This post is part of the series on Learning the Data General Nova with simh.

simh is a fantastic program that simulates more than a dozen computers. Its documentation, although complete, is unfriendly to someone who has never used it. What it lacks is something I consider essential to documentation: The “Hello, world” examples. A “Hello, world” example is something that gets you started even if you know nothing at all about a program. Without that, you’re left with a lot of details about switches and functions, but no idea how to use them to do anything. It’s like having a dictionary but not knowing how to put the words together into a simple sentence.

I’m exploring the Data General Nova using simh. This is my journal of that experience. I’ll document what I learn, including how to use simh to simulate the Nova. At least at first, I’ll try to keep the documentation explicit enough and detailed enough that you could retrace my steps.

My environment

I run Linux, so any shell commands I provide will need to be changed to work with Windows.

Install simh

My OS, Debian Linux 8.2 (aka “jessie”), has a package for simh, so I installed it:

sudo apt-get install simh

This installed simh version 3.8.1.

Download the operating system

On the “Software Kits” page of the simh web site, you will find a file titled “RDOS V7.5 for the Nova” and named rdosswre.tar.Z. Download it:

wget http://simh.trailing-edge.com/kits/rdosswre.tar.Z

And unpack it:

tar xzf rdosswre.tar.Z

This created the directories and files:

• Disks/
• Disks/rdos_d31.dsk
• Licenses/
• Licenses/rdos_license.txt
• Licenses/README.txt

Move the disk image and get rid of the rest:

mv Disks/rdos_d31.dsk .
rm -rf Disks Licenses

Starting simh and RDOS

$ dgnova

NOVA simulator V3.8-1
sim> att dkp0 rdos_d31.dsk
sim> set tti dasher
sim> boot dkp0

Filename?

NOVA RDOS Rev 7.50
Date (m/d/y) ? 12/7/2015
Time (h:m:s) ? 5:23:20

R

The R is RDOS’s prompt. The Nova is booted, runing RDOS, and awaiting our command.

Yes, you’ll need to enter the date and time each time you boot. The Nova had no battery powered clock that kept the time while the computer was off.

simh is quite scriptable, so maybe later I’ll see if I can script the entering of the date and time.

Simh devices

Simh calls each of the computer’s elements a “device,” including the CPU. Let’s use some simh commands to examine devices. Although our simulator is running, we can interrupted it and get back to the simh command prompt by typing ^e (control-e):

R
^e
Simulation stopped, PC: 41741 (MOVZL# 1,1,SNC)
sim>

Note: The ^e shown above won’t show on your screen.

The simulation is paused and we’re at the simh prompt. Let’s list all of the devices:

sim> show dev
NOVA simulator configuration

CPU
PTR
PTP
TTI
TTO
TTI1
TTO1
RTC, 60Hz
PLT
LPT
DSK
DKP, 4 units
MTA, 8 units
QTY, disabled
ALM, lines=64

DKP is the controller for the moving head disk controller, which can control up to four disk drives. These disk drives do the same thing as a modern disk drive, although they are far slower, far smaller (in capacity), far larger (in physical size), and much more expensive. Oh, and they are removable–the media is in a “pack” that can be removed. Otherwise, they are exactly the same as a modern hard drive. Let’s look at that controller:

sim> show DKP
DKP, 4 units
DKP0, 1247KW, attached to rdos_d31.dsk, write enabled, 4047 (Diablo 31)
DKP1, 1247KW, not attached, write enabled, autosize
DKP2, 1247KW, not attached, write enabled, autosize
DKP3, 1247KW, not attached, write enabled, autosize

To get out of the simh prompt and back to the simulation:

sim> cont

Displaying the date and time

The GTOD command displays the date and time:

R
GTOD
12/07/115   05:43:58

115 is how it display the year “2015”. It’s interesting that the OS handles the year 2015 at all. After all, RDOS is from the 70’s. Either it’s just a lucky accident that the year 2015 works, or it only appears to work, or someone was thinking 25 years ahead when they created RDOS. I prefer to think someone was thinking 25 years ahead.

Stopping RDOS and existing simh

RDOS should be shut down by unmounting the device that RDOS is mounted on:

R
RELEASE DP0

MASTER DEVICE RELEASED

HALT instruction, PC: 00001 (INC 2,2)
sim>

To exit simh, use the quit command:

sim> quit
$

References

RDOS/DOS Command Line Interpreter User’s Manual

• p. 4-37 - GTOD command
• p. 4-59 - RELEASE command