Modify FAT16 file-system

The FAT-16 file-system consists of:

• Reserved area
• boot sector
• optional file system information sector (FAT-32) 
• FATs
• number of copies is specified in boot sector
• Data area
• a root directory
  FAT-12 & FAT-16 has fixed root dir size, while it's customizable in FAT-32. 
• data clusters

Boot sector

Bytes/Addr
Purpose
0-2/
0x00-0x02
Assembly code instructions to jump to boot code (mandatory in bootable partition)
3-10
0x03-0x0A
OEM name in ASCII
11-12/
0x0B-0x0C
Bytes per sector (512, 1024, 2048, or 4096)
13/
0x0D
Sectors per cluster (Must be a power of 2 and cluster size must be <=32 KB)
14-15/
0x0E-0x0F
Size of reserved area, in sectors
16/ 
0x10
Number of FATs (usually 2)
17-18/
0x11-0x12
Maximum number of files in the root directory (FAT12/16; 0 for FAT32)
19-20/
0x13-0x14
Number of sectors in the file system; if 2 B is not large enough, set to 0 and use 4 B value in bytes 32-35 below
21/ 0x15
Media type (0xf0=removable disk, 0xf8=fixed disk)
22-23/
0x16-0x17
Size of each FAT, in sectors, for FAT12/16; 0 for FAT32
24-25/
0x18-0x19
Sectors per track in storage device
26-27/
0x1A-0x1B
Number of heads in storage device
28-31/
0x1C-0x1F
Number of sectors before the start partition
32-35/
0x20-0x23
Number of sectors in the file system; this field will be 0 if the 2B field above (bytes 19-20) is non-zero

while for our case, 

screenshot of hexedit

===============================================

ADDR : VALUE       MEANING

===============================================

00-02: eb 3c 90    Instructions to jump to boot code

03-0a: 6d 6B 64 6f 73 66 73 00
                   Name string (mkdosfs.)
0b-0c: 00 02       Bytes/sector (0x0200 = 512)
0d   : 20          Sectors/cluster (20)
0e-0f: 20 00       Size of reserved area (20 sector)
10   : 02          Number of FATs (2)
11-12: 00 02       Max. number of root directory entries (0x0200 = 512)
13-14: 00 00       Total number of sectors (value equal to 0, check value in 0x20-0x23)
15   : F8          Media type (fixed disk)
16-17: 00 01       FAT size (0x0100 = 256 sectors)
18-19: 3F 00       Sectors/track (0x003F = 63)
1a-1b: FF 00       Number of heads (0x00FF = 255)
1c-1f: 00 00 00 00 Number of sector before partition (0)
20-23: 00 F8 1F 00 Total number of sectors (0xF800001F = 4160749599)??
24   : 00          Drive number (0)
25   : 00          Unused
26   : 29          Extended boot signature
27-2a: F5 7E 18 E5 Volume serial number (.~..)
2b-35: 20 20 20 20 20 20 20 20 20 20 20
                   Volume label ("           ")
36-3d: 46 41 54 31 36 20 20 20
                   File system type label ("FAT16 ")
3e-1fd : [snip]    Not used
1fe-1ff: 55 aa     Signature value (0xaa55)

to be continue....