PiDP-8/I Software

How to Control SIMH and OS/8 from Python
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While we were building the mkos8 tool (predecessor to os8-run), we built a set of facilities for driving SIMH and OS/8 running under SIMH from the outside using Python, a very powerful programming language well suited to scripting tasks. It certainly beats writing PDP-8 code to achieve the same ends!

When someone on the mailing list asked for a way to automatically drive a demo script he'd found online, it was natural to generalize the core functionality of mkos8 as a reusable Python class, then write a script to make use of it. The result is class simh, currently used by six different scripts in the PiDP-8/I software distribution including os8-run and the teco-pi-demo demo script.

The basis for this work is pexpect the Python Expect library.

This document describes how teco-pi-demo works, and through it, how class simh works, with an eye toward teaching you how to reuse this functionality for your own ends.


Because we do not install these components in the system's Python library path, you must modify that path to allow your script to find these components. Simply copy this invocation block into the top of your script:

import os
import sys
sys.path.insert (0, os.path.dirname (__file__) + '/../lib')
sys.path.insert (0, os.getcwd () + '/lib')

from pidp8i import *
from simh   import *

That adjusts the path, then imports all of the generic functionality from the PiDP-8/I lib directory into the current namespace.

The sys.path.insert business assumes that your script is installed into the PiDP-8/I's bin directory alongside teco-pi-demo. If you've installed it somewhere else, you'll need to adjust these paths.

Starting SIMH

The first thing we'll do is start SIMH as a child process of our Python script under control of an instance of class simh:

s = simh (

We call that instance s for short, because we will be calling its methods a lot in this script.

We pass as the first parameter to the constructor, which tells it how to find the PDP-8 simulator program, derived from the code shipped on GitHub by the SIMH project, configured and modified for the needs of the PiDP-8/I project. We call this the child program, as it is what class simh controls from the outside.

There is an optional second parameter to the constructor, a Boolean flag that controls whether class simh starts the fully-featured PiDP-8/I simulator or falls back to something closer to the pristine upstream SIMH PDP-8 simulator. By default, we do the former, so that the simulator updates front panel LEDs with internal simulator state, and toggling front panel switches affect the internal state of the simulator.

If you don't want the PiDP-8/I GPIO thread to run while your script runs, pass True here instead, since this is the "skip GPIO" flag, and its default is therefore False. We do that from programs like os8-run and os8-cp because we want them to run everywhere, even on an RPi while another simulator is running; we also don't want the front panel switches to affect these programs' operations. If your program never runs on an RPi, passing True here will usuall make it run faster, since the GPIO thread saps computer resources and so shouldn’t be started if it isn’t needed.


The next step is to tell the s object where to send its logging output:

s.set_logfile (os.fdopen (sys.stdout.fileno (), 'wb', 0))

Contrast the corresponding line in os8-run which chooses whether to send logging output to the console or to a log file:

s.set_logfile (open (dirs.log + 'os8-run' + '.log', 'ab') \
    if not VERY_VERBOSE else os.fdopen (sys.stdout.fileno (), 'wb', 0))

Note that this more complicated scheme appends to the log file instead of overwriting it because there are cases where os8-run gets run more than once with different script inputs, so we want to preserve the prior script outputs, not keep only the latest.

Driving SIMH and OS/8

The basic control flow is:

  1. Send to SIMH text to act upon.
  2. Harvest results.
  3. Check results.
  4. Goto 1 or quit.

Checking results

There are a number of helper methods and data structures to help in checking results.

Although pexpect can search replies for a regular expression string, or a list of such strings, the helper methods use an array of compiled regular expressions.

The simh class contains two arrays, _simh_replies and _os8_replies with corresponding arrays of compiled regular expressions, _simh_replies_rex, and _os8_replies_rex.

You can use the simh Class replies or define some for yourself. Example:

my_replies = [
  ["Sample Reply", "Sample Reply\s+.*\n$", "False"],
  ["Fatal Error", "Fatal error was\s+.*\n$", "True"]

my_replies_rex = []
for item in my_replies:

Often you want your replies in addition to the errors you might want from OS/8. In that case you'd do something like:


Of course the extend would appear before the computation of my_replies_rex.

Running SIMH or OS/8 commands

High level calls to run commands in SIMH can be made from

simh_cmd for SIMH commands and os8_cmd for OS/8 commands.

These two methods default to searching results for replies in the relevant arrays. They return an index into the array that says which reply was received.

The test_result method takes a reply number, an expected reply name, an array replies and a string for helping identify the caller of the test. (There's also an optional debug flag. These scripts can be difficult to debug.)

To see if the reply from running a command that would reply with my_replies the code would be:

s.test_result(reply, "Fatal Error", my_replies, "myfunc")

The reply item at the index given by reply is examined. And the desired reply is matched against the first element of that item. If it matches, True is returned, otherwise False is returned. If the caller string is present, (in this case, "myfunc", a message is printed if the reply doesn't match the expected reply. If the caller string is the empty string, no message is printed. This makes it easy to add error diagnostics without a lot of extra work.

