The data processing device maintains two stacks. The length of the basic unit of information is different for each stack. Pops and pushes cause a change in the stack pointer which differs dependent on which stack is used. Instructions refer to different locations on the stack as registers defined by an offset of a number of basic units relative to the top of stack. In an embodiment a reference to registers is normally interpreted as a reference to one of a set of registers (including the top of stack register) without push or pop, but a reference to one register is interpreted as a reference to top of stack including a push or pop.