.GlobalMemory[op.Value] = value
+
 	default:
 		state.Error = fmt.Errorf("Unknown operand type: 0x%02x", op.Type)
 	}
@@ -170,24 +204,29 @@ func (state *State) Call(instr *Instruction) {
 }
 
 func (state *State) call(functionOffset int64) {
+	if state.FunctionIndex + functionOffset >= int64(len(state.FunctionByteCode)) {
+		state.Error = fmt.Errorf("Invalid function call index: %d + %d = %d", state.FunctionIndex, functionOffset, state.FunctionIndex + functionOffset)
+		return
+	}
+
 	stackFrame := &StackFrame{
 		PreviousFunctionIndex:    state.FunctionIndex,
 		PreviousInstructionIndex: state.InstructionIndex,
 	}
 	state.Stack = append(state.Stack, stackFrame)
-	state.StackFrame = stackFrame
 	state.FunctionIndex += functionOffset
 	state.InstructionIndex = 0
 }
 
 func (state *State) Return(instr *Instruction) {
 	fmt.Printf("Return\n")
-	state.FunctionIndex = state.StackFrame.PreviousFunctionIndex
-	state.InstructionIndex = state.StackFrame.PreviousInstructionIndex
+	state.ret()
+}
+
+func (state *State) ret() {
+	state.FunctionIndex = state.StackFrame().PreviousFunctionIndex
+	state.InstructionIndex = state.StackFrame().PreviousInstructionIndex
 	state.Stack = state.Stack[:len(state.Stack)-1]
-	if len(state.Stack) > 0 {
-		state.StackFrame = state.Stack[len(state.Stack)-1]
-	}
 }
 
 func (state *State) Move(instr *Instruction) {
@@ -321,11 +360,9 @@ func main() {
 			"0000000100000000000000000000000000000001000000000000000000000000",
 			"0000030100000000010000000000000000000000000000000000000000000003",
 			"000004010000000000000000fffffffffffffffd000000000000000000000000",
-			"0000000200000000000000000000000000000000000000000000000000000000",
 		},
 		[]string{
 			"0000020000000000020000000000000000000000000000000000000000000001",
-			"0000000200000000000000000000000000000000000000000000000000000000",
 		},
 	}