FAT16 filesystem reader, work in progress.

fat16.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include "fat.h"

#define FS_PATH "sample.fat16"

int main() {
  print_boot_sector();
  return EXIT_SUCCESS;
}

void print_boot_sector() {
  FILE * fs;
  struct fat16_boot_sector bs;
  struct fat_preamble * pre;
  struct fat_bios_parameter_block * bp;
  struct fat16_extended_bios_parameter_block * ebp;

  // presentation variables
  int total_sectors;
  char oem_name[FAT_OEM_NAME_LENGTH + 1];
  char label[FAT_LABEL_LENGTH + 1];
  char fs_type[FAT_FS_TYPE_LENGTH + 1];

  fs = fopen(FS_PATH, "r");
  fread(&bs, sizeof(bs), 1, fs);
  fclose(fs);

  pre = &bs.preamble;
  bp = &bs.bios_params;
  ebp = &bs.ext_bios_params;

  total_sectors = bp->total_sectors == 0 ? (int)bp->total_sectors_large : (int)bp->total_sectors;

  // null terminate strings
  fat_string_copy(oem_name, pre->oem_name, FAT_OEM_NAME_LENGTH);
  fat_string_copy(label, ebp->label, FAT_LABEL_LENGTH);
  fat_string_copy(fs_type, ebp->fs_type, FAT_FS_TYPE_LENGTH);

  putchar('\n');
  printf("Boot sector size: %d bytes\n", (int)sizeof(bs));
  printf("Jump instruction: %X %X %X\n", pre->jump_instruction[0], pre->jump_instruction[1], pre->jump_instruction[2]);
  printf("OEM name: %s\n", oem_name);

  printf("\nBIOS parameters:\n");
  printf("  Bytes per sector: %d\n", bp->bytes_per_sector);
  printf("  Sectors per cluster: %d\n", bp->sectors_per_cluster);
  printf("  Reserved sectors: %d\n", bp->reserved_sector_count);
  printf("  Number of FATs: %d\n", bp->fat_count);
  printf("  Maximum root entries: %d\n", bp->max_root_entries);
  printf("  Total sectors: %d (small: %d, large: %d)\n", total_sectors, bp->total_sectors, bp->total_sectors_large); 
  printf("  Media descriptor: 0x%02X%s\n", bp->media_descriptor, bp->media_descriptor == 0xf8 ? " (fixed disk)" : "");
  printf("  Sectors per FAT: %d\n", bp->sectors_per_fat);
  printf("  Sectors per track: %d\n", bp->sectors_per_track);
  printf("  Number of heads: %d\n", bp->head_count);
  printf("  Sectors before partition: %d\n", bp->sectors_before_partition);

  printf("\nExtended BIOS parameters:\n");
  printf("  Physical drive code: 0x%02X (%s)\n", ebp->physical_drive_number, ebp->physical_drive_number ? "physical" : "removable");
  printf("  Reserved byte (WinNT): 0x%02X\n", ebp->current_head);
  printf("  Extended boot signature: 0x%02X\n", ebp->extended_boot_signature);
  printf("  Volume serial number: 0x%08X\n", ebp->serial_number);
  printf("  Volume label: %11s\n", label);
  printf("  FAT type: %s\n", fs_type);
  printf("  OS boot code: %d bytes\n", (int)sizeof(ebp->os_boot_code));
  printf("  Boot sector signature: 0x%04X\n", ebp->boot_sector_signature);
  putchar('\n');
}

void fat_string_copy(char * output, char * input, int length) {
  memset(output, '\0', length + 1);
  strncpy(output, input, length);
}

fat16.h

#include <stdint.h>

#define FAT_OEM_NAME_LENGTH 8
#define FAT_LABEL_LENGTH 11
#define FAT_FS_TYPE_LENGTH 8
#define FAT_OS_BOOT_CODE_LENGTH 448

#pragma pack(push)
#pragma pack(1)

struct fat_preamble {
  unsigned char jump_instruction[3];
  char oem_name[FAT_OEM_NAME_LENGTH]; // e.g. "mkdosfs\0" or "BSD  4.4"
};

struct fat_bios_parameter_block {
  uint16_t bytes_per_sector; // commonly 512
  uint8_t sectors_per_cluster; // powers of 2, 1 ~ 128
  uint16_t reserved_sector_count; // sectors before first FAT
  uint8_t fat_count; // almost always 2, sometimes 1, rarely more.
  uint16_t max_root_entries;
  uint16_t total_sectors; // if 0, check total_sectors_large
  unsigned char media_descriptor; // 0xF8 is fixed disk. First byte of each FAT should match this.
  uint16_t sectors_per_fat;
  uint16_t sectors_per_track; // for disks with geometry
  uint16_t head_count; // for disks with geometry, e.g. 2 for double-sided floppy.
  uint32_t sectors_before_partition; // zero for non-partitined disk.
  uint32_t total_sectors_large; // only used if total_sectors == 0, value > 65535
};

struct fat16_extended_bios_parameter_block {
  unsigned char physical_drive_number; // 0x00 = removable, 0x80 = hard disk
  unsigned char current_head; // reserved, Windows NT uses bits one and two as flags.
  unsigned char extended_boot_signature; // 0x29 indicates the following three entries exist.
  uint32_t serial_number;
  char label[FAT_LABEL_LENGTH]; // volume label, padded with spaces (0x20)
  char fs_type[FAT_FS_TYPE_LENGTH]; // padded with spaces (0x20) e.g "FAT16   "
  unsigned char os_boot_code[FAT_OS_BOOT_CODE_LENGTH];
  unsigned char boot_sector_signature; // hex 55AA
};

struct fat16_boot_sector {
  struct fat_preamble preamble;
  struct fat_bios_parameter_block bios_params;
  struct fat16_extended_bios_parameter_block ext_bios_params;
};

#pragma pack(pop)

void print_boot_sector();

void fat_string_copy(char * output, char * input, int length);