Sometimes you want to try a couple different expected values, and don't want to print anything if there isn't a match. That's why we special case an empty caller string.

For SIMH and OS/8 command testing, there are convenience wrappers, simh_test_result and os8_test_result that use the relevant array so you don't have to keep typing it. So the following two are equivalent:

s.test_result(reply, "Prompt", s._simh_replies, "myfunc", debug=True)

s.simh_test_result(reply, "Prompt", "myfunc", debug=True)

Armed with an understanding of how we make calls into SIMH and OS/8, and how we test results, we're ready to continue our exploration.

Finding and Booting the OS/8 Media

If your program will use our OS/8 boot disk, you can find it programmatically by using the dirs.os8mo constant, which means "OS/8 media output directory", where "output" refers to the worldview of os8-run. Contrast dirs.os8mi, which points to the directory holding the input media for os8-run.

This snippet shows how to use it:

rk = os.path.join (dirs.os8mo, 'v3d.rk05')
if not os.path.isfile (rk):
    print "Could not find " + rk + "; OS/8 media not yet built?"
    exit (1)

Now we attach the RK05 disk image to the PiDP-8/I simulator found by the simh object and boot from it:

print "Booting " + rk + "..."
s.simh_cmd ("att rk0 " + rk)
s.simh_test_result (reply, "Prompt", "main 1")
reply = s.simh_cmd ("boot rk0", s._os8_replies_rex)
s.os8_test_result (reply, "Monitor Prompt", "main 2")

A couple subtle points: We issued a command to SIMH to attach the rk0 device. If we didn't get the SIMH prompt back, simh_test_result would have said,

main 1: Expecting Prompt. Instead got: Fatal Error

Then we issued the SIMH command to boot that device. We used the os8_test_result method instead of the simh_test_result method because we expected the panoply of replies would more likely be from the OS/8 list.

After the simulator starts up, and we've confirmed we've got our OS/8 monitor prompt as a result, we send the first OS/8 command to start our demo.

s.os8_cmd ("R TECO")
s.os8_test_result (reply, "Command Decoder Prompt", "main 2")

The bulk of teco-pi-demo consists of more calls to simh.os8_cmd and simh.cmd. Read the script if you want more examples.

IMPORTANT: When you specify the regular expression strings for result matching, and want literal matches for characters that are special to regular expressions such as dot ., asterisk *, etc., you need to be preface the characterpair of backslashes. Example: To match a literal dollar sign you would say \\$.


The operation of OS/8 under SIMH requires awareness of who is getting the commands: SIMH, the OS/8 Keyboard Monitor, the OS/8 Command Decoder, or some read/eval/print loop in a program being run.

Your use of the simh class needs to be mindful of this. Throughout this document every attempt has been made to be clear on which methods keep track of context switches for you and which do not.

Context Within a program under OS/8

If you've forgotten to exit a sub-program, that program will still be getting your subsequent commands instead of OS/8.

You may have a program that keeps running and asking for more input, for example OS/8 PIP returns to the command decoder after each action.

There is a subtle issue with program interrupts: You need to check for the string that gets echoed when you do an interrupt. Otherwise pexpect can get confused.

Two methods that abstract this for you are provided: os8_ctrl_c and os8_escape which send those interrupt characters, ask pexpect to listen for their echo back ($ comes back from escape), and confirms a return to the OS/8 monitor.

Here is the implementation of os8_ctrl_c as an example if you need to run a sub-program with a different interrupt character:

  #### os8_ctrl_c ##################################################
  # Return to OS/8 monitor using the ^C given escape character.
  # We need to listen for the ^C echo or else cfm_monitor gets confused.
  # Confirm we got our monitor prompt.
  # Optional caller argument enables a message if escape failed.
  # Note: OS/8 will respond to this escape IMMEDIATELY,
  # even if it has pending output.
  # You will need to make sure all pending output is in
  # a known state and the running program is quiescent
  # before calling this method. Otherwise pexpect may get lost.

  def os8_ctrl_c (self, caller = "", debug=False):
    self.os8_send_ctrl ("c")
    return self.os8_cfm_monitor (caller)

Sending Control Characters

Several OS/8 programs expect an Escape (a.k.a. ALTMODE) keystroke to do things. Examples are TECO and FRTS.

(Yes, Escape is Ctrl-[. Now you can be the life of the party with that bit of trivia up your sleeve. Or maybe you go to better parties than I do.)

The os8_send_ctrl method enables you to send arbitrary control characters but it does not keep track of whether you're in the OS/8 or SIMH context. Note also that the e control character escapes to SIMH. So avoid writing programs that need that control character as input.

Context Between SIMH and OS/8

It is important to make sure that commands intended for SIMH go there, and not to OS/8 or any programs running under SIMH. The os8_cmd amd simh_cmd methods keep track of context. If you call the simh_cmd method but aren't actually escaped out to SIMH, an escape will be made for you, and the context change will be recorded.

