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timesh[w83,jmc]		Reminiscences on the history of time sharing

	I remember thinking about time-sharing about the time of
my first contact with computers and being surprised that this wasn't
the goal of IBM and all the other manufacturers and users of computers.
This might have been around 1955.

	By time-sharing, I meant an operating system that permits each
user of a computer to behave as though he were in sole control of a
computer, not necessarily identical with the machine on which the
operating system is running.  Christopher Strachey may well have been
correct in saying in his letter to Donald Knuth that the term was already
in use for time-sharing among programs written to run together.  This idea
had already been used in the SAGE system.  I don't know how this kind of
time-sharing was implemented in SAGE.  Did each program have to be sure to
return to an input polling program or were there interrupts?  Who invented
interrupts anyway?  I thought of them, but I don't believe I mentioned the
idea to anyone before I heard of them from other sources.

	My first attempts to do something about time-sharing was in the
Fall of 1957 when I came to the M.I.T. Computation Center on a Sloan
Foundation fellowship from Dartmouth College.  It was immediately clear to
me that the time-sharing the IBM 704 would require some kind of interrupt
system.  I was very shy of proposing hardware modifications, especially as
I didn't understand electronics well enough to read the logic diagrams.
Therefore, I proposed the minimal hardware modification I could think of.
This involved installing a relay so that the 704 could be put into
trapping mode by an external signal.  It was also proposed to connect the
sense switches on the ccnsole in parallel with relays that could be
operated by a Flexowriter (a kind of teletype based on an IBM typewriter).

	When the machine went into trapping mode,
an interrupt to a fixed location would occur the next time the
machine attempted to execute a jump instruction (then called a
transfer).  The interrupt would occur when the Flexowriter had
set up a character in a relay buffer.  The interrupt program would
then read the character from the sense switches into a buffer,
test whether the buffer was full, and if not return to the
interrupted program.  If the buffer was full, the program would
store the current program on the drum and read in a program to
deal with the buffer.

	It was agreed (I think I talked to Dean Arden only.) to install the
equipment, and I believe that permission was obtained from IBM
to modify the computer.  The connector to be installed in the
computer was obtained.

	However, at this time we heard about
the "real time package" for the IBM 704.  This RPQ (request
for price quotation was IBM jargon for a modification to
the computer whose price wasn't guaranteed),
which rented for $2,500 per month had been
developed at the request of Boeing for the purpose of allowing
the 704 to accept information from a wind tunnel.  Some element
of ordinary time-sharing would have been involved, but we did not seek
contact with Boeing.  Anyway it was agreed that the real time
package, which involved the possibility of interrupting after
any instruction, would be much better than merely putting
the machine in trapping mode.  Therefore we undertook to beg
IBM for the real time package.  IBM's initial reaction was
favorable, but nevertheless it took a long time to get the
real time package - perhaps a year, perhaps two.

	It was then agreed that someone, perhaps Arnold Siegel,
would design the hardware to connect one Flexowriter to the
computer, and later an installation with three would be designed.
Siegel designed and build the equipment, the operating system
was suitably modified (I don't remember by whom), and demonstration
of on-line LISP was held for a meeting of the M.I.T. Industrial
Affiliates.  This demonstration, which I planned and carried out,
had the audience in a fourth floor lecture room and me in the
computer room and a rented closed circuit TV system.  Steve
Russell, who worked for me, organized the practical details including
a rehearsal.  This demonstration was called time-stealing,
and was regarded as a mere prelude to proper time-sharing.
It involved a fixed program in the bottom of memory that collected
characters from the Flexowriter in a buffer while an ordinary batch
job was running.  It was only after each job was run that a job
that would deal with the characters typed in would be read in
from the drum.  This job would do what it could until more
input was wanted and would then let the operating system go
back to the batch stream.  This worked for the demonstration,
because at certain hours, the M.I.T. Computation Center operated
at certain hours a batch stream with a time limit of one minute on any job.

