How To Emulate Unix V7 Using Simh (2015)
John David Pressman
Unix v7, released for the PDP-11 line of minicomputers in 1979 is one of computings classics. Just a year before The C Programming Language had been released in its first edition by Bell Labs. In this release is the first appearance of pcc, the Portable C Compiler which began C’s ascendancy as the de facto systems language in use today. This is the same year that Bill Joy stepped down as lead developer of vi. Many of the programs you use on a daily basis can trace their ancestry here.
The emulator we’ll be using, SIMH, was written in 1993 to preserve the dying minicomputer culture that spawned Unix, VMS and other operating systems of the seventies and eighties. As a result, most of the images to boot are old, and their instructions have not kept up with the times. This is a short tutorial to correct this for V7.
SIMH can be downloaded from its website for free. Precompiled executable are available for Windows, if you use a Unix or Unix-like platform such as Macintosh or Linux downloading from the site means compiling the sources yourself. However it is highly likely that alternative, more convenient means exist.
On Macintosh, SIMH can be installed using homebrew with the command:
brew install simh
On Linux, your distribution probably includes a copy of SIMH in its repositories. With Debian you would run the command:
sudo apt-get install simh
After you’ve installed the emulator it’s time to boot a copy of Unix.
Booting An Image
Before we continue, it’s best practice to specify what versions of software are being used in a tutorial. The author is using:
SIMH version 3.8-1
Unix Version 7
As a further disclaimer the author has only tried this on Linux, and cannot speak in further detail for Windows or Macintosh operating systems.
Before we can boot Unix we need to get a copy of it. Unix boot images have been preserved on multiple sites across the net. We’ll be using the one provided on SIMH’s website here. Another site has an outdated README that provided critical material for this tutorial. If you use a different image these instructions will not work. Once you have downladed the image it would probably be prudent to copy it into a directory of its own and then change directories to it. In this case we will name our directory unix-emulation:
cp uv7swre.zip unix-emulation/;
This will help keep boot files and other artifacts in a contained clean workspace. Unpack the file.
unzip -d uv7 uv7swre.zip
Now run the emulator with the ‘pdp11’ command:
PDP-11 simulator V3.8-1
Once at the sim prompt you’ll want to run the following commands:
Set the cpu type to that of a PDP-11/45:
set cpu 11/45
Set the teletype output to seven bits.
set tto 7b
Next you’ll want to attach the boot image to the rl device, which emulates a cartridge disk controller:
att rl unix_v7_rl.dsk
Boot the image from the drive:
The next prompt you get will be a single ‘@’ character, type ‘boot’:
New Boot, known devices are hp ht rk rl rp tm vt
Next you will want to type in the device to boot from and what directory:
And if all goes well you should see:
mem = 177856
This prompt means you have successfully booted the system, but don’t start using it just yet…
Completing The Installation
Before you can start using your new Unix install you must make a /tmp directory, if you do not do this essential programs such as Portable C Compiler (pcc) will fail to execute. Furthermore the default user included with the system, the deceased Dennis MacAlistair Ritchie is missing a home directory. Kudos goes to Markus Wandel for writing a tutorial about this in 2002 describing these steps.
First we want to create a tmp directory.
chmod 777 /tmp
Next we want to create the user directory and change the ownership of it to dmr:
chown dmr /usr/dmr
chgrp 3 /usr/dmr
Then press control-D to exit the current prompt and login as dmr, the account requires no password.
The SIMH FAQ (PDF) (Explains how to get network connectivity.)
First Edition Copy Of The C Programming Language (PDF) (The provided C compilers predates ANSI C, so to write new programs on the system either the compiler must be upgraded or K&R as described here must be used.)