This command shows which areas of the flash memory are currently not in use. When a block contains non-erased contents it is considered in use. Since it is possible to force an image to be loaded at a particular flash location, this command can be used to check whether that location is in use by any other image.
Note: There is currently no cross-checking between actual flash contents and the FIS directory, which mans that there could be a segment of flash which is not erased that does not correspond to a named image, or vice-versa.