1. In a processor including a register stack, the register stack including a plurality of register stack frames, each of the register stack frames including zero or more registers, one of the plurality of register stack frames comprising a current register stack frame, a method comprising steps of: (A) receiving an instruction whose execution requires writing to a register referenced by the instruction; (B) determining whether the register referenced by the instruction is in the current register stack frame; and (C) executing the instruction only if the register is determined to be in the current register stack frame.