create a file system using a single file

#dd if=/dev/zero of=/fat100MB.fs bs=1024 count=102400
#mknod /dev/loop0 b 7 0
#mknod /dev/loop1 b 7 1
#losetup /dev/loop0 /fat100MB.fs
#mkdosfs -F 32 /dev/loop0
#mount -t vfat -o loop /fat100MB.fs /mnt/fat