From: (Lars Persson)
Newsgroups: alt.folklore.computers
Subject: Me and my PDP... (long)
Message-ID: <>
Date: 2 Dec 92 02:02:41 GMT
Organization: Chalmers University of Technology
Lines: 234

Way back in the old days when terminal bells went BZWT and computers
were REAL computers and not some pizza-box like thingy with an oversize
TV-set on top, I became system manager for a PDP11/34a, real macho 19"
full size black coloured mini computer. Now, children, mini means that
you can not move it because it is housed in a full size 19" rack that
weighs more than your mortageplan for this and the following years.
Here are some interesting facts about the PDP11. 
The Programmable Data Processor MARK 11 is a general purpose 16 bit
overlay machine with a wast array of operating systems. It came 1970 and
the architecture is still being made today. Zillions of hackers and
system managers out there were introduced to the wonders of computers
and took the first step on a long and twisty path with occasional
beautiful scenery of late night hours. The PDP11 is very suitable for
Real Time applications. It is sometimes refered to as the fastest
parallel port in the west.. DEC (the creator of the PDP11 and of some
other less significant gadgets as the VAX11, LMF and the BI corner) 
has tried to kill this little critter but the critter is virtually 
indestructable. There are many flavours of PDP11-erism. Some has gone to
the big swapper in the sky but other trudges on. The really old ones had
blinkenlights and tastendrukks on the front panel. Later on there was
rubber buttons and BSD (Big Shining Digits) and eventually just a few
toggle switches. The operating systems became more complex as the
buttons grew fewer (the few-switch-more-fuzz syndrom). Yes, I said
operating systems. Here a few:
RT11 - Real Time 11. A single user, multi programming, multi terminal, 
multi user application operatins system. 
Awsome, huh? Single user - multi terminal..
You can start up the system on one terminal, hack along and be happy and
then TRANSFER your session to ANOTHER terminal and continue there. The
initial terminal then becomes inactive. Multi programming means that you
can have more than one job going at the same time. Like forground and 
How about single user, multi user APPLICATION? Well.. you can write neat
and shimmering little programs that ACCESSES other devices thru and
application. This means that even though the operating system ITSELF
only converses with one terminal, the thrifty little PROGRAM you did in
Fortran 4 speaks with the other three (or what you have on your system).
This is neat and complicated... But fast...

RSX-family. Common. This is the four wheel drive operating system. It
does it all. Multi user, multi terminal, multi programming, queues, real time
pretty small and has queues. The operating system is totally flat. The
directories lies beside eachother and there is only one level. You log
into your own little bag where you keep all your file. Only wimps need a
structure on files. A real hacker know what he got and can find it. Naw
Cute feature: You say hello when you log in. Like hello [1,54] means I
wanna log in as the system administrator. Aboard you can choose from two
CLI (Command Language Interpretors). The DCL (Digital Command Language,
a CLI that exists on most of DEC's operating systems save Ultrix) or MCR
(Monitor Console Routine). You REALLY run MCR but when you have choosen
DCL. The DCL is INTERPRETED into MCR. DCL is therefore a CLII (Command
Language Interpretor-Interpretor). There are some incomprehensible
things on the RSX11. One is PIP...
Peripheral Interchange (or Intercommunication) Program is what you use.
You might not understand it first but after a while it becomes a nice
tool to confuse novices, baboons (another world for YOUR stupid and
ignorant superiors) and sometimes even yourself.
For a starter: You use PIP to transfer data from point A to point B.
Here is a copy command:

Now you are not lucky enough to have copied myfile into yourfile. Oh no.
That would be to easy. You have done the reverse. There is a reverse
syntax on PIP compared with what you are used to.

Here is another feature: PIP TI:=DK1:[7,77]YOURFILE.TXT;3 
This one types yourfile on your screen.

Now lookee here: PIP DK1:[7,77]YOURFILE.TXT;3=TI:
Guess what children.. You have just begun to copy text from your
terminal into yourfile... Neat huh?

PIP is a massively useful program. PIP /FR tells how much space you have
left on your disk and PIP *.*;*/RM erase all you got. No questions

RSX is as old as the hills. The family is wide and confusing. SOME of 
the members are: 

RSX11/M. Needs 18 bit wide adress range on the bus. CAN be made to run on a 
more narrow bus, but poorly.

