diff --git a/stackframe.go b/stackframe.go
index 006cce3..b91fe73 100644
--- a/stackframe.go
+++ b/stackframe.go
@@ -1,9 +1,15 @@
 package vm
 
-const functionMemoryEntries = 16
-
 type stackFrame struct {
 	previousFunctionIndex    int64
 	previousInstructionIndex int64
-	functionMemory           [functionMemoryEntries]uint64
+	functionMemory           *memory
+}
+
+func newStackFrame(state *State) *stackFrame {
+	return &stackFrame{
+		previousFunctionIndex:    state.functionIndex,
+		previousInstructionIndex: state.instructionIndex,
+		functionMemory:           newMemory(16),
+	}
 }
diff --git a/state.go b/state.go
index 944ab53..9bf4c57 100644
--- a/state.go
+++ b/state.go
@@ -4,8 +4,6 @@ import "fmt"
 
 import "github.com/pkg/errors"
 
-const globalMemoryEntries = 16
-
 type State struct {
 	running bool
 	err     error
@@ -22,7 +20,7 @@ type State struct {
 func NewState(functions [][]*Instruction) (*State, error) {
 	return &State{
 		functions:    functions,
-		globalMemory: newMemory(globalMemoryEntries),
+		globalMemory: newMemory(16),
 	}, nil
 }
 
@@ -91,11 +89,11 @@ func (state *State) readUnsigned(op *Operand) uint64 {
 		return op.Value
 
 	case FunctionMemoryIndex:
-		if op.Value >= functionMemoryEntries {
-			state.setError(fmt.Errorf("Invalid function memory index: %016x", op.Value))
-			return 0
+		value, err := state.stackFrame().functionMemory.readUnsigned(op.Value)
+		if err != nil {
+			state.setError(err)
 		}
-		return state.stackFrame().functionMemory[op.Value]
+		return value
 
 	case GlobalMemoryIndex:
 		value, err := state.globalMemory.readUnsigned(op.Value)
@@ -120,11 +118,10 @@ func (state *State) writeUnsigned(op *Operand, value uint64) {
 		state.setError(fmt.Errorf("Write to literal operand"))
 
 	case FunctionMemoryIndex:
-		if op.Value >= functionMemoryEntries {
-			state.setError(fmt.Errorf("Invalid function memory index: %016x", op.Value))
-			return
+		err := state.stackFrame().functionMemory.writeUnsigned(op.Value, value)
+		if err != nil {
+			state.setError(err)
 		}
-		state.stackFrame().functionMemory[op.Value] = value
 
 	case GlobalMemoryIndex:
 		err := state.globalMemory.writeUnsigned(op.Value, value)
@@ -147,10 +144,7 @@ func (state *State) call(functionOffset int64) {
 		return
 	}
 
-	stackFrame := &stackFrame{
-		previousFunctionIndex:    state.functionIndex,
-		previousInstructionIndex: state.instructionIndex,
-	}
+	stackFrame := newStackFrame(state)
 	state.stack = append(state.stack, stackFrame)
 	state.functionIndex += functionOffset
 	state.instructionIndex = 0