The operating firmware of a portable flash memory storage device is stored in the relatively large file storage memory, which is non executable. It is logically parsed into overlays to fit into an executable memory. The overlays can be of differing sizes to organize function calls efficiently while minimizing dead space or unnecessarily separating functions that should be within one or a group of frequently accessed overlays. Eviction of the overlays is preferably carried out on a least recently loaded basis. These features minimize latency caused by calling overlays unnecessarily and minimize fragmentation of the random access memory used for the overlays.