In general, in one aspect, the disclosure describes a processing unit that includes a memory, an arithmetic logic unit, and control logic having access to program instructions of a control store. The control logic includes logic to access multiple sets of variables, variables in the different sets of variables being identically referenced by instructions, associate a one of the sets of variables as the current set of variables to be used in instructions that are executed by the arithmetic logic unit, change the set of variables associated with the current set of variables in response to a procedure call or exit, and alter the value of a variable of a set of the variables other than the set of variables associated with the current set of variables in response to an instruction.