Emulating a DEC PDP-8 with SimH

The DEC PDP-8 mini-computer was launched on 22 March 1965 and was a great success. It was fairly cheap for the day and could easily be expanded. What attracts me most to the PDP-8 is its simple design. I therefore decided to experiment with the SimH emulation of this machine, but found that the documentation wasn't always straightforward. This article intends to show how easy SimH is and how powerful it can be.

Obtaining SimH

SimH can be obtained from The Computer History Simulation Project. It is available as a Windows executable or as source code which can be compiled on many different systems. In addition it is included with most Linux distributions. SimH actually emulates a number of computer systems, and all are included.

Starting the PDP-8 SimH Emulator

SimH is very easy to use once you get the hang of the way that it works. The PDP-8 emulator is started with the command: `pdp8`. The first thing that you generally see is the version of the simulator followed by the `sim>` prompt. At this prompt you can control the simulated hardware of the machine, configure various options and once you start a program it will be taken over by the Console Input/Output of the emulated computer.

SimH will allow you to run a PDP-8 just using the front panel switches, or you can use simulated paper tape and a teletype, as well as a number of other devices. Some programs run directly on the PDP-8 and there are a few operating systems available with their associated software.

Once you have had enough, you can exit the emulator with the following command at the sim> prompt:

exit                         Returns to host Operating System

Configuring the Emulator

SimH is highly configurable and has a number of options which can be set from the sim> command prompt. Below are the most useful to start with.

Controlling the Emulation Speed

On some host machines, the emulator can be made more realistic by slowing down the speed of execution. This is done using one of the following commands:
set throttle xM              Set execution rate to x mips
set throttle xK              Set execution rate to x kips
set throttle x%              Limit simulator to x% of host time
set nothrottle               Turn off throttling

Reducing Resource Use on the Host Machine

Some host machines support the following command to control how resources are used.

set cpu idle                 Reduces drain on host machine when emulated cpu is idle
set cpu noidle               Constant drain on host machine

Connecting the Console to a Telnet Port

If you wish to connect the output of the console to a telnet port, this can be done with the following:

set console telnet=<port>    Connect console terminal to Telnet session on port
set console notelnet         Disable console Telnet

Automating the Configuration

The above configuration options can be stored in a file so that each time the PDP-8 emulator is started, they will take effect. To do this put any of the options above into a file called pdp8.ini. Now when the PDP-8 emulator is started, it will look to see if the file pdp8.ini is in the current directory, if it is then the lines from it will be read in just as if you had typed them at the sim> prompt.

Example pdp8.ini file

If the following lines are put in a file called pdp8.ini, the emulator will not hog so many resources on the host system, will emulate at 500,000 Instructions Per Second and the console will be reachable through telnet on port 18977. However, not all of these settings will work on all machines.

set cpu idle
set throttling 500k
set console telnet=18977


SimH can emulate a large number of the devices that were attached to the PDP-8. Each of these devices can be connected to a file on the host system, e.g. The line Printer (LPT) could be connected to a file called `print.out` and then if anything was printed from the PDP-8, it would go to that file. The most important devices for the purpose of this article and up-coming ones are:
PTRHigh-Speed Paper Tape Reader
PTPHigh-Speed Paper Tape Punch
LPTLine Printer
RXFloppy Disk (RX01 and RX02)
RFFixed Head Disk
DTDECtape (DT00 to DT07)

Devices can be connected to files or disconnected with the following commands:

attach <unit> <filename>
detach <unit>

Octal Addressing

The natural way of abbreviating binary numbers on the PDP-8 is in octal. This is a base 8 number system where each octal digit represents 3 bits. The machine uses 12-bit addressing and hence 4 octal digits represent these 12 bits. When these bits are referred to it must be remembered that the PDP-8 uses big-endian bit numbering (that is, bit 0, the left most bit, is the most significant).

Low-level Access to the Machine

The PDP-8 often required you to access registers and memory addresses directly and it was particularly common for software to require you to configure certain option by setting the front panel switches. Below are the most important registers that can be set through SimH:
NameSize in bitsDescription
PC15Program Counter, including IF as high 3 bits
MQ12Multiplier Quotient
IF3Instruction Field
DF3Date Field
SR12Switch Register (Front Panel Switches)

Commands to Change/Examine Memory/Registers

There are two main commands that can be used to change or examine memory locations and registers:
examine <Register|Memory Location>              Examine a Register|Memory Location
deposit <Register|Memory Location> <Value>      Deposit Value in the Register|Memory Location

To make this easier, examine can be abbreviated to e and depost can be abbreviated to d.

Controlling Programs

There are a number of commands to start a program running and control its execution:

run [<address>]              Resets all devices and starts execution at address
                             or PC if address not specified
go [<address>]               As above, but doesn't reset devices
cont                         Does not reset devices and continues execution at PC
boot <unit>                  Resets all devices and bootstraps the device and unit given by its argument.
                             If no unit is supplied, unit 0 is bootstrapped

On a real PDP-8 to load a program the first step is often to enter a short program via the front-panel switches that loads paper tape in RIM format. This is then generally used to load a Binary Loader program via paper tape. This Binary Loader can then load programs via paper tape in BIN format. The BIN format is normally used for programs as it is more compact. You could follow this process with SimH, but there are some shortcuts to make things easier:

load -r <filename>           Loads a file from the host system in RIM format
load <filename>              Loads a file from the host system in BIN format

You can use CTRL+E to get back to the sim> prompt when talking to the emulated PDP-8 via the Console Input/Output. This is defined by the WRU (Where are you) console option and is initially set to 005 (CTRL+E). To continue execution you would just use the cont command above.


To give a quick demonstration of SimH, I have chosen to use FOCAL-69. FOCAL-69 was an important version of the FOCAL (FOrmula CALculator) programming language, created by Richard Merrill. It would run even on systems with only 4K of memory but could use more if available. A copy of the FOCAL-69 Promotional Booklet is available from Douglas W. Jones's PDP-8 page and a FOCAL-69 binary paper tape image is available from the SimH Software Kits Page.

To try out FOCAL-69 in the PDP-8 emulator, first uncompress the file containing the FOCAL-69 binary paper tape image downloaded above. This will give you a file called focal69.bin. Now run, pdp8 in the directory in which focal69.bin is located.

To load the paper tape image into memory type:

sim> load focal69.bin

Programs loaded from paper tape in binary format often start at Octal address 200 and this is the case here. To start the program run:

sim> run 200

FOCAL-69 will now start and present you with the * prompt. To say hello to the world type the following excluding the * which is the prompt:


Where Now?

More information is available at the The Computer History Simulation Project, particularly on the Simulator Documentation page. There are several software kits available to run on the PDP-8 and documentation for these can be found on the Simulator Documentation page. Another good source of software and documentation is at bitsavers.org.

I will shortly be writing some more articles on the PDP-8, so stay tuned.

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Emulating a DEC PDP-8 with SimH by Lawrence Woodman is licensed under a Creative Commons Attribution 4.0 International License.

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