1. A processor, comprising: an instruction fetch unit that, during operation, issues instructions for execution, wherein the instructions are programmer-selectable from a defined instruction set architecture (ISA); an instruction execution unit that, during operation, receives instructions for execution from the instruction fetch unit and executes a large-operand instruction defined within the ISA, wherein execution of the large-operand instruction is dependent upon a plurality of registers arranged within a plurality of register windows, wherein each of the register windows comprises a plurality of input registers and a plurality of output registers, and wherein the register windows are arranged such that for at least a given one of the register windows, one or more output registers of the given register window form one or more input registers of a successive register window; and control circuitry that, during operation, determines whether one or more of the register windows depended upon by the large-operand instruction are not present; wherein in response to determining that one or more of the register windows depended upon by the large-operand instruction are not present, the control circuitry during operation causes to be restored one or more of the register windows that were determined not to be present.