Hercules Version 4: Frequently-Asked Questions

1.01What is Hercules?
1.02So what exactly does that mean?
1.03Is it functional enough to run production work?
1.04What are the licensing restrictions for Hercules?
2.01Can it run z/OS, z/VM, z/VSE?
2.02What operating systems can I run legally?
2.03What other programs will run under Hercules?
2.04Where can I obtain OS/360 ?
2.05Where can I obtain MVS ?
2.06Where can I obtain VM/370 ?
2.07Where can I obtain DOS/VS ?
2.08Where can I obtain Linux/390 ?
2.09Where can I find documentation?
3.01What PC hardware do I need to run Hercules?
3.02What sort of MIPS rate can I expect?
3.03What PC software do I need to run Hercules?
3.04What software do I need to build Hercules on Linux and Unix?
3.05What software do I need to build Hercules on Windows?
3.06Can Hercules be ported to run on other platforms?
4.01How can I create a virtual DASD volume?
4.02Can I read a tape which was created on a mainframe?
4.03Can I attach a PC tape drive to Hercules?
4.04Can I process mainframe tapes with Hercules?
4.05Can I create Assembler programs without a mainframe?
5.01What architectural features are implemented?
6.01Who are the Herculeans?
7.01Where can I obtain technical support?

1.01 What is Hercules?

Hercules is a software implementation of the System/370, ESA/390 and z/Architecture mainframe architectures. Hercules runs under Windows and Linux, as well as under various other Unix or Unix-like systems on Intel Pentium and other hardware platforms including Alpha, Sparc, and Mac.

1.02 So what exactly does that mean?

It means that your PC can emulate an IBM mainframe processor. The mainframe can range from a System/360 to a z10 - running in "S/370" mode, "ESA/390" mode, or "z/Architecture" mode.

Hercules executes S/370, ESA/390, and z/Architecture instructions and channel programs. It emulates mainframe I/O devices by using PC devices. For example, 3390 DASD devices are emulated by large files on your hard disk, and local 3270 screens are emulated by tn3270 sessions. (Note: Not all 370 and 390 features have been implemented in Hercules. See the list of particulars later in this document. Also, certain non-standard models, 360/20s, and the 360/67 virtual memory mode are not emulated.)

Hercules implements only the raw S/370, ESA/390, and z/Architecture instruction set; it does not provide any operating system facilities. This means that you need to provide an operating system or standalone program which Hercules can load from an emulated disk or tape device. You will have to write the operating system or standalone program yourself, unless you can manage to obtain a license from IBM to run one of their operating systems on your PC, or use IBM programs and operating systems which have been placed in the public domain.

1.03 Is it functional enough to run production work?

Hercules has never claimed to be a production-capable system. It was always meant to be a system programmer's toy. Having said that, it's now become good enough to run a wide range of software without problems, and there are reports that it has been used to run production work in some parts of the world.

1.04 What are the licensing restrictions for Hercules?

Hercules is a copyright work which has been made generally available, subject to the terms of the Q Public License. In essence this allows free use and distribution of the program for personal and commercial use. You may not distribute modified copies of the program, but you may distribute your own patches along with the program, provided that you also grant the maintainer permission to include those patches in future versions of the program. You may not copy any portion of the source code for use in any other program.

Hercules is not, repeat, not GPL software! The GNU General Public License is a Unix/Linux software licensing agreement, which we, the authors, will not participate in. We believe that the QPL, which has been certified as compliant with the Open Source Definition, provides the benefits and protections of open source for both users and developers, without the political baggage that has come to be associated with the GPL.

2.01 Can it run z/OS, z/VM, z/VSE?

Yes. Hercules is a software implementation of z/Architecture, and so it is capable of running z/OS, z/VM, and z/VSE. Hercules also implements ESA/390 (including SIE) and so it can run OS/390, VM/ESA, and VSE/ESA, as well as older versions of these operating systems such as MVS/ESA, MVS/XA, VM/SP, VSE/SP, etc.

But (and this is a big but), these operating systems are all IBM Licensed Program Products, whose conditions of use generally restrict their usage to specific IBM machine serial numbers. So you cannot just copy these systems from work and run them on your PC.

2.02 What operating systems can I run legally?

Most 3rd party operating systems like Linux/390, z/Linux and TELPAR are covered under their own free license and can therefore run under Hercules without any legal problems.

OS/360 (PCP, MFT and MVT) is in the public domain, as far as we know. The status of OSes for which IBM did not charge a license fee is somewhat murky; these include MVS 3.8, VM/370 release 6, and DOS/VS release 34.