	Around the time of this demonstration, Herbert
Teager came to M.I.T. as an assistant professor of Electrical Engineering
 and expressed interest in the time-sharing
project.  Some of the ideas of time-sharing overlapped some ideas
he had had while on his previous job, but I don't remember what
they were.  Philip Morse, the Director of the Computation Center,
asked me if I was agreeable to turning over the time-sharing project
to Teager, since artificial intelligence was my main interest.
I agreed to this, and Teager undertook to design the three Flexowriter
system.  I'm not sure it was ever completed.
There was a proposal for support for time-sharing submitted to
NSF and money was obtained.  I don't remember whether this preceded
Teager, and I don't remember what part I had in preparing it or
whether he did it after he came.
This should be an important document, because it will contain that
year's conception of and rationale for time-sharing.

	Besides that, IBM was persuaded to make substantial modifications
to the IBM 7090 to be installed at the M.I.T. Computation Center.
These included memory protection and relocation and an additional
32,768 words of memory for the time-sharing system.  Teager was the
main specifier of these modifications.  I remember my surprise when
IBM agreed to his proposals.  I had supposed that relocation
and memory protection would greatly slow the addressing of the computer,
but this turned out not to be the case.

	Teager's plans for time-sharing were ambitious and, it
seemed to many of us, vague.  Therefore, Corbato undertook an
"interim" solution using some of the support that had been obtained
from NSF for time-sharing work.  This system was demonstrated some
time in 1962, but it wasn't put into regular operation.  That wasn't
really possible until ARPA support for Project MAC permitted
buying a separate IBM 7090.

	Around 1960 I began to consult at BBN on artificial intelligence
and explained my ideas about time-sharing to Ed Fredkin and
J. C. R. Licklider.  Fredkin, to my surprise, proposed that time-sharing
was feasible on the PDP-1 computer.  This was D.E.C.'s first computer,
and BBN had the prototype.  Fredkin designed the architecture of
an interrupt system and designed a control system for the drum to
permit it to be used in a very efficient swapping mode.  He convinced
Ben Gurley, the chief engineer for D.E.C. to build this equipment.
It was planned to ask NIH for support, because of potential medical
applications of time-sharing computers, but before the proposal could
even be written, Fredkin left BBN.  I took technical charge of the
project as a one-day-a-week consultant, and Sheldon Boilen was hired
to do the programming.  I redesigned the memory extension system
proposed by D.E.C. and persuaded them to build the modified system
instead of the two systems they were offering, but fortunately
hadn't built.  I also supervised Boilen.

	Shortly after this project was undertaken, D.E.C. decided
to give a PDP-1 to the M.I.T. Electrical Engineering Department.
Under the leadership of Jack Dennis, this computer was installed
in the same room as the TX-0 experimental transistorized computer
that had been retired from Lincoln Laboratory when TX-2 was built.
Dennis and his students undertook to make a time-sharing system
for it.  The equipment was similar, but they were given less memory
than the BBN project had.  There wasn't much collaboration.

	My recollection is that the BBN project was finished first
in the summer of 1962, but perhaps Corbato remembers earlier
demonstrations of CTSS.  I left for Stanford in the Fall of 1962,
and I hadn't seen CTSS, and I believe I hadn't seen Dennis's system
operate either.  BBN didn't operate the first system and didn't
even fix the bugs.  They had few computer users and were content
to continue the system whereby users signed up for the whole
computer.  They did undertake a much larger follow-on project
involving a time-shared PDP-1 that was installed in Massachusetts
General Hospital, where it was not a success.  The computer was
inadequate, there were hardware and software bugs, and
there was a lack of application programs, but mainly the project
was premature.

	At the same time that CTSS, the BBN system, and the EE Department
systems were being developed, M.I.T. had started to plan for
a next generation computer system.  The management of M.I.T. evidently
started this as an ordinary university planning exercise and
appointed a high level committee consisting of Philip Morse, Albert
Hill and Robert Fano to supervise the effort.  However, the actual
computer scientists were persuaded that a revolution in the
way computers were used - to time-sharing - was called for.
The lower level committee was chaired by Teager, but after his
ideas clashed with those of everyone else, the committee was
reconstituted with me as chairman.  The disagreement centered around
how ambitious to be and whether to go for an interim solution.
Teager wanted to be very ambitious, but the rest of us thought
his ideas were vague, and he wanted M.I.T. to acquire an IBM 7030
(Stretch) computer as an interim solution.  As it turned out, acquiring a
Stretch would have been a good idea.

