PiDP8 boot script for MULTOS/8

The PiDP8 has one really neat feature; by setting the first left three white switches, you can control which of 8 boot scripts are used to start the SimH session running the PDP8/I emulation on the Pi.

Oscar provided 8 scripts, from 0.script to 7.script. According to the description file, 5.script is blank. It actually contains assembly instructions for ??? but I just saved that file as, then created a new 5.script which automatically boots the PiDP8 into MULTOS8 maintenance mode if the switches are set to octal 5 = 101.

Here’s my new 5.script:

set cpu 32k
set cpu noidle
att rk0 ../imagefiles/multos8/rsh.rk05
att rk1 ../imagefiles/multos8/multos8.rk05
boot rk1

If you created a running MULTOS/8 system based on my last blog post, you can name the RKA0: disk pack you create to anything you want. I chose rsh.rk05, but any name will do. The .rk05 extension is not mandatory, but appropriate to keep things straight.T800_PiDP8 completed 2016-05-26

MULTOS/8 – a timeshare operating system for the PDP/8

As stated previously in my blog, I’ve been having tons of fun playing with my PDP8/I replica, the PiDP8 designed by Oscar Vermeulen and sold as a kit. The engine for the replica is a Raspberry Pi (3, in my case) running Simh, an awesome ‘old iron’ simulator that  runs on many platforms.

I was intrigued by the idea of MULTOS/8 after reading an inquiry about it on Oscar’s PiDP8 forum. I did a bit of digging, and found that a few folks were interested, but none (apparently) had it running on Simh.

I was able to obtain a copy from the PDP repository at and began.

The disk image is multos8.rk05, indicating it’s a disk pack much like the standard OS8 operating system. After some playing, I have been able to get MULTOS/8 running successfully on my PiDP8.

The general procedure is to boot OS8, then enter SIMH and attach the multos disk image to RK1, while detaching the OS8 image. In place of the OS8 disk image on RK0, you attach a new file as disk image (mine was called rsh.rk05). Then you boot to MULTOS8 in a maintenance mode. In that mode, you format (ZERO) the blank disk image and copy the MULTOS files to the blank disk. Now you can boot MULTOS into timeshare mode and play.

One quick note on paths for SIMH on the PiDP8:  I put my image files in /opt/pidp8/imagefiles/rsh to keep them separate from Oscar’s default files. SIMH finds files relative to /opt/pidp8, hence the ‘../imagefiles/rsh/rsh.rk05’ path. You are free to put your files where you want as long as SIMH can see them.

Here are the detailed steps:

Begin with the normal OS8 boot.
<ctrl-e> (to get into simh)

*Command                                                     Notes & comments

det rk0                                                     Detach the OS8 boot disk from rk0
att rk1 ../imagefiles/rsh/multos8.rk05                      Attach the MULTOS8 boot disk to rk1
att rk0 ../imagefiles/rsh/rsh.rk05                          Attach rk0 to a new file ‘rsh.rk05’. This will be the timeshare boot drive.
boot rk1                                                    Boot MULTOS/8 in maintenance mode

*Now booted to MULTOS/8 maintanance mode                     Looks and acts just like OS8 standard boot)

DIR RKA1:                                                   Directory listing of MULTOS/8 disk, first partition
DIR RKB1:                                                   Directory listing of MULTOS/8 disk, second partition
DIR RKA0:                                                   Will show nothing (or error) as the disk has not been ‘created’ yet.
ZERO RKA0:/Y                                           This initializes the RKA0: disk (rsh.rk05). You will be asked to confirm. **
COPY RKA0:<RKB0:*.*                           Copy all the files on the MULTOS/8 primary disk to the new disk.
DIR RKA0:                                                   Directory listing of RKA0:, now the same as RKA1:

DATE 23-JUN-70                                        Only dates in the ’70s work. This is needed to boot MULTOS8.
R MULTOS                                                  It will ask for a time in hhmm format, then boot to timeshare mode.

<ctrl-h>                                                        Log in user 1. There is no password so just press enter to get the ‘.’ prompt.
DIR                                                                This will show RKA0: listing, which is now the boot/default drive for timeshare.
R SYSTAT                                                      A command to show system status.
R LOGOFF                                                    A command to log off the user.

With this I was able to play on terminal 1 (the console) as user 1. I don’t have serial terminals set up for my PiDP8,  so can’t check other terminals. That’s a future project.

Critical NOTE: The /Y option on the zero command is critical as it makes the new disk bootable. If you omit the /y flag, then MULTOS will simply stall after you try and log in.