The legal status outside the USA, where something like public domain or software without copyright doesn't exist, is "copyrighted software provided at no charge". It is a known fact that vendors like Amdahl, Hitachi, Nixdorf and others modified those operating systems, and distributed them as their own OS for their own hardware, without asking IBM for permission. But law had been changed over that time, so its not clear if the same legal status applies in your country right now.

Rick Fochtman managed to obtain a letter from IBM that he is allowed to distribute OS/360. Try to ask your salesdroid for a similar letter for VM/370, MVS 3.8j or DOS/VS next time they want to sell you a major upgrade.

OS/390, z/OS, and other ESA or z/Architecture operating systems are definitely licensed to a particular machine. Therefore, in practice you cannot run any classic ESA or z/Architecture operating system on your PC unless you can obtain a license from IBM allowing you to do so. It is believed that there are, however, four ways you could run z/OS, z/VM, z/VSE, OS/390, VM/ESA, or VSE/ESA under Hercules using currently available licenses:

  1. Running under Linux on the Pentium processor of a P/390 which is licensed to run the OS.
  2. Running under Linux/390 on a mainframe which is licensed to run the OS.
  3. Running under the terms of a disaster recovery provision of the OS license (but I really don't recommend depending on Hercules to be your disaster recovery solution!).
  4. Using the IBM OS/390 or z/OS DemoPkg, which is available only to IBM employees and IBM Business Partners.

2.03 What other programs will run under Hercules?

Any program which uses the S/370, ESA/390, or z/Architecture instruction set, as implemented in Hercules. Some special utilities in the form of standalone programs are known to run well. I can particularly recommend Jan Jaeger's excellent standalone editor (ZZSA) which is included in the Hercules distribution, or it can be downloaded from http://www.cbttape.org/~jjaeger; I use it regularly to look at DASD while debugging an OS installation, which is just what it's designed to do. Note: ZZSA runs in ESA/390 mode. See Jan Jaeger's website for more information and special logon procedures.

2.04 Where can I obtain OS/360 ?

  1. Rick Fochtman's OS/360 archive CD is now obtainable by download from these locations:
  2. At the cbttape site, you can download the OS/360 Y2K Starter package from ftp://cbttape.org/pub/OS360/OS360RICK1.zip, which contains a full featured MVT system on a 3330 image with some minimal documentation. The configuration is that of a 370/158 with 4 megabytes of main storage, running OS/MVT Release 21.0. The same site also offers a MVTDBL volume and a Builder package for those who like to participate in OS/360 nucleus hacking.

    Please note that the previously mentioned cbttape OS/360 download link is an "ftp://" protocol link, and some web browsers (most notably Google Chrome) no longer support the "ftp://" protocol. If such is the case for you, you will need to use a proper FTP client to download the mentioned OS/360 .zip file.

  3. Jay Maynard's "IBM Public Domain Software Collection" at http://www.ibiblio.org/jmaynard/ contains copies of the OS/360 Release 21.8 distribution tapes.

2.05 Where can I obtain MVS 3.8 ?

The Original MVS 3.8 distribution as it was first used came from

who advise using the mirrors at

Several people have generated a functional MVS system from this archive :

2.05 Where can I obtain VM/370 ?

The VM/370 page at cbttape.org contains download links for the Andy Norrie VM 4-pack system and the Bob Abeles VM/370 R6 distribution.

2.07 Where can I obtain DOS/VS ?

I've put the DOS/VS r34 install tape on my site. It'll expand to a 21 MB AWSTAPE file, dosrel34.aws. You need the Coverletter to install it. Read the relevant postings of the Hercules mailing list first, as the install process is quite obscure.

You can grab those files at :

2.08 Where can I obtain Linux/390 ?

The best starting point for information about Linux for S/390 and Linux for zSeries is http://www.linuxvm.org/

2.09 Where can I find documentation?

The Creating Hercules DASD document describes various methods of creating and loading virtual DASD volumes.

I've produced a document describing how to build an OS/360 system on Hercules, called "OS/360 on Hercules". It can be found at

This will build an MVT system without TCAM/TSO, but with two 3270 consoles. You will need Malcolm Beattie's "Guide to Using 3270 Consoles and Terminals for Hercules" with this MVT version.

The N.U.D.E guides can be found at :

IBM provides only current documentation, ...
but many things haven't changed since 1964 :

3.01 What PC hardware do I need to run Hercules?

Classic IBM operating systems (OS/360, MVS 3.8, VM/370) are very light by today's standards and will run satisfactorily on a 300Mhz Pentium with as little as 32MB RAM.

