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SIMH v4.0 - Beta

WHAT'S NEW Coverity Scan Build Status

New Simulators

Seth Morabito has implemented a AT&T 3B2 simulator.

Leonid Broukhis and Serge Vakulenko have implemented a simulator for the Soviet mainframe BESM-6 computer.

Matt Burke has implemented new VAX model simulators:

VAX/11 730
VAX/11 750
VAX 8600/8650
MicroVAX I & VAXStation I
MicroVAX II & VAXStation II
rtVAX 1000 (or Industrial VAX 620)

Howard Harte has implemented a Lincoln Labs TX-0 simulator.

Gerardo Ospina has implemented a Manchester University SSEM (Small Scale Experimental Machine) simulator.

Richard Cornwell has implemented a Burroughs B5500.

Richard Cornwell has implemented the IBM 701, IBM 704, IBM 7010/1410, IBM 7070/7074, IBM 7080/702/705/7053 and IBM 7090/7094/709/704 simulators.

Dave Bryan has implemented an HP-3000 Series III simulator.

Updated AltairZ80 simulator from Peter Schorn.

Updated HP2100 simulator from Dave Bryan.

Beta Sigma 5, 6 & 7 simulator from Bob Supnik

Beta SAGE-II and PDQ-3 simulators from Holger Veit

Intel Systems 8010 and 8020 simulators from Bill Beech

CDC 1700 simulator from John Forecast

Hans-Åke Lund has implemented an SCELBI (SCientic-ELectronics-BIology) simulator.

New Host Platform support - HP-UX and AIX

Simulator Front Panel API

The sim_frontpanel API provides a programatic interface to start and control any simulator without any special additions to the simulator code.

New Functionality

Remote Console Facility

A new capability has been added which allows a TELNET Connection to a user designated port so that some out of band commands can be entered to manipulate and/or adjust a running simulator. The commands which enable and control this capability are SET REMOTE TELNET=port, SET REMOTE CONNECTIONS=n, SET REMOTE TIMEOUT=seconds, and SHOW REMOTE.

The remote console facility has two modes of operation: 1) single command mode. and 2) multiple command mode.
In single command mode you enter one command at a time and aren't concerned about what the simulated system is doing while you enter that command. The command is executed once you've hit return. In multiple command mode you initiate your activities by entering the WRU character (usually ^E). This will suspend the current simulator execution. You then enter commands as needed and when you are done you enter a CONTINUE command. While entering Multiple Command commands, if you fail to enter a complete command before the timeout (specified by "SET REMOTE TIMEOUT=seconds"), a CONTINUE command is automatically processed and simulation proceeds.

A subset of normal simh commands are available for use in remote console sessions. The Single Command Mode commands are: ATTACH, DETACH, PWD, SHOW, DIR, LS, ECHO, HELP The Multiple Command Mode commands are: EXAMINE, IEXAMINE, DEPOSIT, EVALUATE, ATTACH, DETACH, ASSIGN, DEASSIGN, STEP, CONTINUE, PWD, SAVE, SET, SHOW, DIR, LS, ECHO, HELP

A remote console session will close when an EOF character is entered (i.e. ^D or ^Z).

VAX/PDP11 Enhancements

RQ has new disk types: RC25, RCF25, RA80
RQ device has a settable controller type (RQDX3, UDA50, KLESI, RUX50)
RQ disks default to Autosize without regard to disk type
RQ disks on PDP11 can have RAUSER size beyond 2GB
DMC11/DMR11 DDCMP DECnet device simulation.  Up to 8 DMC devices are supported.  Packet transport is via TCP or UDP connections.
KDP11 on PDP11 for DECnet
DUP11 on PDP11 for DECnet connectivity to talk to DMC, KDP or other DUP devices
DZ on Unibus systems can have up to 256 ports (default of 32), on 
    Qbus systems 128 port limit (default of 16).
DZ devices optionally support full modem control (and port speed settings 
    when connected to serial ports).
TU58 device support for all PDP11 and VAX systems.
DHU11 (device VH) on Unibus systems now has 16 ports per multiplexer.
XQ devices (DEQNA, DELQA and DELQA-T) are bootable on Qbus PDP11 simulators
XQ and XU devices (DEQNA, DELQA, DELQA-T, DEUNA and DELQA) devices can now 
    directly communicate to a remote device via UDP (i.e. a built-in HECnet bridge).
XQ and XU devices (DEQNA, DELQA, DELQA-T, DEUNA and DELQA) devices can now 
    optionally throttle outgoing packets which is useful when communicating with
    legacy systems (real hardware) on a local LAN which can easily get over run 
    when packets arrive too fast.
MicroVAX 3900 has QVSS (VCB01) board available.
MicroVAX 3900 and MicroVAX II have SET CPU AUTOBOOT option
MicroVAX 3900 has a SET CPU MODEL=(MicroVAX|VAXServer|VAXStation) command to change between system types
MicroVAX I has a SET CPU MODEL=(MicroVAX|VAXSTATION) command to change between system types
MicroVAX II has a SET CPU MODEL=(MicroVAX|VAXSTATION) command to change between system types

