CTSS was one of the earliest time-sharing operating systems, developed on the IBM 7094 computer at MIT in 1961. It allowed multiple users to simultaneously use the system via teletype terminals. Key features included virtual memory management, protection of user memory spaces, and a preemptive multilevel scheduling algorithm. It also introduced several pioneering applications like the first email system and text editors. The success of CTSS demonstrated the viability of time-sharing as a paradigm for computer usage.

1. CTSS
Compatible Time Sharing System
Sebastian Woinar

2. IBM 7094 at MIT

3. Area of a triangle

4. Area of a triangle
1 0 0 0 0 9 9 9 9 9 0 0 0 0 0

5. X28-7327-5
®
FORTRAN Coding Form Printed in U.S.A.
PROGRAM GRAPHIC PAGE OF
PUNCHING
INSTRUCTIONS CARD ELECTRO NUMBER*
PROGRAMMER DATE PUNCH
CONT.
STATEMENT IDENTIFICATION
NUMBER
FORTRAN STATEMENT SEQUENCE
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* A standard card form, IBM electro 888157, is available for punching statements from this form

6. ®
FORTRAN C
PROGRAM
PROGRAMMER DATE
CONT.
STATEMENT
NUMBER
FORTRAN ST
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

7. Key Punch Room

9. Punched Card

10. Punched Z(1) = Y + W(1)
Card

12. Binary formated

15. Vision
John McCarthy

16. Fernando José Corbató
Fernando José Corbató

17. IBM 709

18. IBM 709T/7090

19. Operating System
• FMS - FORTRAN Monitor System
• compile FORTRAN programs
• IBSYS
• compile FORTRAN, COBOL etc.

20. IBM 7094 configuration

21. IBM 7094 configuration
Basic Cycle Time: 2.18 µSecs
Float. Pt Add Time: 4.36µSecs

22. IBM 7094 configuration

23. IBM 7094 configuration

26. IBM 7094 configuration

27. IBM 7094 configuration

28. IBM 7094 configuration

29. Console
IBM 709

31. Max Mathews
John Kelly

32. Max Mathews
John Kelly

33. CTSS

34. c

35. Hardware Problems
• Different user programs (and supervisor) may interfere
with each other
• Input-output equipement may be initiated by a user and
read words in on another user program
• Time sharing supervisor must be able to interrupt user
program after a quantum of computation
• Large core memories (e.g.. a million words) would ease
the system programming complications immensely
since the different active user programs, systems
programs could remain in core memory at all times.

36. Hardware Problems
• Different user programs (and supervisor) may interfere
Memory Protection
with each other
• Input-output equipement may be initiated by a user and
Trap all input-output
read words in on another user program instructions
• Time sharing supervisor must be able to interrupt user
Time controlled
program after a quantum of computation interrupt
• Large core memories (e.g.. a million words) would ease
the system programming complications immensely
8 MB RAM ;-)
since the different active user programs, systems
programs could remain in core memory at all times.

37. Programming Problems
• Supervisor should charge user usage
accounting automatically
• Supervisor should coordinate all user
input-output
• Good system programs tooling

38. Usage Problems
• Too large a computation or excessive
typewriter output may be requested
• Since real time is not usage-time,
supervisor must inform user
• Computer malfuntions must be expected

39. 7094 Modifications
• Standard 60 cycle accounting and interrupt
clock
• Memory boundary and relocation register
• 2 32,768 Core memory bank for the
nd
supervisor
• 7094 has become a two-mode machine

40. Supervisor
• Provided virtual machines
• 3 foreground machines which run 7094
instructions and could invoke supervisor
services
• 1 background machine with tape access
• Scheduling
• Managed I/O

41. Memory protection
• 32k memory is managed in 128 blocks of 256
words
• 3 7 Bit registers
5 7 5
base end relo
• Protection check
base [ 5* 256 ]<= addr <= end [ 7 * 256 ]

42. Instruction protection
• User mode: subset of 7094 instructions
• Memory access only to defined region by
memory relocation and protection
• Certain instructions are forbidden and invoke
protection trap

43. Instruction protection
Privileged
instructions
• all
I/O
instructions
(RDS,
WRS,
BSR,
BSF,
User mode: subset of 7094 instructions
• Memory access only
SDN,
RUN,
REW,
etc)
to defined region by
memory relocation and protection
all
channel
instructions
(RCHx,
LCHx,
SCHx,
• Certain instructions are forbidden and invoke
etc)
protection trap
all
I/O
transfer
instructions
(TEFx,
TRCx,
TCOx,
TCNx)
plus
and
minus
sense
(+0760...
and
-­‐0760...)

44. Scheduling algorithm
• Goal:
• optimize turnaround time
• minimize response time

45. Multi Level Feedback Queue
• Task is put into level l0
dependent on its actual size (calculation time)
• Execution time in level x is 2x quanta
• If task is not completed, it is put at the end of
level x+1 queue
• MLFQ is preemptive

46. Simulator

47. Simulator

48. Simulator

49. Simulator

50. Simulator

51. Simulator

52. Simulator

53. Simulator

54. TYPSET & RUNOFF
• TYPSET for creating and editing files
• High speed input mode
• Edit mode
• RUNOFF
• Control words in the text
• Line-length, indention, alignment

55. MAIL
• Described in Programming Staff Note 39
• Created file MAIL BOX in directory
• User has been informed about new mail

56. MAIL
• Described in Programming Staff Note 39
• Created file MAIL BOX in directory
• User has been informed about new mail

57. MAIL
• Described in Programming Staff Note 39
• Created file MAIL BOX in directory
• User has been informed about new mail

58. MAIL
• Described in Programming Staff Note 39
• Created file MAIL BOX in directory
• User has been informed about new mail

59. RUNCOM
• File-system file of commands to be
executed in a batch
• Commands are usable like library
subroutine

60. MAC Project

61. Quellen
• REMINISCENCES ON THE HISTORY OF TIME SHARING
• Fernando J. Corbato, Marjorie Merwin Daggett, Robert C. Daley AN EXPERIMENTAL
TIME-SHARING SYSTEM, Proceedings of Spring Joint Computer Conference, May 3, 1962
• R.M. Fano, The MAC system: a progress report, MAC-TR-12
• http://www.ibiblio.org/apollo/Documents/CTSS_ProgrammersGuide.pdf
• http://www.multicians.org/thvv/7094.html
• http://www.multicians.org/thvv/mail-details.html
• http://www.frobenius.com/7090.htm
• http://motherboard.vice.com/2011/2/17/watson-s-hardest-question-what-is-life--2
• http://u-tx.net/media/fano-on-ctss.png
• http://mason.gmu.edu/~tbell5/page2.html
• http://www-03.ibm.com/ibm/history/exhibits/storage/storage_1301.html
• http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP7090.html
• http://simh.trailing-edge.com/docs/ctss_hardware.pdf
• http://mit.edu/saltzer/www/publications/ctss/CC-244.html
• http://en.wikipedia.org/wiki/Daisy_Bell
Simulator
• http://www.cozx.com/~dpitts/ibm7090.html
Videos
• http://www.youtube.com/watch?v=Q07PhW5sCEk
• http://www.youtube.com/watch?v=8edfFH0rklc