1. A method of operating a data processing system, comprising: executing a first instruction by an N-bit processor, wherein the first instruction is a memory access instruction with update, wherein the execution of the first instruction includes: accessing at least one source register of a plurality of registers, wherein the accessing the at least one source register includes accessing a first register of the plurality of registers, wherein each register of the plurality of registers includes a main register portion of N bits and an extension register portion of M bits, wherein the main register portion of the first register includes a first address operand; forming a memory access address using the first address operand; using the memory access address as an address for a memory access; producing an updated address operand, wherein the updated address operand is based on the first address operand; writing the updated address operand to the main portion of the first register; wherein the producing includes accessing an extension portion of a source register of the at least one source register to obtain modifying information and using the modifying information in the producing an updated address operand.