Anything more up-to-date, such as Linux/390 or OS/390, requires much more processing power. Hercules is CPU intensive, so you will want to use the fastest processor you can get. A 2GHz Pentium, preferably with hyperthreading, will probably provide acceptable performance for a light workload. If you can afford a multiprocessor system, so much the better. Hercules makes extensive use of multi-threading to overlap I/O with CPU activity, and to dispatch multiple emulated CPU's in parallel.

For the latest 64-bit operating systems such as zLinux and z/OS, be aware that there is a performance penalty when Hercules emulates z/Architecture on a 32-bit processor such as the Pentium. If you are serious about running 64-bit then you will probably want to build Hercules for a 64-bit processor such as Alpha (DEC/Compaq/HP), or AMD64 (AMD Opteron, Athlon-64, Turion 64) together with a 64-bit version of Linux or PPC (Power Mac G5) with OS X.

Hercules does not depend on the Pentium architecture. I've built and run it successfully on a 500 MHz Alpha 21164, and others have run it on SPARC and S/390 (!) Linux systems. One guy has even run OS/360 under Hercules under Linux/390 under Hercules under Linux/390 under VM/ESA! The prize for the world's smallest mainframe probably goes to Ivan Warren, who claims to have run VM/370 under Hercules on an iPAQ 5450 handheld PDA.

You should provide enough RAM to accommodate your S/390 real storage (main storage plus expanded storage) in addition to the normal requirements of your PC's operating system. For maximum throughput, you should set your main and expanded storage sizes high enough to eliminate S/390 paging. S/390 storage is allocated out of your PC's virtual storage.

You also need enough hard disk space to accommodate the emulated DASD. A virtual "3330 model 1" disk drive will need about 100 megabytes of space for emulation (a 3330-11 will need about 200 megabytes). A 3380 "single density" model will need about 650MB, a 3390 model 2 needs about 2GB, and a 3390 model 3 needs about 3GB. If you use the compressed CKD DASD feature, these sizes will shrink dramatically, usually to about 20 to 30 percent of the original size.

3.02 What sort of MIPS rate can I expect?

Thanks to the cumulative work of many individuals, including Valery Pogonchenko, Juergen Dobrinski, Albert Louw, Gabor Hoffer, Jan Jaeger, Paul Leisy, Clem Clarke, and Greg Smith, the performance of Hercules today is vastly better than it was 5 years ago.

Even on a Celeron 300 you should see an execution speed of 1 to 2 MIPS, which is enough to run OS/360 (MFT or MVT) or MVS 3.8 with a response time better than that of a 3033 from the 1970's. It's also fast enough to run VSE/ESA with an acceptable response time. On a more recent system with a 2GHz Pentium processor, you may see the system peak at around 30 MIPS which is enough to run Linux/390 or z/OS with a light workload.

Performance on server class machines is now fairly respectable. For example, on a dual-core Intel Xeon with hyperthreading (4 CPUs) running at 3.46GHz, you might expect to see a sustained MIPS rate of 40 to 60 MIPS. A dual-processor quad-core Mac Pro (8 cores, 3 GHz) will sustain over 150 MIPS. For anyone who is prepared to spend a considerable amount of money on their Hercules system, there are reports that a sustained 300+ MIPS has been achieved on an Intel Core i7 processor running at 3.75GHz using all four cores plus hyperthreading (8 CPUs).

Typical I/O rates of around 50 EXCP/second are reported on average hardware, with rates over 500/second achievable with hardware RAID.

3.03 What PC software do I need to run Hercules?

The following software platforms are supported:

  • Linux (kernel 2.4 or later)
  • Windows Vista or greater (Windows 7 preferred)
  • Mac OS X 10.3 or later
  • Solaris 2.9 or later (Sparc or Intel)
  • FreeBSD

    You will also need tn3270 client software for the virtual 3270 console. The tn3270 client can run on the same machine as Hercules, or on any Unix or Windows box with a TCP/IP connection to the Hercules machine.

    The supported and recommended tn3270 clients for Hercules are:

    x3270 for Unix
    x3270 is included with most Linux distributions, or you can download it from http://x3270.bgp.nu/
    Vista tn3270 for Windows
    Vista tn3270 can be obtained from www.tombrennansoftware.com. The very modest license fee charged for this excellent 3270 emulator helps to support an independent software developer.
    Brown University tn3270 for Macintosh
    Brown University tn3270 is freely available. You can download it from http://www.brown.edu/Facilities/CIS/tn3270/. There is one setting that must be changed to use this program with some operating systems, especially MVS 3.8: Open a connection to Hercules, but before IPLing the system, go to the Session->Features menu and set "Change embedded nulls to blanks" to "No". Click on "OK". Now, click on File->Save default settings... to make the setting permanent.