RSX11/S. Single user, memory based operating system. Used for stand
alone measure-control-no-nonsens-factory-hard-hat computers. Runs
nicely on 64K core or less. Loads mainly thru downline loading from a
load host. Yes you can build stand alone Space Invaders.

RSX11/M+ Needs 22 bit wide adress range on the bus. It is RSX11/M with
extra options like named directories, batch and some other niceties.
Real hackers consider this a sissy operating system since things are
less obscure and complicated. Real users gets the shivers from this babe
too though.

uRSX. Horrible and ghastly! You can not run MCR but only DCL. A life
without PIP is a life in misery. This is really a scaled down RSX11M+
where the sissy parts are all there but the macho stuff is gone. 

P/OS. This is evil in concentration. It is a sales person approach to
operating systems made to run on a special type of PDP11 namely the
DEC/PRO. Without the so called TOOLKIT you can not reach the operating
system but is doomed to mess around in a bundle of menues to get your
jobs run. The PRO is in itself a very interesting machine. It is perhaps
the first true workstation. You have a PC like box and a bit map
graphics display. Inside is a small PDP11 (the 23 or 73 depending on
model of the PRO). This is a single user multi user operating system
with a semi flat directory structure. My is this evil!
The P/OS is a modified uRSX. You log in thru a menue, you run your
space ivaders thru a menue and you get a stroke thru a menue. The menues
are controlled by a series of function buttons above the numeric row.
You need a special stripe to know which one of the buttons are what.
The buttons supply niceties like DO, RESUME, NEXT, INSERT and other
things we all can live without. However, there is a toolkit... This is
basically DCL. With it you can reach your operating system and even
hook up a second terminal on the printer port (undocumented feature) and
have time sharing.
With clever assignments and logical names, the operating system APPEARS
to have a tree structure but realy does not. You will be amazed how lost
you can get on a 10 meg winchester!
As I believe the P/OS is released as public domain and aviable thru
DEC's user club DECUS for a price that is tagged so that you barely

There are more RSX11s around but we leave it here.
Another colourful operating systems is the RSTS-family.
The most used one is the RSTS/E and the uRSTS/E.
Here you can choose from three modes when dealing with the OS. You can
do it in RT11 mode, RSX-mode or BASIC. Yes, children, BASIC.
The other two modes are rather uncomplicated and the orientation of
RT11 and RSX are dealt with above. BASIC-mode is another thing. You
actually do all your fiddling with files, queues, fellow users and space
invader runnings thru a BASIC interpretor.
In some areas BASIC is pretty much like swearing at church. You can get
away with it but it is sociable not very acceptable. For those people I
do not recomend that they log in on an RSTS/E system in basic mode. The
trauma that would cause could be pretty devestating.

Other OS-es are IAS, MUMPS, CTS, CAPS11, TAP11 and various types of
UNIXes like BSD2.11, VENIX and so on and so forth. Shortly, you have
freedom of choice with your pet PDP11.

How about hardware then? Well.. There is some hardware about...
BUT there are also some errors to be made...
Basically what you have are two major types of buses and three adress
widths. Some stuff is interchangeable within the bus-types and some is not.

Here the two major bus types. UNIBUS and QBUS. The UNIBUS is a
replacement of the OMNIBUS (!) from the forerunner the PDP8. The UNIBUS
is a very usefull bus with lots of niceties that goes with it. The
oldest machines used UNIBUS. A UNIBUS is six slots wide and a UNIBUS
backplane is always wired. There are two types of slots. The UD and the
MUD. A card that has DMA needs a MUD. You can easily transform an UD to
a MUD by cutting the right strap on the wired backplane. *SHIVER*. To
retransfrom it, you wire back the strap. If a wrong type of card is
placed in an UD or a MUD, the PDP will croak at once on boot and report
BUS ERROR. This is not a fatal condition and just MUDify your UD or
DEMUD your MUD and you will be back in business.

