### The IMG file format

The IMG file is internally structured like a file-system containing a number of files and a directory. It is common to refer to the whole thing as a file and the contained files as sub-files. However I use the terms file system and file respectively, because it is more descriptive of what is going on I think.

There are two main sub-packages here.

fs
  This deals with filesystem as a whole. The individual files are just blobs of data. You could handle any filename at this level.
app
  Code for actual application files that are used. Code in here knows about the internal structure of individual files.

As the purpose of this application is to write IMG files (there are plenty of programs that read it), the focus is on writing and not reading. The code to read at the filesystem level will probably be implemented at some stage so that we can combine several .img files into one gmapsupp.img file.