[TUHS] Lions laboratory and a short excursion into DOS-11

[Clem Cole via TUHS]

below

On Sun, Aug 9, 2026 at 6:11 AM Lars Brinkhoff via TUHS <tuhs at tuhs.org>
wrote:

> Aron Insinga wrote:
> > Calling the operating system the Monitor is, I think, from the
> > PDP-6/PDP-10 Monitor (later renamed TOPS-10)
>
> There's also BBN's TENEX monitor (ObTuhs: tcsh is the "TENEX csh"), of
> which there there is a resident part and a swappable (demand paged) part.
>
> I wager there were many other operating systems - also outside DEC -
> around that time using monitor in a similar sense.
>

Lots of systems used that term.  As John pointed out, the IBM 709x IBSYS
manuals describe the System Monitor; IBJOB has the Program Monitor, *etc*.
Aron and Lars point out that TOPS, Tenex, and WAITS all use that term,
which I suspect they inherited from the earlier PDP-6 Monitor.   IIRC, even
the ITS folks called it a Monitor.  But I think that was a historical
artifact, and by the time many of us started working as OS developers in
the early/mid 1970s, the world had moved on; although some, like Doug, who
lived the transitions, probably have a better understanding.

As I understand the history, the term "Monitor" stems from the idea of a
program that "monitors" or supervises user execution. In early systems like
DEC's RT-11, you can see the explicit breakdown: ISTR, the user-interface
command-line component, was called the Keyboard Monitor (KMON), while the
resident (in-core) portion that remained in memory to route I/O interrupts
was called the RMON (Resident Monitor). A key point is that this was for
OS-like systems before MMUs were on all systems and timesharing, or at
least multitasking, was considered "de rigueur."   By the mid-to-late
1970s, as architectural theories had all matured to require protected
memory spaces and a greater level of hardware abstraction (which was
obviously accelerated by the realization of security models that
timesharing required), such that the term "Kernel" emerged to describe the
innermost, privileged core of software stripped of its outer user-facing view
of the system (which also begets the term "shells").

I also suspect a piece of this was that, in early systems, the command
interpreter was built into the core "executive."   But that started to
change after Dijkstra presented at the inaugural ACM Symposium on Operating
Systems Principles and published it in 1968 in the *CACM *(Volume 11, Issue
5, pages 341–346), his paper "The Structure of the 'THE'-Multiprogramming
system" described the system being built in layers and in particular the
"console layer" as being separate from the "processor allocation" and
"memory segment control."  By the mid-to-late 1970s, except on the smallest
microprocessors (*i.e.* CP/M and later MS DOS), some amount of Dijkstra's
layering and modularity in the OS was pretty much standard; thus the term
"monitor" fell out of use.

Of course, as OS developers, many of us later took Dijkstra's idea further
and developed the concept of a microkernel and a fully layered OS (which I
still think are better than "monolithic ones", even if they don't eke out
the absolute maximum from the hardware because of their overhead).

Clem