Methods and structures for providing patches or updates to embedded ROM firmware simply and inexpensively while avoiding imposition of execution or memory fetch overhead. A patch memory includes locations storing addresses and optional alternate data values. Read/fetch operations addressed to a firmware ROM memory are applied in parallel to the patch memory. All locations of the patch memory may be compared in parallel to the supplied address to determine if a match is found in patch memory. If no match is found, the read/fetch memory cycle completes normally retrieving data from the ROM memory. If a match is found, the alternate data value is applied to the data bus in place of the ROM memory data retrieved. Any ROM location may therefore be patched regardless of whether the location stores instruction or data.