Now UNIBUSes are big boxes that use a lot of power. They are expensive
to make and bulky. DEC took the UNIBUS under reconstruction and came out
with a DUAL to QUAD bus called the Qbus. The QBUS is more simple in its
construction but there are some DEAP DEAP pits to fall into.
As a starter the QBUS was made for PDP with 16 bit adress range. That
means that they can in theory adress 64 K of mem. The first only
swallowed around 56 or so. They run RT11 and RSX11/S. Somebody at DEC
said: Hey! Let us open up the range! First came 18 bit and then 22 bit.
This ment that true multi user systems could be made as QBUS machines.
Pretty dang good. The draw back was that the 22 bit and the 18 bit
machines are not TOTALLY compatible. Some 18 bit cards will go FUMP when
stuck into a 22 bit machine. Not ALL though. Just some... Nice feeling
there... I wonder if this expensive and custom built card will burn down
when put into a 22 bit machine... *FUMP* Yes it did....

The Qbus seeked its identity for a long time. There were all kinds of
Qbuses out there. Dual Q-buses, Quad Q-buses, dual Q-buses in quad boxes
with only powersupply and ground on the two right slots and so on.
The most awsome quirk I ever saw was a pirate PDP11 made by DATARAM. It
had a row of 22 bit duals to the left and a ZIGZAG 18 bit quad assembly
to the right. Between the two bus systems was a *MEMORY PATH MODULE*
that shoved the adresses on the 18 bit side up to highbyte on the left
side. Awsome! To bad the little bugger had burned to cinder when I got
my hands on it... Eventually DEC came out with the modern QBUS. This
babe has duals to the left and power and such to the right for a while
down the box. Then the guy went on vacation and Georgie to the right of
him took over and produced quad zigzags for the rest of the box. The
first cabinet was called BA23 and all so called MicroPDP systems are
housed in one or in the bigger brother the BA123. 
The great thing with both Q-bus and UNIBUS was that they do not give a
hoot on how many bits the processor processes. This means that in theory
devices are interchangeable between PDP11 systems (16 bit overlay
machine) and VAX11 systems (32 bit virtual machine). 

You can easily see if you have a Q-bus or a UNIBUS in your
documentation. Here some hints:'

PDP11 ending on 0 or 5 are old UNIBUS machines. Example: PDP11/45,
PDP11/60. Usually PDP11 ending on 05 are OEM machines and identical to
the next higher decimal. PDP11/05 and PDP11/10 (not to be confused with
the PDP10 that is a TOTALLY other architecture that sometimes HAS PDP11/10 as
communication computers). This would be to easy though. So the PDP11/55
and the PDP11/60 are totally different.

If the PDP11 ends on a 4 it is a newer UNIBUS machine. Here is a gallery
of PDP11/x4-s.
PDP11/34a 18 bit adress range. No separate I/D space.
PDP11/24 22 bit adress range. No separate I/D space.
PDP11/44 22 bit adress range. Separate I/D space. You can run UNIX on
this one and the rest of the UNIBUS list.
PDP11/84 Ditto but faster.
PDP11/94 This is the latest. Order from DEC thru YOUR fav'rite sales rep.

The PDP11/x3 is a Qbus machine. Here are some:

PDP11/03 First Q-bus. Came in -73. 16 bit adress range. SOOOO SLOOOOOW.
PDP11/23 18 bit. Compatible with the 34.
PDP11/23 Rev C. 22 bit. Pretty compatible with the 24.
PDP11/23+ 22 bit. Compatible with the 24. Two serial lines on the CPU card!
PDP11/73 22 bit. Separate I/D space. Unixable! Faster than the 23.
PDP11/53 Pretty simular in function to the 73. Has another CPU card and
is slower. Unixable as the rest of the list.
PDP11/83. Same card as the 73 but faster clock and, hardware floating
point as standard and a different prom that identifies it as an 83.
PDP11/93. The Q-bus 94. Fast and expensive.

Well, friends. This has been a little orientation in PDP11 trivia. There
is still much to be said like a comprehensive overview of Peripherals of
the Past and My favorite SLUs, How To Strap A DL11 without consuming a
sixpack of beer first or MacroAssembler, the COMPLETE guid in just a
double sided leaflet saying PDP11 in big friendly letters on the top of
the first page. These tasks has to be preformed by somebody else. It is
cofee time over here now and I am leaving you.
Happy hacking!
/L. Persson, Harlosa PD computer center, Sweden.