Atari TOS is based on GEMDOS which uses a modified FAT12 (or, on hard disks, FAT16) file system. The major differences are the fact that the boot sector does not need to contain the IBM compatible jump sequence at the beginning (typically or ), the lack (before TOS 1.04) of an OEM identifier compatible with PC-based systems, and the fact that a checksum is used to mark the boot sector as executable (the PC format uses the signature word instead). Executable boot sectors for the Atari platform typically start with an MC68K jump opcode (e.g. , and the last two byte word must sum with the rest of the boot sector (in big-endian word form) to in order to be bootable.