Atari Forum FireBee FAQ
The Atari ST Operating System is called The Operating System or better known as TOS.
The first version of Atari ST requires TOS to be loaded from Floppy disk. Internally on the motherboard is just a Boot Loader program in the Read Only Memory (ROM). Later version of Atari ST came with TOS already burned on to the ROM on the motherboard and is able to boot with any Floppy loaded TOS.