The Xbox 360 system uses a series of bootloaders to load the hv/kernel and dashboard. These loaders are explained at Boot Process.

Header

The header is at the start of the bootloader and is usually 0x20 bytes long.

Offset	Length	Type	Information
0x0	0x2	ASCII characters	Loader name
0x2	0x2	unsigned short	Loader build number
0x4	0x2	unsigned short	Loader pairing data
0x6	0x2	unsigned short	Loader flags
0x8	0x4	unsigned integer	Loader entrypoint
0xC	0x4	unsigned integer	Loader length (including header)
0x10	0x10	bytes	HMAC salt (used for decrypting the loader)

Bootloaders