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PDP-8 Frequently Asked Questions (posted every other month)
Section - What operating systems were written for the PDP-8?

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A punched paper-tape library of stand-alone programs was commonly used
with the smallest (diskless and tapeless) configurations from the
beginning up through the mid 1970's.  This included a paper-tape based
text editor, the PAL III and MACRO-8 assembler, and a FORTRAN compiler,
as well as a library of support routines.  Many paper tapes from this
library survive to the present at various sites!  The minimum
configuration expected by these tapes is a CPU with 4K memory and a
teletype ASR 33 with paper tape reader and punch.  Note that much of this
paper-tape-based software is based on memory-use and I/O conventions that
are incompatible with later disk-based systems.

The DECtape Library System was a DECtape oriented save and restore system
that was available from the start.  The resident portion of this system
occupies only 17 words of memory (7600-7625 octal), and it allowed saving
and restoring absolute core images as named files on a reel of DECtape.
Initially, program development was still done with paper tape, and only
executable memory images were stored on DECtape, but eventually, a limited
DECtape-based text editor was introduced, along with a DECtape based

The 4K Disk Monitor System provided slightly better facilities.  This
supported on-line program development and it worked with any device that
supported 129 word blocks (DECtape, the DF32 disk, or the RF08 disk).
It was quite slow, but it also used very little of the available memory.

MS/8 or the RL Monitor System, was developed starting in 1966 by
Richard F. Lary; it was submitted to DECUS in 1970.  This was a disk
oriented system, faster than the above, with tricks to make it run
quickly on DECtape based systems.

POLY BASIC was a BASIC only system submitted to DECUS and later sold by
DEC as part of its EduSystem marketing program.  EduSystem 25 Basic
is available from:

P?S/8 was developed starting in 1971 from an MS/8 foundation.  It runs
on minimal PDP-8 configurations, supports somewhat device independant
I/O and requires a random-access device for the file system (DECtape is
random-access!).  P?S/8 runs compatibly on most PDP-8 machines including
DECmates, excepting only the PDP-8/S and PDP-5.  P?S/8 is still being

Richard F. Lary developed a system called the Fully Upward Compatible
Keyboard Monitor; and between a Wednesday and the following Friday, a
prototype was up and running from DECtape.  The original intention of
this project was to build a programming environment for the PDP-8 that
looked like TOPS-10 on the PDP-10.  A year later, this was released as
Programming System/8 (or PS/8), and then renamed OS/8 in 1971 because
Eli Glaser (a salesman from Long Island) said he could sell more systems
with an operating system than with a programming system, and because, by
renaming the system, DEC could increase the price despite Nixon's
wage-price freeze.

OS/8, developed in parallel with P?S/8, became the main PDP-8 programming
environment sold by DEC.  The minimum configuration required was 8K words
and a random-access device to hold the system.  For some devices, OS/8
requires 12K.  There are a large number of OS/8 versions that are not
quite portable across various subsets of the PDP-8 family.  RX01 images of
OS/8 Version 3Q are available, with DEC's free non-commercial use licence,

The second site above also includes an incomplete but useful RK05 image
of OS/8 Version 3R.  Parts of the OS/8 source can be found in:

OS/8 V3D was renamed OS/78 (to match the VT78), and in followups to this
distribution, support for Omnibus machines was no longer important.  OS/78
V4 was developed for the DECmate I, and the name OS/278 used for the
versions released with later DECmate machines.  These have unnecessary
incompatabilities with earlier versions of OS/8.  OS/278 and related
material is available from DECUS as catalog item 800941, or from:

A growing collection of OS/8 documentation, including the OS/8 software
support manual on the internals of the system is available on line from:

OS8 (no slash) may still be viable.  It requires 8K of main memory, an
extended arithmetic unit, and DECtape hardware.  Unlike most PDP-8
operating systems, it uses a directory structure on DECtape compatible
with that used on the PDP-10.

