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SIMH FAQ, 31-Mar-2004

clipped for general and 1401

1 General

2 Operational

3 Writing and Debugging New Code

4 VAX [clipped]
5 PDP-11 [clipped]

1. General Questions

1.1 What is SIMH?

SIMH is the Computer History Simulation system. It consists of simulators for approximately 20 different computers, all written around a common user interface package and set of supporting libraries. SIMH can be used to simulate any computer system for which sufficient detail is available, but the focus to date has been on simulating computer systems of historic interest.

1.2 Why was SIMH written?

Significant portions of the computing past are being irretrievably lost, as old systems are scrapped, documentation and software is thrown out, media become obsolete or unreadable, and inventors and pioneers die. SIMH was written as a vehicle to allow the computing past to be made accessible to a wider audience, for recreational and educational purposes. SIMH preserves historic computers as portable software, that can be run on any modern system. SIMH also preserves representative software packages for these systems. With SIMH, anyone with a desktop computer can call up and run significant samples from the computing past, at any time.

1.3 What is the history of SIMH?

The SIMH project started in 1993, at the suggestion of Larry Stewart of DEC. Its immediate purpose was to preserve the fading hardware and software record of early minicomputers. Since then, the project has been expanded to include other important systems, spanning the history of computing from the late 50's to the late 80's.

SIMH's core design is based on an earlier simulation system called MIMIC. MIMIC was written in the late 1960's at Applied Data Research, by Mike McCarthy, Len Feshkens, and Bob Supnik. MIMIC was a mini-computer simulator that ran on the PDP-10. Its purpose was to facilitate the development and debugging of real-time embedded systems by using the the PDP-10 timesharing environment for program development, instead of the limited facilities of the native minicomputer environments. Ironically, given SIMH's mission to preserve the computing record, all machine-readable copies of MIMIC have been lost.

1.4 Who writes and maintains SIMH?

Many people have contributed, and continue to contribute, to SIMH. The full list of contributors can be found on the SIMH web site. Bob Supnik coordinates SIMH development.

1.5 How is SIMH licensed?

SIMH is licensed under a modified X-Windows license. This license allows more or less unrestricted use of the sources and binaries. The license is included with the documentation and is also included in every source module. The software packages are available under various terms and conditions; see the documentation included with each software package.

1.6 How is SIMH distributed?

SIMH is distributed in source form from the SIMH web site, in the form of a Zip archive. For Windows users, pre-compiled binaries are also available.

1.7 Which computer systems does SIMH simulate?

SIMH simulates the following computer systems:
Manufacturer Model
Digital Equipment Corporation PDP-1, PDP-4, PDP-7, PDP-8, PDP-9, PDP-10, PDP-11, PDP-15, VAX
Data General Corporation Nova, Eclipse
IBM Corporation 1130, 1401, 1620, System 3
GRI Corporation GRI-909
Honeywell Corporation H316/516
Hewlett Packard Corporation HP2116, HP2100, HP21MX
Interdata Corporation 16b systems, 7/32, 8/32
Scientific Data Systems SDS-940
MITS Altair 8080, Altair Z80
Royal-Mcbee LGP-30, LGP-21
The documentation contains more details on supported models and peripherals.

1.8 Which host systems does SIMH run on?

Host System Compiler comments
OpenVMS/VAX DEC C no 64b support; no Ethernet support
OpenVMS/Alpha DEC C Ethernet support provided in pcap-vms
Windows 9x or
Windows 2000
Windows XP
Mingw/gcc or
or Visual C++ or
Borland C++
requires WinPcap for Ethernet support
Mac OS/X . requires libpcap for Ethernet support
Linux gcc requires libpcap for Ethernet support
Tru64 UNIX DEC C no Ethernet support
AIX . no Ethernet support
Solaris . requires libpcap for Ethernet support
HP/UX . no Ethernet support
NetBSD gcc requires libpcap for Ethernet support
OpenBSD gcc requires libpcap for Ethernet support
FreeBSD gcc requires libpcap for Ethernet support
OS/2 EMX no Ethernet support

1.9 What software packages are available to run on SIMH?

The list of available software packages can be found on the SIMH web site.

