You wrote "i know that file are stored in one or more than one clusters and
information about the clusters and files are stored in File Allocation
Table".
That is true of a hard disk with a FAT file system, but not for NTFS, ZFS, EXT-2, Reiser, etc.
Search on the Web for FAT file system documentation. On an MSDOS 2.11 floppy, for example, two copies of the FAT were kept in dedicated sectors.
You can read about mmap() by googling it. It's a unix function, and if you use windows, you'll have to find the corresponding function that performs memory mapping.