The timesharing system TSS-8 was developed by Don Witcraft and John Everett
at DEC, starting in late 1967, and with the first beta sites up and running
in the fall of 1968.  This was based on a protection architecture proposed by
Adrian Van Der Goor, a grad student of Gordon Bell's at Carnegie-Mellon.
It requires a minimum of 12K words of memory and a swapping device; on a
24K word machine, it could give good support for 17 users.  It was
the standard operating system on the EduSystem 50 which was sold to many
small colleges and large public school systems.  The first installation was
at Lexington High School in Massachusetts, and the second was at Northern
Arizona University.  Each user gets a virtual 4K PDP-8; many of the
utilities users ran on these virtual machines were only slightly modified
versions of utilities from the Disk Monitor System or paper-tape
environments.  Internally, TSS8 consists of RMON, the resident monitor, DMON,
the disk monitor (file system), and KMON, the keyboard monitor (command shell).
BASIC was well supported, while restricted (4K) versions of FORTRAN D and
Algol were available.

Significant parts of TSS-8 have been found, but at this time, nobody has
managed to recover a complete working system.  Much of the available TSS/8
code can be found at:

Jim Dempsey, an alum of the OS/8 group at DEC, developed ETOS for Educomp
(later Quodata) for the PDP-8/E; this was a true virtual machine operating
system in the spirit of IBM's VM/370, and a special board was required
to optionally trap JMP and JMS instructions; this was enabled after an
emulated CIF instruction so that the actual change of instruction field
could be emulated when the JMP or JMS was attempted.  After leaving Quodata
and founding Network-Systems Design in 1976, Dempsey went on to develop
OMNI-8, first installed at Ripon College; initially it was priced at $4900,
several hundred copies were sold.  The OMNI-8 operating system supported
the enlarged PDP-8 address space of the CESI (Computer Extension Systems
Inc) memory cards, and when CESI began making PDP-8 clones, OMNI-8 was
extended to support asymmetric multiprocessors (one CPU handled the I/O).
The end of OMNI-8 development came around 1990.  Dumps of the ETOS kernel
and drivers survive in various places, and Jim Dempsey still has the full
source for OMNI-8.

Other timesharing systems developed for the PDP-8 include MULTI-8
and MULTOS.  The source for MULTOS is available from:

CAPS-8 was a cassette based operating system supporting PAL and BASIC.
There are OS/8 utilities to manipulate CAPS-8 cassettes, and the file
format on cassette is compatible with a PDP-11 based system called

RTS/8 was a real-time system developed by DEC, developed from an earlier
system, SRT8, dating back to at least 1974.  Curiously, even the last
versions of RTS/8 continued to support paper-tape and DECtape.  RTS/8 also
offered a virtual PDP-8 for background processing, unlike ETOS, this did
not require special hardware; instead, software emulation was used to retain
control of the machine between the CIF instruction and a following
JMP or JMS.  Source code for most of the versions of RTS and SRT is
available from:

WPS was DEC's word processing system, developed for the 8/E with a VT50
terminal with special WPS keycaps replacing the standard keycaps, and
widely used on the 1980's vintage machines.  It was heavily promoted on
the VT-78, and when the DECmates came out, DEC began to suppress knowledge
that DECmates could run anything else.  WPS-11 was a curious distributed
system using a PDP-11 as a file server for a cluster of VT-78 WPS systems.
DECmate/WPS Version 2.3 is available from DECUS for the DECmate II and
DECmate III under the catalog entry DM0114.

COS-310, DEC's commercial operating system for the PDP-8, supported the
DIBOL language.  COS-310 was a derivative of MS/8 and OS/8, but with a
new text file format.  The file system is almost the same as OS/8, but
dates are recorded differently, and a few applications can even run under
both COS and OS/8.  COS was the last operating system other than WPS
promoted by DEC for the DECmates.

SCPSYS, developed by D. C. Amoss prior to 1974 at Clemson University, is
a system that copies most of the features of LAP (the LINC Assembly
Program) for a pure PDP-8 based system.  A DECtape of this system has
recently come to light, with one known application, Spacewar.

AMOS, an operating system for the PDP-8/E with TD8E DECtape interface,
was a very small system developed in Australia or New Zealand and supporting
assembly and text editing on a 4K machine.

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