	Our second report to M.I.T. proposed that M.I.T. send out
a request for proposals to computer manufacturers.  On the basis
of the responses, we would then ask the Government for the money.
The RFP was written, but M.I.T. stalled perhaps for two reasons.
The first reason was that our initial cost estimates were very
large for reasons of conservatism.  Secondly, IBM asked M.I.T. to
wait saying that they would make a proposal to meet M.I.T.'s needs
at little or no cost.  Unfortunately, the 360 design took longer
than IBM management expected, and along about that time, relations
between M.I.T. and IBM became very strained because of the patent
lawsuit about the invention of magnetic core memory.

	As part of the stall, President Stratton proposed a new
study with a more thorough market survey to establish the demand
for time-sharing among M.I.T. computer users.  I regarded this
as analogous to trying to establish the need for steam shovels
by market surveys among ditch diggers and didn't want to do it.
About this time George Forsythe invited me to come back to
Stanford with the intention of building a Computer Science
Department, and I was happy to return to California.

	In all this, there wasn't much publication.  I wrote a
memo to Morse dated January 1, 1959 proposing that we time-share
our expected "transistorized IBM 709".  It has been suggested
that the date was in error and should have been 1960.  I don't
remember now, but I believe that if the memo had been written
at the end of 1959, it would have referred to the 7090, because
that name was by then current.  In that memo I said the idea of
time-sharing wasn't especially new.  I don't know why I said that,
except that I didn't want to bother to distinguish it from what
was done in the SAGE system with which I wasn't very familiar.

	Most of my argumentation for time-sharing was oral, and when
I complained about Fano and Corbato crediting Strachey with time-sharing
in their 1966 Scientific American article, Corbato was surprised to
find my 1959 memo in the files.  Their correction in Scientific
American was incorrect, because they supposed that Strachey and I
had developed the idea independently, whereas giving each user
continuous access to the machine wasn't Strachey's idea at all.
In fact, he didn't even like the idea when he heard about it.

	Teager and I prepared a joint abstract for an ACM meeting shortly
after he arrived, and I gave a lecture in an M.I.T series called
Management and the Computer of the Future.  In this lecture I referred to
Strachey's paper "Time-sharing of large fast computers" given at the 1959
IFIP Congress in Paris.  I had read the paper carelessly, and supposed he
meant the same thing as I did.  As he subsequently pointed out, he meant
something quite different that did not involve a large number of users,
each behaving as though he had a machine to himself.  As I recall, he
mainly referred to fixed programs, some of which were compute bound and
some input-output bound.  He did mention debugging as one of the
time-shared activities, but I believe his concept involved one person
debugging while the other jobs were of the conventional sort.

	My 1959 memo advertised that users generally would get the
advantage of on-line debugging.  However, it said nothing about how
many terminals would be required and where they would be located.
I believe I imagined them to be numerous and in the users' offices, but 
I cannot be sure.  Referring to an "exchange" suggests that I had
in mind many terminals.  I cannot now imagine what the effect was on
the reader of my failure to be explicit about this point.  I'm afraid
I was trying to minimize the difficulty of the project.

	The major technical error of my 1959 ideas was an underestimation
of the computer capacity required for time-sharing.  I still don't
understand where all the computer time goes in time-sharing installations,
and neither does anyone else.

	Besides M.I.T.'s NSF proposal, there ought to be some letters
to IBM and perhaps some IBM internal documents about the proposal,
since they put more than a million dollars worth of equipment into
it.  Gordon Bell discusses D.E.C.'s taking up time-sharing in Bell
and Newell book, but I don't recall that they discuss Ben Gurley's
role.  Fredkin and perhaps Alan Kotok would know about that.

	After I came to Stanford, I organized another PDP-1 time-sharing
project.  This was the first time-sharing system based on display
terminals.  It was used until 1969 or 1970 for Suppes's work on
computer aided instruction.