The original author of JSIM-51 is:
AFAIK he has never written (or chosen) a special software license for his simulator.
However, his documentation contains some statements expressing his will:
"JSIM51 is a powerful tool for software development with 8051-controllers
and its derivatives.
The program simulates the processor kernel and some of the hardware functions.
I wrote it because all commercial products are too expensive for private users.
Obviously, as for all freeware, there is no guarantee for the correct function in all cases."
"Unfortunately I've no time for further support of the program.
So if anybody wants to have anything changed, he must do it himself.
Here are the sources.
I would expect that useful changes will be made accessible for other people!"
For my understanding this means:
JSIM-51 has been developed with MS-VisualC++ 5.0.
The class library MRCEXT, used for the docking windows,
is free software, which was released by Micro Focus Inc.
under the terms of the GNU General Public License (GPL).
Of course this doesn't answer every subtle question.
JSIM-51 was distributed separately, together with or as part of
several other freeware programs and tool chains for many years.
Jens Altmann has started a new career as DJ JensA.
He has always been a nice guy with good humour and common sense.
I don't think that anybody has to expect copyright problems.
W.W. Heinz, April 16, 2018