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

[Bakul Shah via TUHS]

You're perhaps thinking of Hoare's '74 paper:
https://www.engineering.upenn.edu/~lee/07cis505/Lec/74hoa.pdf
Monitors: An Operating System Structuring Concept
From it:

This paper develops Brinch-Hansen's concept of a monitor as a
method of structuring an operating system.  It introduces a
form of synchronization, describes a possible method of
implementation in terms of semaphores and gives a suitable
proof rule.
...
Thus the structure of a class of monitors is identical to
that described for a data representation in [13], except for
addition of the basic word monitor. Brinch-Hansen uses the
word shared for the same purpose [3].

where [3] refers to Brinch-Hansen's "Operating System Principles"
1973 book. One can think of Unix on a uni-core machine essentially
as a monitor in this sense. AFAIK this use of "monitor" came later.

> On Aug 15, 2026, at 3:04 PM, Luther Johnson via TUHS <tuhs at tuhs.org> wrote:
> 
> I think I read a book by C. A. R. Hoare in the mid-70's or early 80's that used the term "monitor" as something like a monolithic kernel, where, once a thread entered it, it was not re-scheduled and nothing could happen behind its back, the monitor was a critical region, and only one thread could be in at a time - therefore whatever it did inside was guaranteed to finish, all the data it managed was left in a coherent state on exit, and so, on the next entry by any other thread. Similar but a little different than the idea of an ground-level interactive program, I have seen the term used in both ways, since.
> 
> On 08/15/2026 02:36 PM, Clem Cole via TUHS wrote:
>> 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
>