A system code is stored in a first nonvolatile memory. The first nonvolatile memory and a second nonvolatile memory are heated during assembly of an electronic device including the first nonvolatile memory and a second nonvolatile memory. The heating is to a temperature sufficient to change a state of at least some memory cells in the second nonvolatile memory device. After the heating, the system code stored in the first nonvolatile memory is copied into the second nonvolatile memory. The first nonvolatile memory may he less vulnerable to temperature-related data alteration than the second nonvolatile memory. For example, the first nonvolatile memory may include a NAND flash memory and the second nonvolatile memory may include a variable resistance memory.