PDP10 Enhancements

KDP11 (from Timothe Litt) for DECnet connectivity to simulators with DMC, DUP or KDP devices
DMR11 for DECnet connectivity to simulators with DMC, DUP or KDP devices on TOPS10.

SDS 940 Enhancements

Support for SDS internal ASCII character encoding during display and data entry.
Allow breakpoints to be qualified by normal, monitor or user mode.
Fix CPU, RAD, MUX and I/O bugs that prevented SDS Time Share System Monitor and Executive from executing properly.

Terminal Multiplexer additions

Added support for TCP connections using IPv4 and/or IPv6.
Logging - Traffic going out individual lines can be optionally logged to 
Buffering - Traffic going to a multiplexor (or Console) line can 
        optionally be buffered while a telnet session is not connected
        and the buffered contents will be sent out a newly connecting 
        telnet session.  This allows a user to review what may have 
        happened before they connect to that session.

Serial Port support based on work by J David Bryan and Holger Veit
Serial Console Support
Separate TCP listening ports per line
Outgoing connections per line (virtual Null Modem cable).
Packet sending and reception semantics for simulated network device support using either TCP or UDP transport.
Input character rates reflect the natural character arrival time based on the line speed.

Video Display Capabilities

Added support for monochrome displays with optional keyboards and mice.
The VAXstation QVSS device (VCB01) simulation uses this capability. Host platforms which have libSDL available can leverage this functionality.

Asynchronous I/O

* Disk and Tape I/O can be asynchronous.  Asynchronous support exists 
  for pdp11_rq, pdp11_rp and pdp11_tq devices (used by VAX and PDP11 
* Multiplexer I/O (Telnet and/or Serial) can be asynchronous.  
  Asynchronous support exists for console I/O and most multiplexer 
  devices.  (Still experimental - not currently by default)

Clock/Timer Enhancements

* Asynchronhous clocks ticks exist to better support modern processors 
  that have variable clock speeds.  The initial clock calibration model 
  presumed a constant simulated instruction execution rate.  
  Modern processors have variable processor speeds which breaks this 
  key assumption.  
* Strategies to make up for missed clock ticks are now available
  (independent of asynchronous tick generation).  These strategies
  generate catch-up clock ticks to keep the simulator passage of 
  time consistent with wall clock time.  Simulator time while idling 
  or throttling is now consistent.  Reasonable idling behavior is 
  now possible without requiring that the host system clock tick be
  10ms or less.

Ethernet Transport Enhancements

* UDP packet transport.  Direct simulator connections to HECnet can be 
  made without running a local packet bridge program.
* NAT packet transport.  Simulators which only speak TCP/IP (No DECnet)
  and want to communicate with their host systems and/or directly to 
  the Internet can use NAT packet transport.  This also works for WiFi 
  connected host systems.
* Packet Transmission Throttling.  When connected to a LAN which has 
  legacy network adapaters (DEQNA, DEUNA) on legacy systems, it is very
  easy for a simulated system to overrun the receiving capacity of the
  older systems.  Throttling of simulated traffic delivered to the LAN 
  can be used to mitigate this problem.
* Reliable MAC address conflict detection.  
* Automatic unique default MAC address assignment.  

Disk Extensions

RAW Disk Access (including CDROM)
Virtual Disk Container files, including differencing disks

Embedded ROM support

Simulators which have boot commands which load constant files as part of 
booting have those files imbedded into the simulator executable.  The 
imbedded files are used if the normal boot file isn't found when the 
simulator boots.  Specific examples are:

	VAX (MicroVAX 3900 - ka655x.bin)
	VAX8600 (VAX 8600 - vmb.exe)
	VAX780 (VAX 11/780 - vmb.exe)
	VAX750 (VAX 11/750 - vmb.exe, ka750_old.bin, ka750_new.bin), 
	VAX730 (VAX 11/730 - vmb.exe)
	VAX610 (MicroVAX I - ka610.bin)
	VAX620 (rtVAX 1000 - ka620.bin)
	VAX630 (MicroVAX II - ka630.bin)

Control Flow

The following extensions to the SCP command language without affecting prior behavior:

