XOM (eXecute-Only-Memory) is a secure zone of the ROM used to save secure application instructions; the memory address where these instructions are located prohibits any type of data access from the AHB bus and can only be executed by the CPU. Users can use standard program memory programming instructions to call functions (APIs) to execute the program codes saved in the XOM zone, but these contents cannot be accessed through any other means, as shown in the figure below:

Users only need to save the key program codes that need to be protected in the XOM zone; for example, the encryption and decryption algorithms of security keys, firmware update programs, and voice recognition algorithms. Not only can it be used as complete development projects, but it can also even be used as function modules and provided to third-parties for further application development. Once users place the voice recognition algorithm into the XOM, it can protect the algorithm so it cannot be accessed by any other means; it can then be made into voice recognition modules and sold to manufacturers for standard application program development such as the biometric smart door locks, smart appliances, smart speakers, and home security systems in the smart home field.

Nuvoton’s NuMicro M487 Flash 256KB series supports up to four XOM zones with freely configurable space; when setting the XOM zones, users must first set the base address and then set the XOM page size, as shown in the figure below:

This XOM function provides secure environments for project development; users can set program code zones as XOM zones to protect important source codes and prevent the source codes inside the XOM zone from being stolen from external attacks. Nuvoton provided the XOM function for its M487 Flash 256KB series to ensure that the applications developed by users are protected more securely.