1. A data processing apparatus operating under program control in a first state, having a first domain of facilities within the apparatus including first domain storage space, or in a second state having a second domain of facilities within the apparatus including second domain storage space, for processing a sequence of instructions normally in the first state, the sequence of instructions including an instruction that results in a branch to the second state for execution of a program of instructions in the second domain, comprising: an instruction register, connected to receive instructions in the first and second states, including a plurality of fields, at least one field identifying a location for address information for an operand; decoding means, responsive to an instruction in the first state, for decoding the instruction to generate a control code, the control code including a branch signal and a domain access activation signal; means, connected to receive and responsive to the branch signal, for branching program control of the apparatus to the second state to enter the program of instructions in the second domain; interface means, responsive to the domain access activation signal and the one field in each instruction in the program of instructions, for generating a first domain access signal; addressing means, in communication with the instruction register, for supplying an address of an operand; and storage means, connected to receive the first domain access signal and the address, for storing operands at locations identified by addresses from the first and second domain storage spaces including means, responsive to the first domain access signal, for translating the address to first domain address space during processing of an instruction in the second state.