GOTO <Label>                 Command is now available.  Labels are lines 
                             in which the first non whitespace character 
                             is a ":".  The target of a goto is the first 
                             matching label in the current do command 
                             file which is encountered.  Since labels 
                             don't do anything else besides being the 
                             targets of goto's, they could be used to 
                             provide comments in do command files, for 
                             example (":: This is a comment")
SET ON                       Enables error trapping for currently defined 
                             traps (by ON commands)
SET NOON                     Disables error trapping for currently 
                             defined traps (by ON commands)
RETURN                       Return from the current do command file 
                             execution with the status from the last 
                             executed command
RETURN <statusvalue>         Return from the current do command file 
                             execution with the indicated status.  Status 
                             can be a number or a SCPE_<conditionname> 
                             name string.
ON <statusvalue> commandtoprocess{; additionalcommandtoprocess}
                             Sets the action(s) to take when the specific 
                             error status is returned by a command in the 
                             currently running do command file.  Multiple 
                             actions can be specified with each delimited 
                             by a semicolon character (just like 
                             breakpoint action commands).
ON ERROR commandtoprocess{; additionalcommandtoprocess}
                             Sets the default action(s) to take when any 
                             otherwise unspecified error status is returned 
                             by a command in the currently running do 
                             command file.  Multiple actions can be 
                             specified with each delimited by a semicolon 
                             character (just like breakpoint action 
ON <statusvalue>                   
ON ERROR                     Clears the default actions to take when any 
                             otherwise unspecified error status is 
                             returned by a command in the currently 
                             running do command file.

Error traps can be taken for any command which returns a status other than SCPE_STEP, SCPE_OK, and SCPE_EXIT.

ON Traps can specify any status value from the following list: NXM, UNATT, IOERR, CSUM, FMT, NOATT, OPENERR, MEM, ARG, STEP, UNK, RO, INCOMP, STOP, TTIERR, TTOERR, EOF, REL, NOPARAM, ALATT, TIMER, SIGERR, TTYERR, SUB, NOFNC, UDIS, NORO, INVSW, MISVAL, 2FARG, 2MARG, NXDEV, NXUN, NXREG, NXPAR, NEST, IERR, MTRLNT, LOST, TTMO, STALL, AFAIL. These values can be indicated by name or by their internal numeric value (not recommended).

Interactions with ASSERT command and "DO -e":

DO -e		is equivalent to SET ON, which by itself it equivalent 
            to "SET ON; ON ERROR RETURN".
ASSERT		failure have several different actions:
   * If error trapping is not enabled then AFAIL causes exit from 
     the current do command file.
   * If error trapping is enabled and an explicit "ON AFAIL" 
     action is defined, then the specified action is performed.
   * If error trapping is enabled and no "ON AFAIL" action is 
     defined, then an AFAIL causes exit from the current do 
     command file.

Other related changes/extensions: The "!" command (execute a command on the local OS), now returns the command's exit status as the status from the "!" command. This allows ON conditions to handle error status responses from OS commands and act as desired.

Scriptable interactions with running simulators.

The EXPECT command now exists to provide a means of reacting to simulator output and the SEND command exists to inject data into programs running within a simulator.

EXPECT {HALTAFTER=n,}"\r\nPassword: "
SEND {AFTER=n,}{DELAY=m,}"mypassword\r"


EXPECT {HALTAFTER=n,}"\r\nPassword: " SEND {AFTER=n,}{DELAY=m,}"mypassword\r"; GO


HELP dev
HELP dev SET  (aka HELP SET dev)
HELP dev SHOW (aka HELP SHOW dev)

Generic scp support Clock Coscheduling as opposed to per simulator implementations.

New SCP Commands:

