A method and system for overriding selected ROM code functions or adding new ROM code functions within a processing system. A system designer determines an existing ROM address for the selected existing code function or a desired ROM address for the new code function. The system designer then programs a patch to replace the selected existing code function or programs a new code function. The patch or new code function is then loaded into a first memory. A loader module is also programmed and loaded into the first memory. Upon system boot-up, the loader module transfers any patches or new code functions within the first memory into a second memory that is memory-mapped to the ROM. This second memory can be accessed by the processor at a faster rate than the processor can access ROM. During a processor request cycle, the processor first examines the second memory for the presence of a desired ROM code function. If a patch or a new code function that has been transferred into the second memory maps to the location of the desired code function in ROM, then the processor will fetch the patch or new code function from the second memory.