To run the emulator a TOS ROM image is needed. EmuTOS, a free implementation
of TOS, is shipped with hatari.

EmuTOS is a single-user single-tasking operating system for the 32 bit Atari
computers and emulators. It can be used as a replacement for the TOS images
typically needed today for using emulators and it is also running on some
real hardware, like the Atari Mega STE. All the source code is open and
free, licensed under the GNU General Public License (GPL). A copy of the GNU
GPL can be found in the gpl.txt file.

EmuTOS has its home at sourceforge:
http://sourceforge.net/projects/emutos

Hatari is shipped with tos.img ROM image that is identical to etos256us.img
from the binary package of version 0.8.1 available at sourceforge:
http://dl.sourceforge.net/sourceforge/emutos/emutos256k-0.8.1.zip