Here’s a sample session:



PHONE 801-942-8000

(E.G. 0925 FOR 9:25 AM AND 1935 FOR 7:35 PM):





MULTOS-8   SYSTEM STATUS       1-JUN-1978    AT    12:27:10


JOB       ACTIVE        BUSY

SELF        1             0       TTY  I/O WAIT AT PC 6651

LPT         1             1






PiDP8 – still fun after all these days…

I’m still having a blast with my PiDP8, which is a PDP8/I replica that uses a Raspberry Pi (3 in my case) to provide PDP8/I emulation inside a very nice box with replica PDP8/I switches and blinking leds. The PiDP8 was created by Oscar Vermeulen in the  Netherlands and is available from him as a kit. If you are interested, his website is!pidp-8/cbie

I’ve attached a picture of my finished kit to this post as well.  I’m very happy with the build and with the overall unit. I only had one build problem – There’s a 40-pin header for the Raspberry Pi, and I always solder things in alternate pins/rows to spread the soldering heat evenly about the build. In this case, I managed totally miss one connection. Of course it was the main power connection between Pi and PiDP8, so when I finished, checked & double checked and finally connected it all up and applied power… nothing. After a careful re-scanning under a large magnifier knowing it had to be something really simple, I found the bare connection. Once soldered everything worked perfectly.

As I’ve said in other posts, I’ve been having a blast resurrecting my Engineering graduate studies FORTRAN programs and getting them to run on the PiDP8. I now have over 30 programs running, and all but a couple give results comparable to the same programs’s original output  and my modern 64-bit PC tests. In those cases, it’s due to single-precision floating point rounding errors that cannot be avoided. The originals were double precision, but the PDP8/I FORTRAN IV compiler does not support double precision. Still, I am satisfied with my results, and it was a lot of fun to do.

T800_PiDP8 completed 2016-05-26

PiDP8 (PDP8 replica) and FORTRAN: How suite it is :-)

Sorry for the pun, but I couldn’t resist. (the pun comes from program testing usually being done with a test framework and a “test suite”).

The past few days I have all of my smaller Engineering FORTRAN programs compiled and running on the PiDP8 – my PDP8 replica.  It was time to “work smarter” as they say. The basics of this involve replacing hand-typed commands with batch processes so that dozens of commands are stored in a file and invoked with one short command.

I began by reading all about the OS8 BATCH command and  batch files. I then created some smaller  test batch files to see how things worked, especially program input which usually requires keystrokes such as the following: “EXE RKB0:GAUSS.LD<esc>RKB0:GAUSS.DA/1<esc>” where <esc> means “press the escape key”. Batch files don’t typically allow control characters, so they rely on another mechanism. In the case of OS8, it’s the ‘$’ which tells BATCH that ‘the esc key was pressed’. In the end, the command line





This works perfectly.

With such things resolved, I created two large batch files – one that compiles and loads all of my programs to date, and another which runs the programs. I no longer have to keep load files around as I can recreate them with one command:


and likewise can run all but 3 programs with the command SUB RUNALL. It turns out one set of programs requires too much memory as the DIMENSION statements for some arrays consume 4000 bytes, just under the FORTRAN 4192 bytes allowed. However, this is enough to prevent the programs running under the BATCH system as it requires enough memory to prevent the programs from running (“INSUFFICIENT CORE”).

Still, pretty good stuff.


Playing with FORTRAN IV on the PiDP8

I’ve been spending a lot of my research time playing with my PIDP8. This is a replica of the PDP8 (slightly smaller form factor) complete with blinking lights and operational switches, but using a Raspberry Pi as the processing engine. The actual simulation of the PDP8 is handled by Simh, a  remarkable ‘ancient tech’ simulation package.

I’ve been resurrecting all my old Engineering grad school FORTRAN programs and getting them to compile and run on the PiDP8. It’s been a blast!

The PiDP8 runs OS8 as the nominal operating system, and the version of FORTRAN IV available is quite old and a bit quirky. The limits include no double precision, no lower case (at all!), strict adherence to column placement of code and strict 6 character limits on both file names (OS8) and variable names (FORTRAN IV). The quirks come when these limits are violated. Sometimes you get a compile error, sometimes a run-time error, but some times nothing except a hung program. As they say, “the lights are flashing, but no-one’s home”.

Some quick examples. Although there’s no double precision, the compiler accepts “DABS” and “DEXP” library function calls. No error messages anywhere, but the program will just hang waiting for the non-existent library. Using too-long variable names results in compiler error messages that have no apparent meaning until you ‘fine tooth’ the code and realize a variable name is longer than 6.

FORTRAN IV also does not allow free-format read & writes. “read(5,,end=9999,err=9999) x,y,z” is just full of not-allowed stuff. You need a proper format statement for every read & write.

In the end it took quite a while to get all of my programs running; around 35 in total. These include various matrix solvers, mass transfer and heat transfer programs, wellbore programs and flash calculations. As I said, much fun.

There were some differences in output between the original 1980’s runs (on a Honeywell MULTICS system), runs on a modern 64 bit PC running the GNU fortran compiler (f77) and the 12-bit PiDP8. However, with the exception of a couple of programs that I have not had time to scrutinize, all results were well within expected ranges given the differences in word size for single-precision calculations. The two cases that were substantially different were cases where the original program was double-precision on a 32-bit machine and convergence of the iterative solution was difficult, even in the original cases.