Titokone is a TTK-91 machine language simulator written in Java.

http://www.cs.helsinki.fi/group/titokone/

Titokone is free software (free as in freedom), published under the GNU
Lesser General Public License.  You are allowed to use, modify and
redistribute your modified or unmodified version, provided the license
is kept with it and followed.  The users' rights are protected by
copyright held by the University of Helsinki Computer Science
department; the software is not in the public domain. See COPYING
for details.