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filetype-conversionbytefat16

Reading a FAT16 file system

I am trying to read a FAT16 file system to gain information about it like number of sectors, clusters, bytespersector etc...

I am trying to read it like this:

FILE *floppy;
unsigned char bootDisk[512];
floppy = fopen(name, "r");
fread(bootDisk, 1, 512, floppy);

int i;
for (i = 0; i < 80; i++){
    printf("%u,",bootDisk[i]);  
}

and it outputs this:

235,60,144,109,107,100,111,115,102,115,0,0,2,1,1,0,2,224,0,64,11,240,9,0,18,0,2,0,0,0,0,0,0,0,0,0,0,0,41,140,41,7,68,32,32,32,32,32,32,32,32,32,32,32,70,65,84,49,50,32,32,32,14,31,190,91,124,172,34,192,116,11,86,180,14,187,7,0,205,16,

What do these numbers represent and what type are they? Bytes?

Solution

  • You are not reading the values properly. Most of them are longer than 1 byte.

    From the spec you can obtain the length and meaning of every attributes in the boot sector:

    Offset  Size (bytes)    Description
0000h   3 Code to jump to the bootstrap code.
0003h   8 Oem ID - Name of the formatting OS
000Bh   2 Bytes per Sector
000Dh   1 Sectors per Cluster - Usual there is 512 bytes per sector.
000Eh   2 Reserved sectors from the start of the volume.
0010h   1 Number of FAT copies - Usual 2 copies are used to prevent data loss.
0011h   2 Number of possible root entries - 512 entries are recommended.
0013h   2 Small number of sectors - Used when volume size is less than 32 Mb.
0015h   1 Media Descriptor
0016h   2 Sectors per FAT
0018h   2 Sectors per Track
001Ah   2 Number of Heads
001Ch   4 Hidden Sectors
0020h   4 Large number of sectors - Used when volume size is greater than 32 Mb.
0024h   1 Drive Number - Used by some bootstrap code, fx. MS-DOS.
0025h   1 Reserved - Is used by Windows NT to decide if it shall check disk integrity.
0026h   1 Extended Boot Signature - Indicates that the next three fields are available.
0027h   4 Volume Serial Number
002Bh   11 Volume Label - Should be the same as in the root directory.
0036h   8 File System Type - The string should be 'FAT16 '
003Eh   448 Bootstrap code - May schrink in the future.
01FEh   2   Boot sector signature - This is the AA55h signature

    You should probably use a custom struct to read the boot sector.

    Like:

    typedef struct {
unsigned char jmp[3];
char oem[8];
unsigned short sector_size;
unsigned char sectors_per_cluster;
unsigned short reserved_sectors;
unsigned char number_of_fats;
unsigned short root_dir_entries;
[...]
} my_boot_sector;

    Keep in mind your endianness and padding rules in your implementation. This struct is an example only.

    If you need more details this is a thorough example.