SCREENSHOT filename.bmp          Save video window to the specified file
SET ENV Name=Value               Set Environment variable
SET ENV -p "Prompt" Name=Default Gather User input into an Environment Variable
SET ASYNCH                       Enable Asynchronous I/O
SET NOASYNCH                     Disable Asynchronous I/O
SET VERIFY                       Enable commang display while processing DO command files
SET NOVERIFY                     Enable commang display while processing DO command files
SET MESSAGE                      Enable error message output when commands complete (default)
SET NOMESSAGE                    Disable error message output when commands complete
SET QUIET                        Set minimal output mode for command execution
SET NOQUIET                      Set normal output mode for command execution
SET PROMPT                       Change the prompt used by the simulator (defaulr sim>)
SET THROTTLE x/t                 Throttle t ms every x cycles
SET REMOTE TELNET=port           Specify remote console telnet port
SET REMOTE NOTELNET              Disables remote console
SET REMOTE CONNECTIONS=n         Specify the number of concurrent remote console sessions
SHOW FEATURES                    Displays the devices descriptions and features
SHOW ASYNCH                      Display the current Asynchronous I/O status
SHOW SERIAL                      Display the available and/or open serial ports
SHOW ETHERNET                    Display the available and/or open ethernet connections
SHOW MULTIPLEXER                 Display the details about open multiplexer devices
SHOW CLOCKS                      Display the details about calibrated timers
SHOW REMOTE                      Display the remote console configuration
SHOW ON                          Display ON condition dispatch actions
SET ON                           Enable ON condition error dispatching
SET NOON                         Disable ON condition error dispatching
GOTO                             Transfer to lable in the current DO command file
CALL                             Call subroutine at indicated label
RETURN                           Return from subroutine call
SHIFT                            Slide argument parameters %1 thru %9 left 1
NOOP                             A no-op command
ON                               Establish or cancel an ON condition dispatch
IF                               Test some simulator state and conditionally execute commands
CD                               Change working directory
SET DEFAULT                      Change working directory
PWD                              Show working directory
SHOW DEFAULT                     Show working directory
DIR {path|file}                  Display file listing
LS {path|file}                   Display file listing
NEXT                             Step across a subroutine call or step a single instruction.
EXPECT                           React to output produced by a simulated system
SEND                             Inject input to a simulated system's console
SLEEP time                       Pause command execution for specified time
SCREENSHOT                       Snapshot the current video display window
RUN UNTIL breakpoint             Establish the breakpoiunt specified and run until it is encountered
RUN UNTIL "output-string" ...    Establish the specified "output-string" as an EXPECT and run until it is encountered.
GO UNTIL breakpoint              Establish the breakpoiunt specified and go until it is encountered
GO UNTILE "output-string" ...    Establish the specified "output-string" as an EXPECT and go until it is encountered.

Command Processing Enhancements

Environment variable insertion


Token "%0" expands to the command file name. Token %n (n being a single digit) expands to the n'th argument Token %* expands to the whole set of arguments (%1 ... %9)

The input sequence "%%" represents a literal "%". All other character combinations are rendered literally.

Omitted parameters result in null-string substitutions.

Tokens preceeded and followed by % characters are expanded as environment variables, and if an environment variable isn't found then it can be one of several special variables:

      %DATE%              yyyy-mm-dd
      %TIME%              hh:mm:ss
      %DATETIME%          yyyy-mm-ddThh:mm:ss
      %LDATE%             mm/dd/yy (Locale Formatted)
      %LTIME%             hh:mm:ss am/pm (Locale Formatted)
      %CTIME%             Www Mmm dd hh:mm:ss yyyy (Locale Formatted)
      %UTIME%             nnnn (Unix time - seconds since 1/1/1970)
      %DATE_YYYY%         yyyy        (0000-9999)
      %DATE_YY%           yy          (00-99)
      %DATE_MM%           mm          (01-12)
      %DATE_MMM%          mmm         (JAN-DEC)
      %DATE_MONTH%        month       (January-December)
      %DATE_DD%           dd          (01-31)
      %DATE_WW%           ww          (01-53)     ISO 8601 week number
      %DATE_WYYYY%        yyyy        (0000-9999) ISO 8601 week year number
      %DATE_D%            d           (1-7)       ISO 8601 day of week
      %DATE_JJJ%          jjj         (001-366) day of year
      %DATE_19XX_YY%      yy          A year prior to 2000 with the same
                                      calendar days as the current year
      %DATE_19XX_YYYY%    yyyy        A year prior to 2000 with the same 
                                      calendar days as the current year
      %TIME_HH%           hh          (00-23)
      %TIME_MM%           mm          (00-59)
      %TIME_SS%           ss          (00-59)
      %STATUS%            Status value from the last command executed
      %TSTATUS%           The text form of the last status value
      %SIM_VERIFY%        The Verify/Verbose mode of the current Do command file
      %SIM_VERBOSE%       The Verify/Verbose mode of the current Do command file
      %SIM_QUIET%         The Quiet mode of the current Do command file
      %SIM_MESSAGE%       The message display status of the current Do command file

Environment variable lookups are done first with the precise name between the % characters and if that fails, then the name between the % characters is upcased and a lookup of that valus is attempted.

The first Space delimited token on the line is extracted in uppercase and then looked up as an environment variable. If found it the value is supstituted for the original string before expanding everything else. If it is not found, then the original beginning token on the line is left untouched.