1.10 Where can I get more information on SIMH?

The SIMH web site is

2 Operational Questions

2.1 How do I install SIMH on Windows?

The simplest way is to download the pre-compiled binaries. Unzip these into the directory where you want to run SIMH. You can then run whichever binary that you want.

If you want to run the VAX emulator, you will also need files ka655.bin and ka655x.bin from the source kit.

2.2 How do I install SIMH with Ethernet support on Windows?

Separate pre-compiled binaries contain Ethernet support. Before running these binaries, you must download download and install the WinPCAP AutoInstaller from

This creates a network packet driver in Windows for SIMH to attach to.

To use network support, you must either be an administrator on the Windows machine (implied in Windows 9X), or you must set the windows packet driver to autostart when the system boots; see the WinPCAP FAQ page for detailed information on how to do this.

2.3 How do I install SIMH on Unix?

- Unzip the archive of sources to a new directory. You must specify the -a switch to unzip for proper conversion of Windows cr-lf sequences to UNIX newline sequences.

- If your system supports gmake, you can compile the simulators with the command:

% gmake all

- If you want Ethernet support in the PDP-11 and VAX, you should compile the simulators with the command:

% gmake USE_NETWORK=1 all

Note that Ethernet support is available ONLY on Linux, NetBSD, and OpenBSD.

2.4 How do I install SIMH on VMS?

Download the SIMH source kit, and UNZIP it using the /TEXT=AUTO qualifier to the directory that you want SIMH to reside in. Use MMK or MMS and the descrip.mms file to build the binaries.

On a VAX use:

$ MMx

On a Alpha use:
$ MMx/MACRO=("__ALPHA__=1")              !Without ethernet support
$ MMx/MACRO=("__ALPHA__=1","__PCAP__=1") !With    ethernet support
UNZIP can be found on the VMS freeware CDs, or from
MMK can be found on the VMS freeware CDs, or from
MMS can be licensed from HP/Compaq/Digital.

Note that the PDP-10 emulator cannot be built and used on VAX/VMS, because the DEC C compiler for VAX/VMS does not support 64-bit integers. DEC C on on Alpha VMS has the required 64-bit capability to build and run all of the emulators. Ethernet support is only available on Alpha VMS 7.3-1 and above.

2.5 How do I transcribe a real CD for use with SIMH?

- On UNIX, you can copy a CD to an ISO file with the dd command:

% dd /if=/dev/raw_cd_device /out=/path/cdimage.iso

Linux, and many Unix variants, support direct access to the CD ROM from SIMH:

sim> set rq1 cdrom
sim> att rq1 /dev/cdrom_drive

- On Windows, there are quite a few products that can do this. The two most common products are detailed below. Make sure to disable any antivirus software before proceeding. Antivirus software tends to interfere with the smooth flow of data from the CD and will occasionally transform the data in strange and unexpected ways to 'protect' you.

  1) Roxio EZ-CD Creator 5.x
     Go to the the Disc menu and select Disc Info (there will be a delay).
     Select the track shown, then click the Read Track button.
     Enter the Save file name, then OK.

  2) Nero 5.5
     Select Recorder|Save Track
     Select the track, set the output filename
     In Options, you may need to set the Read Speed down;
      the VMS Hobbyist CD didn't work after a 52x read, but worked fine at 8x
     Click GO

2.6 How do I transcribe other archival media for use with SIMH?

You must have access to a real system that can read the media to be transcribed (e.g., a system with a working DECtape drive to read a DECtape). Most systems have utilities to copy raw data to a disk file; that file can then be transferred over the console serial line to a system with an Internet link. Utility programs are available to convert raw data streams to SIMH format.

2.7 How can I get text files in and out of SIMH?

Since SIMH supports the universal serial interface using TELNET, text can be transferred using one of the serial line transfer protocols (X/Y/Zmodem, Kermit) or using standard cut and paste techniques, if the host's TELNET program supports it.

