A system comprises a main stack, a local data stack and plurality of flags. The main stack comprises a plurality of entries and is located outside a processor's core. The local data stack is coupled to the main stack and is located internal to the processor's core. The local data stack has a plurality of entries that correspond to entries in the main stack. Each flag is associated with a corresponding entry in the local data stack and indicates whether the data in the corresponding local data stack entry is valid. The system performs two instructions. One instruction synchronizes the main stack to the local data stack and invalidates the local data stack, while the other instruction synchronizes the main stack without invalidating the local data stack.