Code to be modified is identified and modified by a dynamic code optimizer (DCO) or other optimizer executing in the same or different address space as the application being modified. The modified code is stored in memory regions separate from the memory regions of the unmodified or pre-existing code. This allows instruction fetches by the application to access the modified code, while data accesses by the application access the unmodified code. An optimizer may also access the code, but accesses by the optimizer are considered data accesses, and therefore, such accesses would be to the unmodified code, instead of the modified code. Thus, an address mapping capability is provided in which the optimizer may access the modified code by having multiple addresses point to the same physical memory location. One address is used by the application in an instruction fetch and the other address is used by the optimizer in a data access to access the modified code.