To use the TELNET feature, connect to the SIMH machine using TELNET, and set the target environment into a 'receive' mode. This is usually something like running a text editor. Then tell the TELNET program to 'send', 'transfer', or 'paste' the text that you want sent into the SIMH system.

To get text out of the system, have the TELNET program either log the output, or if the TELNET program supports a backscroll region you can use that. Tell the SIMH system to 'type' or 'cat' the text file, sending the output to the TELNET device, where you can edit it into a text file.

Many TELNET programs also support transferring large files via X/Y/ZModem or Kermit, which you can use as long as the SIMH system has the appropriate matching program.

C-Kermit from Columbia University ( is probably the most universal way to transfer files in and out of SIMH systems.

If the SIMH system supports Ethernet connectivity (PDP-11, VAX), you can also use the various network copy programs (FTP, DECNET) to transfer files.

Finally, you can "print" text files to the simulated line printer. Printer output is automatically formatted as an ASCII text file.

2.8 How can I get binary files in and out of SIMH?

Since SIMH supports the universal serial interface using TELNET, binary files can be transferred using one of the serial line transfer protocols (X/Y/ZModem, Kermit) or by converting the binary to a text-encoded file (HEXify, UUENCODE, VMShare, etc.) and transferred in text mode (see section 2.7).

Many TELNET programs also support transferring large files via X/Y/ZModem or Kermit, which you can use as long as the SIMH system has the appropriate matching program.

C-Kermit from Columbia University ( is probably the most universal way to transfer files in and out of SIMH systems.

If the SIMH system supports Ethernet connectivity (PDP-11, VAX), you can also use the various network copy programs (FTP, DECNET) to transfer files.

2.9 Can I connect real devices on the host computer to SIMH?

At the moment, Ethernet is the only supported real device.

2.10 My Windows host can't communicate with the PDP-11 or VAX over Ethernet; why?

Due to the inherent limitations of WinPCAP, the SIMH system _CANNOT_ communicate with the host on the primary interface. To establish communications between SIMH and a PC host, add a second Ethernet controller, attach both controllers to the same hub, and attach SIMH to the second controller. The host and SIMH will now be able to communicate across the physical network connection.

3 Writing and Debugging New Code

3.1 What resources are available for writing new simulators?

The SIMH web site contains documentation on the internals of SIMH, as well as specific help for writing new peripherals for several of the popular simulators.

3.2 What debugging facilities are available in SIMH?

Most simulators provide the following debugging capabilities:

- Symbolic assembly and disassembly of memory contents.
- Numeric examination and modification of the data store of any simulated
- Numeric search on both memory and device data.
- Visibility to simulator internal structures, such as the event queue.
- An unlimited number of instruction breakpoints.
- Proceed counts on breakpoints.
- Automatic execution of simulator commands on a breakpoint.
- Stepped execution (from single step to 'n' steps).
- A PC change queue, usually 64 instructions deep.
Specific simulators may provide additional features, such as an instruction history buffer, CPU and/or device logging, and breakpoints on memory reads and writes.

3.3 When do I need to use the host debugger for debugging a simulator?

While a simulator is being debugged, its execution of instructions or debugging support code may be unreliable. During this process, the programmer may need to use the host debugger to stop in the middle of an instruction execution, or to trap an error condition. Host debugger breakpoints should be invisible to the simulator; with the exception of clock calibration, all simulator events are driven off the event queue rather than real-world events.

If the programmer needs to force a simulator stop from the host debugger, most simulators provide an "address stop" global variable. Setting this variable to 1 will cause the simulator to stop after completing the current instruction.

3.4 What is the release process for SIMH?

SIMH is released whenever a significant number of new features, or important bug fixes, has accumulated. This has averaged every 4-8 weeks. The major version number only changes when there is a major restructuring of SIMH's internal structures. The minor version number is changed when the format of the save/restore file must be updated.

the rest is clipped - simualted processor specific, not 1401