_BootSec Structure

typedef struct {
  union {
    _BPB_FAT32 FAT_32;
    _BPB_FAT16 FAT_16;
    _BPB_FAT12 FAT_12;
  } FAT;
  BYTE Reserved[MEDIA_SECTOR_SIZE-sizeof(_BPB_FAT32)-2];
  BYTE Signature0;
  BYTE Signature1;
} _BootSec;

The _BootSec structure has the same form as a boot sector. When the boot sector is loaded from the device, it will be cast as a _BootSec structure so the boot sector elements can be accessed.