size of FAT12 boot sector structure

[AndrewAPrice]

sizeof(FATBootStructure) reports this struct is 520 bytes long:

Code: Select all

struct FATBootSector
{
	unsigned char BootStrapCode[3]; // code to jump to the bootstrap code
	unsigned char OEMID[8]; // name of the formatting OS
	unsigned short BytesPerSector;
	unsigned char SectorsPerCluster;
	unsigned short ReservedSectors;
	unsigned char FATCopies;
	unsigned short NumberOfPossibleRootEntries;
	unsigned short TotalSectors;
	unsigned char MediaDescriptor;
	unsigned short SectorsPerFAT;
	unsigned short SectorsPerTrack;
	unsigned short NumberOfHeads;
	unsigned int HiddenSectors;
	unsigned int LargeNumberOfSectors;
	union
	{
		struct
		{
			unsigned char FATDriveNumber;
			unsigned char FAT_Reserved;
			unsigned char FATExtendedBootSignature;
			unsigned int FATVolumeSerialNumber;
			unsigned char FATVolumeLabel[11];
			unsigned char FATFileSystemType[8];
			unsigned char FATBootstrapCode[448];
		};
		struct
		{
			unsigned int FAT32SectorsPerFAT;
			unsigned short FAT32FATFlags;
			unsigned short FAT32Version;
			unsigned int FAT32ClusterNumberOfRootDirectoryStart;
			unsigned short FAT32SectorNumberOfFSInformationSector;
			unsigned short FAT32SectorNumberOfFSInformationSectorCopy;
			unsigned char FAT32_Reserved[12];
			unsigned char FAT32DriveNumber;
			unsigned char FAT32_Reserved1;
			unsigned char FAT32ExtendedBootSignature;
			unsigned int FAT32ID;
			unsigned char FAT32VolumeLabel[11];
			unsigned char FAT32FileSystemType[8];
			unsigned char FAT32BootstrapCode[420];
		};
	};
	unsigned char FAT32BootSectorSignature;
};

If I use __attribute__((packed)) it becomes 511 bytes long. It is annoying since data near the beginning of the structure is aligned, but not near the end.

[AndrewAPrice]

FAT32BootSectorSignature should occupy 2 bytes not 1.