A file is stored in fat 16 (any fat) like so:

You find the root directory of the volume (which is in the first data
sectors of the volume after the fat copies), you find the filename, you
find the first data sector in its record. You get that first data sector,
it's the first sector of the file. When you want the next one, you go to
the fat (one of the copies), to the cell corresponding to the file you
were just reading, and read its contents. It will be a marker (bad,
last sector, etc) or the number of the next data sector on the disk.
Continue. fat16 is easy, fat12 is a little harder because it is packed in
a way that numbed my brains at the time.

Peter

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.