Roger Bowler's Hercules Page

IBM 360/30 from Ed Thelen's Computer History Museum site Ever since I saw my first IBM mainframe (a 360/30 at Charter Consolidated in Ashford, Kent in about 1970), it has been my dream to own and operate a “real” computer.

Thirty years later, I still didn't have my own mainframe, so I created Hercules instead. It's a little toy which will turn your PC into an IBM mainframe. Well, almost!

Although the germ of Hercules dates back to 1994, most of the work was done during a nine month period in 1999 while I was between contracts. By the autumn of that year I had implemented enough of the S/360 and ESA/390 architecture to be able to IPL and run OS/360 (MFT) and Jan Jaeger's ZZSA standalone program in ESA mode. Following this, I gradually filled in the missing parts of the architecture, so that at the start of the year 2000, Hercules was (according to reports from IBMers) quite capable of running VSE/ESA, and could even IPL OS/390 (albeit somewhat slowly!)

During spring 2000, others began to make significant contributions to the work:

In May 2000, the need to get a job meant that I could no longer continue full-time work on Hercules, and I handed over control of the project to Jay Maynard, who continued until 2012 as official maintainer and champion of Hercules. Jay's regular Hercules presentations at SHARE conferences attracted large audiences, and his session at SHARE 99 in San Francisco received a Best Session Award for a session in the MVS Program.

In autumn 2000, IBM announced a new 64-bit z/Architecture (also known as ESAME or ESA Modal Extensions). Using publicly available information together with his deep knowledge of the evolution of S/360/370/390, Jan Jaeger was able to predict the likely form that the 64-bit architectural extensions would take. This enabled him to design preliminary support for the new architecture, and to implement many of the new instructions in advance of the publication of the full technical details in January 2001. During some busy weekends which followed, I added support in Hercules for 64-bit mode IDAW, Cross Memory and DAT, with the result that at the end of February 2001, only 5 weeks after publication of the z/Architecture Principles of Operation manual, Hercules became the first (and, for 18 months, the only) non-IBM implementation of the new 64-bit mainframe architecture.

A series of performance enhancements by Greg Smith, Gabor Hoffer, Juergen Dobrinski, and Paul Leisy during the period 2002-2004, coupled with a significant increase in the power of entry level Intel processors, brought Hercules up to a more respectable MIPS level (around 25 mainframe MIPS on a 3GHz Pentium CPU in 2005). During the same period Hercules also kept pace with the evolution of the mainframe architecture, adding support for new features introduced by the latest IBM z990 and System z9 processors.

In October 2005 the Hercules project was honored by NaSPA, the Network and Systems Professionals Association, with three Herculeans (myself, Jay Maynard, and Volker Bandke) sharing the NaSPA 2005 Award for Technical Excellence.

Hercules is by far the world's most popular S/390 emulator, with the number of installed systems now estimated at over 6000. That's more than ten times the combined total of commercial S/390 emulators installed world-wide! Looked at from another perspective, it means that over 20% of the world's mainframes are now Hercules emulators! And what's more, Hercules is the most widely-used S/390 emulator within IBM itself, where many IBM systems engineers and marketing representatives use Hercules as an inexpensive and readily available platform for testing and demonstration purposes.

Follow these links to find out more:

What people are saying about Hercules

“ Never in my wildest dreams did I expect to see MVS running on a machine that I personally own. Hercules is a marvelous tool. My thanks to you all for a job very well done. ”
— Reed H. Petty

“ I do miss my mainframe a lot, and playing with Herc sure brings back memories. Just seeing the IBM message prefixes, and responding to console messages again was a wonderful bit of nostalgia! ”
— Bob Brown

“ I have installed your absolutely fantastic /390 emulator. You won't believe what I felt when I saw the prompt. Congratulations, this is a terrific software. I really have not had such a fascinating and interesting time on my PC lately. ”
— IBM Large Systems Specialist

“ Such simulators have been available for a long time. One of the most complete (up to modern 64-bit z/Architecture) is hercules. ”
— Michel Hack, IBM Thomas J. Watson Research Center

“ An apparently excellent emulator that allows those open source developers with an "itch to scratch", to come to the S/390 table and contribute. ”
— Mike MacIsaac, IBM

“ BTW grab a copy of Hercules and you can test it at home. It's a very good S/390 and zSeries (S/390 64bit) emulator.. ”
— Alan Cox

“ It works even better than I imagined. Hercules is a fine piece of software! ”
— Dave Sienkiewicz

“ Hercules is a systems programmer's dream come true. ”
— René Vincent Jansen

“ Aside from the electric trains my parents got me in 1953, this is the best toy I've ever been given, bar none.”
— Jeffrey Broido

“ Congratulations to you and your team on a fine piece of work! ”
— Rich Smrcina

“ Congratulations on a magnificent achievement! ”
— Mike Ross

“ For anyone thinking running Hercules is too much trouble or too hard or whatever, I came home from work one day and my 13 year old 8th grade son had MVS running under VM under Hercules on Linux. He had gotten all the information about how to do this from the Internet. When he complained about MVS console configuration and figuring out how to get it to work with VM, I knew he had felt all the pain he ever needed to feel about mainframes. ”
— Scott Ledbetter, StorageTek

“ I am running a fully graphical Centos z/Linux environment on my desktop. The Hercules emulator is an amazing feat of engineering. I just wanted to send my compliments to the team for an excellent job! Thanks much for making this product part of the open-source community! ”
— Roby Gamboa

“ I have DOS and DOS/VS running on Hercules with some demo applications, both batch and on-line. It does bring back some good memories. My compliments go to the Hercules team. Thank you. ”
— Bill Carlborg

“ This is stunning piece of work. To say that I am blown away is an understatement. I have a mainframe on my notebook!!!!!! P.S. Now if I can just remember my JCL ”
— Roger Tunnicliffe

Read Hesh Wiener's Technology News article about Hercules at

Read Moshe Bar's article about Hercules at

For eighteen months, the IBM Redbook SG24-4987 Linux for S/390 at contained a chapter written by Richard Higson describing how to run Linux/390 under Hercules. Then suddenly, all mention of Hercules was mysteriously removed from the online edition of the book! Read the story of the disappearing Redbook chapter at

Other Hercules-related sites

If you have any questions or comments, join the hercules-390 discussion group at

Last updated 11 June 2013