# Header: Boot Sector

```c
 struct fat_boot_sector {
         __u8    ignored[3];     /* Boot strap short or near jump */
         __u8    system_id[8];   /* Name - can be used to special case
                                    partition manager volumes */
         __u8    sector_size[2]; /* bytes per logical sector */
         __u8    sec_per_clus;   /* sectors/cluster */
         __le16  reserved;       /* reserved sectors */
         __u8    fats;           /* number of FATs */
         __u8    dir_entries[2]; /* root directory entries */
         __u8    sectors[2];     /* number of sectors */
         __u8    media;          /* media code */
         __le16  fat_length;     /* sectors/FAT */
         __le16  secs_track;     /* sectors per track */
         __le16  heads;          /* number of heads */
         __le32  hidden;         /* hidden sectors (unused) */
         __le32  total_sect;     /* number of sectors (if sectors == 0) */

         /* The following fields are only used by FAT32 */
         __le32  fat32_length;   /* sectors/FAT */
         __le16  flags;          /* bit 8: fat mirroring, low 4: active fat */
         __u8    version[2];     /* major, minor filesystem version */
         __le32  root_cluster;   /* first cluster in root directory */
         __le16  info_sector;    /* filesystem info sector */
         __le16  backup_boot;    /* backup boot sector */
         __le16  reserved2[6];   /* Unused */
 };
```

To use it, first declare a`fat_boot_sector`structure, then use`fread()`to read the first n bytes of data to that structure, where`n`is the size of`fat_boot_sector`structure. After reading all the parameters are read into the variables respectively.

```c
// Declaration
struct fat_boot_sector boot_entry;
```

## Getting the information <a href="#getting-the-information" id="getting-the-information"></a>

The following can be retrieved directly from the structure:

1. Number of FATs (`fats`)
2. Number of sectors per cluster  (`sec_per_clus`)
3. Number of Reserved Sector(`reserved`)

And the following information requires some manipulations/calculations

1. Bytes Per Sector (`sector_size[0] + sector_size[1] << 8`)
2. First FAT Starting Position (Bytes Per Sector \* Number of Reserved Sector)
3. Data Area Starting Position (Bytes Per Sector \* (Number of Reserved Sector + Number of FATs \* `fat_length`))

Here, `fat_length` is the number of sectors per FAT.

![](/files/-Lp-5bPGBDzANDiYSuXA)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://eric-lo.gitbook.io/lab9-filesystem/accessing-fat-using-c/c-header-of-fat/header-boot-sector.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.