    Other tn3270 clients, such as QWS3270, IBM Personal Communications, Attachmate Extra, or Dynacomm Elite should also work in most cases, but be aware that some tn3270 clients have bugs which make them unusable as OS/360 or MVS consoles.

    3.04 What software do I need to build Hercules on Linux and Unix?

    To build Hercules for Linux and other Unix-like environments (including Cygwin under Windows), you need to use the gcc compiler, version 3.x or above. You will also need a full set of GNU development tools, including recent versions of autoconf, automake, flex, gawk, gcc, grep, m4, make, perl, and sed. Refer to the util/bldlvlck file in the Hercules distribution for details.

    3.05 What software do I need to build Hercules on Windows?

    To build Hercules for the Windows native environment (without Cygwin), you need to use the Microsoft C/C++ compiler (MSVC) version 15.x or later (i.e. Visual Studio 2008 or greater). The 32-bit compiler and SDK were packaged as Microsoft Visual C++ 2008 Express Edition at one point in the past, but unfortunately Microsoft no longer offers it for download from their web site.

    Fish's "Software Development Laboratories" web site however, has a "Hercules Windows Build Instructions" web page that contains download links that are still working should you wish to use Visual Studio 2008 Express Edition.

    The good news is Microsoft's latest offering of their Visual Studio product comes with a free "Community Edition" which should be able to successfully build Hercules.

    Which version of Visual Studio (Microsoft Visual C++) you use to build Hercules with is largely unimportant as long as it works. More recent versions may or may not provide better performance and may or may not successfully build Hercules. The only way to know is to try.

    3.06 Can Hercules be ported to run on other platforms?

    With the introduction of autotools, we do make efforts to ensure Hercules builds and run on several different operating system platforms (mostly Linux, Windows, MAC, Solaris, and FreeBSD right now), but we of course simply cannot guarantee that it will run on every operating system platform out there.

    If you want to make Hercules run on AS/400, OS/2, or whatever, then by all means go ahead. I welcome reports of any bugs or problems you find, but I probably won't fix problems if it means introducing platform-specific code, and I will not be able to test new releases against other platforms. Folks who have gotten it compiled on the BSDs report that the hardest part is removing the Linux-specific tape support.

    The Hercules code is not intended to be specific to Intel hardware, so if you find any issues or faults related to running on other hardware (SPARC, Alpha, PPC, ...) under Linux, then I'm likely to be receptive to fixing that sort of problem. Issues related to Unix variants are less likely to be fixed however.

    4.01 How can I create a virtual DASD volume?

    The Creating Hercules DASD document describes various methods of creating and loading virtual DASD volumes.

    4.02 Can I read a tape which was created on a mainframe?

    Yes, indirectly. The mainframe tape must be converted to AWSTAPE format and then downloaded to your PC. The tapeconv.jcl file in the Hercules directory contains a sample program which you can run under OS/390 on your mainframe system. It reads a file from tape and converts it to AWSTAPE format. Download the AWSTAPE file to your PC (making sure to choose binary format for the download), and then add the downloaded filename to the Hercules configuration file as a virtual tape device. You will then be able to read the tape file from the virtual tape drive located on your PC.

    Note: the "tapeconv" program will not correctly process input tapes whose block size exceeds 32760! One symptom of this may be the message " ADRY011E I/O ERROR - DEVICE NOT ATTACHED.0000,NA,00...00,0000" when attempting to restore from tape originally created using the default DF/DSS block size. The solution is to recreate the dump tape with DCB=BLKSIZE=32760.

    4.03 Can I attach a PC tape drive to Hercules?

    Yes. Hercules can read and write tapes on SCSI drives. I have tested this with 4mm DAT, QIC-1000, and 9-track drives.

    4.04 Can I process mainframe tapes with Hercules?

    Yes. It is possible to obtain 9-track open reel drives and 3480-type cartridge drives which attach to the SCSI bus. Hercules makes these appear to the operating system as channel-attached 3420 or 3480 devices, making it possible to read and write real mainframe tapes.

    4.05 Can I create Assembler programs without a mainframe?

    Yes. If you want to write Assembler (BAL) programs to run on Hercules, but you don't have access to a mainframe, then there are two interesting products which you can run on your PC to assemble programs:

    The "Tachyon 390 Cross Assembler" ( http://www.tachyonsoft.com/tachyon )
    With this assembler you can produce S/390-compatible object decks using your Linux or Windows PC. A high degree of HLASM compatibility, coupled with the ability to perform complex assemblies at lightning speed, make this a product which is well worth looking at. I have tried this assembler and it is truly amazing.
    The "Dignus Systems/C Compiler" ( http://www.dignus.com )
    This is a C compiler which runs under Windows or Linux and generates mainframe assembler code which you can then assemble using the Tachyon assembler.