If you issue os8_cmd when OS/8 is not running, it will complain and refuse to send the command.

The cleanest way to explicitly escape from OS/8 to SIMH is to call esc_to_simh. It manages the context switch, and tests to see that you got the SIMH prompt. Example:


Subtle points: Calling simh_cmd will leave you in SIMH. You will need to resume OS/8 explicitly. There are a variety of ways to do this.

Getting Back to OS/8 from SIMH

There are several ways to get back to the simulated OS/8 environment from SIMH context, each with different tradeoffs.

FIXME We used to have a lot of trouble with continue commands. We think they're all fixed now, so we can fully flesh out this section.


You saw the first one above: send a boot rk0 command to SIMH. This restarts OS/8 entirely. This is good if you need a clean environment. If you need to save state between one run of OS/8 and the next, save it to the RK05 disk pack or other SIMH media, then re-load it when OS/8 reboots.

It's important to check that you got your OS/8 prompt so the recommended code looks like this:

    reply = s.simh_cmd ("boot rk0", my_replies_rex)
    s.os8_test_result (reply, "Monitor Prompt", "myprog")


The way teco-pi-demo does it is to send a cont command to SIMH:

    s.send_line ('cont')

A previous version of the simh class would sometime hang the simulator unless a small delay were inserted before escaping to the SIMH context. We believe this is no longer necessary. However the problems with cont made implementors gun shy using it. Most code you will see does a restart with an explicit confirmation we are at the OS/8 command level.

Re-starting OS/8

If your use of OS/8 is such that all required state is saved to disk before re-entering OS/8, you can call the simh_restart_os8 method to avoid the need for a delay or a reboot.

It sends the simh command go 7600 which is the traditional "restart at the OS/8 entrypoint" commonly used from the PDP-8 front panel. It then uses os8_test_result to confirm that it got a monitor prompt. simh_restart_os8 has an optional caller argument to make it quick and easy to print an error if returning to the monitor failed.

s.simh_restart_os8 (caller = "myprog")

os8-run uses this option extensively.

Sending without testing results.

At some point you always need to test your results and make sure you are where you think you are. Otherwise some corner case will trip up your use of the simh class, and the error message you will get is a 60 second pause, and a big backtrace.

But often you need to send and receive data in a much less structured way than that used by os8_cmd and simh_cmd. Here is what you need:

Method Description
send_line Send the given line blind without before or after checks.
simh_send_line Like send_line above, but mindful of context. Will escape to SIMH if necessary.
os8_kbd_delay Wait an amount of time proportional to what OS/8 should be able to handle on the hosting platform without overflowing the input buffer and dying.
os8_send_ctrl Send a control character to OS/8. Use os8_kbd_delay to prevent overflowin the input buffer and killing OS/8.
os8_send_str Send a string of characters to OS/8, and wait for os8_kbd_delay afterwards.
os8_send_line Add a carriage return to the given string and call os8_send_str to send it to OS/8.

Other Operations

Quitting the simulator

It is recommended that you use the quit method to exit the simulator. It will make sure it selects the simh context before trying to quit.

Indeed it's further recommended that you send a command to simh to detach all devices to make sure any buffered output is flushed. You need to test for the SIMH prompt to make sure you've succeeded.

  s.simh_cmd ("detach all")
  s.simh_test_result(reply, "Prompt", my_replies_rex, "myprog")

Zero Core

SIMH's PDP-8 simulator doesn't start with core zeroed, on purpose, because the actual hardware did not do that. SIMH does not attempt to simulate the persistence of core memory by saving it to disk between runs, but the SIMH developers are right to refuse to do this by default: you cannot trust the prior state of a PDP-8's core memory before initializing it yourself.

Zero OS/8 Core

Sometimes we want to zero out core, but leave OS/8 in tact. The os8_zero_core method zeros all of core excepting:

We then restart OS/8, which means we absolutely need to do #1 and may need to do #2. We could probably get away with zeroing page 0.

But There's More!

The above introduced you to most of the functionality of class simh used by teco-pi-demo. It is a useful exercise to read through the simh class's source code. There are many useful and interesting methods in the simh class that are documented there not here. Although it started off as a simple class amenable to quick study, some heavy duty drivers for configuration of OS/8 devices under SIMH were added. The source file is organized places the lower level methods first and proceeds through progressively higher level ones, first for simh direct interaction and the OS/8 interaction.

The os8-run script has a whole higher level library built on top of the simh class that includes state machines for executing complex commands like BUILD and applying patches with ODT and FUTIL.

Another useful module is pidp8i.dirs which contains paths to many directories in the PiDP-8/I system, which you can reuse to avoid having to hard-code their locations. This not only makes your script independent of the installation location, which is configurable at build time via ./configure --prefix=/some/path, but also allows it to run correctly from the PiDP-8/I software's build directory, which has a somewhat different directory structure from the installation tree.

Credits and License

Written by and copyright © 2017-2020 by Warren Young and William Cattey. Licensed under the terms of the SIMH license.