diff --git a/hal/instructions.go b/hal/instructions.go
index 543f0fd..710a5db 100644
--- a/hal/instructions.go
+++ b/hal/instructions.go
@@ -160,11 +160,13 @@ var InstructionJump = newJumpInstruction("JUMP", func(accumulator float64) bool
 var InstructionAdd = &Instruction{
 	Name: "ADD",
 	ExecuteWithOperand: func(module *Module, operand float64) error {
-		index := int64(operand)
-		if _, ok := module.programStorage[index]; !ok {
+		index := int(operand)
+		if len(module.register) <= index {
 			return fmt.Errorf("index %d does not exist in program storage", index)
 		}
 
+		module.accumulator += module.register[index]
+
 		return nil
 	},
 }