Command aliases

Commands can be aliases with environment variables. For example:

  sim> set env say=echo
  sim> say Hello there
  Hello there
Do command argument manipulation

The SHIFT command will shift the %1 thru %9 arguments to the left one position.

Building and running a simulator

Use Prebuilt Windows Simulators

Simulators for the Windows platform are built and made available on a regular basis (at least once a week if changes have been made to the codebase).

The prebuilt Windows binaries will run on all versions of Microsoft Windows from Windows XP onward.

They can be accessed at

Several relatively recent versions should be available which you can download and use directly.

Building simulators yourself

First download the latest source code from the github repository's master branch at

Depending on your host platform one of the following steps should be followed:

Linux/OSX other *nix platforms

If you are interested in using a simulator with Ethernet networking support (i.e. one of the VAX simulators or the PDP11), then you should make sure you have the correct networking components available. The instructions in describe the required steps to get ethernet networking components installed and how to configure your environment.

See the 0readme_ethernet.txt file for details about the required network components for your platform. Once your operating system build environment has the correct networking components available the following command will build working simulators:

$ make {simulator-name (i.e. vax)}

The makefile provided requires GNU make, which is the default make facility for most systems these days. Any host system which doesn't have GNU make available as the default make facility may have it installed as 'gmake'. GNU make (gmake) is generally available an installation package for all current operating systems which have a package installation system.

Build Dependencies

Some simulators depend on external packages to provide the full scope of functionality they may be simulating. These additional external packages may or may not be included in as part of the standard Operating System distributions.

OS X - Dependencies

The MacPorts package manager is available to provide these external packages. Once MacPorts is installed, these commands will install the required dependent packages:

# port install vde2
# port install libsdl2
# port install libsdl2_ttf


The HomeBrew package manager can be used to provide these packages:

$ brew install vde
$ brew install sdl2
$ brew install sdl2_ttf
Linux - Dependencies

Different Linux distributions have different package managment systems:


# apt-get install libpcap-dev
# apt-get install vde2
# apt-get install libsdl2
# apt-get install libsdl2_ttf


Compiling on windows is supported with recent versions of Microsoft Visual Studio (Standard or Express) and using GCC via the MinGW environment. Things may also work under Cygwin, but that is not the preferred windows environment. Not all features will be available as well as with either Visual Studio or MinGW.

Required related files.

The file

Visual Studio (Standard or Express) 2008, 2010, 2012, 2013 or Visual Studio Community 2015

The file describes the required steps to use the setup your environment to build using Visual Studio.


The file describes the required steps to use the setup your environment to build using MinGW.


Download the latest source code as a zip file from:

Unzip it in the directory that you want SIMH to reside in. Unpack it and set the file attributes as follows:

$ unzip
$ set default [.simh-master]
$ set file/attri=RFM:STM makefile,*.mms,[...]*.c,[...]*.h,[...]*.txt

Simulators with ethernet network devices (All the VAX simulators and the PDP11) can have functioning networking when running on Alpha or IA64 OpenVMS.

In order to build and run simulators with networking support, the VMS-PCAP package must be available while building your simulator. The file can be downloaded from
This link will return a file called which should be unpacked as follows:

$ unzip -aa
$ rename [.simh-vms-pcap]pcap-vms.dir []

The PCAP-VMS components are presumed (by the descript.mms file) to be located in a directory at the same level as the directory containing the simh source files. For example, if these exist here:

[]descrip.mms []scp.c etc.

Then the following should exist: [-.PCAP-VMS]BUILD_ALL.COM [-.PCAP-VMS.PCAP-VCI] [-.PCAP-VMS.PCAPVCM] etc.

To build simulators:

On a VAX use:

$ MMx

On a Alpha & IA64 hosts use:

$ MMx                        ! With Ethernet support
$ MMx/MACRO=(NONETWORK=1)    ! Without Ethernet support

UNZIP can be found on the VMS freeware CDs, or from MMS (Module Management System) can be licensed from HP/Compaq/Digital as part of the VMS Hobbyist program (it is a component of the DECSET product). MMK can be found on the VMS freeware CDs, or from DEC C can be licensed from HP/Compaq/Digital as part of the VMS Hobbyist program.

Problem Reports

If you find problems or have suggestions relating to any simulator or the simh package as a whole, please report these using the github "Issue" interface at

Problem reports should contain;

  • a description of the problem
  • the simulator you experience the problem with
  • your host platform (and OS version)
  • how you built the simulator or that you're using prebuilt binaries
  • the simulator build description should include the output produced by while building the simulator
  • the output of SHOW VERSION while running the simulator which is having an issue
  • the simulator configuration file (or commands) which were used when the problem occurred.