    Sam Golob wrote a fascinating review of these two products in the September 1999 issue of NaSPA Technical Support magazine.

    5.01 What architectural features are implemented?

    The following features of ESA/390 have been implemented:

    The following features of z/Architecture have been implemented:

    The following features of z/Architecture have not yet been implemented:

    The following standard feature has not yet been implemented:

    The following optional features have been partially implemented:

    The following features are not yet implemented, either due to lack of documentation, limited host system capability, or lack of supporting hardware:

    Hercules is compliant with IBM's ALS-1, ALS-2 and ALS-3 architectural level sets to the degree necessary to run all OS/390 versions through 2.10 and known versions of z/OS in both ARCHLVL 1 and ARCHLVL 2 mode, and Linux and z/VM and z/VSE in both ESA/390 and z/Architecture mode.

    6.01 Who are the Herculeans?

    The following people are among those who have contributed to this project, either as coders or as testers or both:

  • Roger Bowler (original author)
  • Jay Maynard
  • Jan Jaeger
  • Butch Anton
  • Volker Bandke
  • David Barth
  • Malcolm Beattie
  • Mario Bezzi
  • Florian Bilek
  • Gordon Bonorchis
  • Mike Cairns
  • Chris Cheney
  • Marcin Cieslak
  • Clem Clarke
  • Vic Cross
  • Jacob Dekel
  • Guy Desbiens
  • Jacques Dilbert
  • Juergen Dobrinski
  • Fritz Elfert
  • Neale Ferguson
  • Tomas Fott
  • Mike Frysinger
  • Martin Gasparovic
  • Mark Gaubatz
  • Steve Gay
  • Paolo Giacobbis
  • Peter Glanzmann
  • Roland Goetschi
  • Graham Goodwin
  • Paul Gorlinsky
  • Harold Grovesteen
  • John P. Hartmann
  • Glen Herrmannsfeldt
  • Brandon Hill
  • Laddie Hanus
  • Robert Hodge
  • Gabor Hoffer
  • Dan Horak
  • Peter J. Jansen
  • Soren Jorvang
  • Willem Konynenberg
  • John Kozak
  • Nobumichi Kozawa
  • Peter Kuschnerus
  • Paul Leisy
  • Kevin Leonard
  • Bill Lewis
  • Albert Louw
  • Peter Macdonald
  • Lutz Mader
  • Tomas Masek
  • Rick McKelvy
  • John McKown
  • Dave Morton
  • Christophe Nillon
  • Mike Noel
  • Andy Norrie
  • Steve Orso
  • Dutch Owen
  • Max H. Parke
  • Gerd Petermann
  • Reed H. Petty
  • Jim Pierson
  • Richard Pinion
  • Tim Pinkawa
  • Pasi Pirhonen
  • Valery Pogonchenko
  • Bob Polmanter
  • Andy Polyakov
  • Frans Pop
  • Wolfhard Reimer
  • Emerson Santos
  • Jeff Savit
  • Axel Schwarzer
  • Paul Scott
  • Daniel Seagraves
  • Victor Shkamerda
  • Ian Shorter
  • Greg Smith
  • Enrico Sorichetti
  • John Summerfield
  • Peter Sylvester
  • Mark Szlaga
  • Adam Thornton
  • Adrian Trenkwalder
  • "Fish" (David B. Trout)
  • Ronen Tzur
  • Bernard van der Helm
  • Ard van der Leeuw
  • Kris Van Hees
  • Adam Vandenberg
  • Kees Verruijt
  • Giuseppe Vitillaro
  • Ivan Warren
  • James Wekel
  • Juergen Winkelmann
  • Bob Wood
  • Ian Worthington
  • Rod Zazubek
  • Bjoern A. Zeeb
  • Matt Zimmerman

    And thanks for support and encouragement from:

  • Tim Alpaerts
  • Bertus Bekker
  • Giorgio de Nunzio
  • Rick Fochtman
  • Alex Friis
  • Sam Golob
  • Achim Haag
  • Cory Hamasaki
  • Tony Harminc
  • Richard Higson
  • Jim Keohane
  • Sam Knutson
  • Mike Ross
  • Daniel Rudin
  • Rich Smrcina
  • Henk Stegeman
  • Mark S. Waterbury

    If anyone feels they have been unfairly omitted from either of these lists, please let us know.

    7.01 Where can I obtain technical support?

    Please see our Technical Support web page.