1. The UNIX Time-Sharing System
Mosharaf Chowdhury
9/12/16
EECS 582 – F16

2. A Memorandum
“This memorandum is based on the assumption that MIT will be given a transistorized IBM 709 about July l960. I want to propose an operating system for it that will substantially reduce the time required to get a problem solved on the machine. Any guess as to how much of a reduction would be achieved is just a guess, but a factor of five seems conservative. …”
“The proposal requires a complete revision in the way the machine is used, …”
“… I think the proposal points to the way all computers will be operated in the future, and we have a chance to pioneer a big step forward in the way computers are used. …”
“…The ideas expressed in the following sections are not especially new, but they have formerly been considered impractical with the computers previously available. …”
9/12/16
EECS 582 – F16

3. A Memorandum on Time Sharing
Divide a shared resource between multiple programs and users
John McCarthy wrote the memorandum in 1959
As opposed to batch processing
Submit jobs to queues to be processed one at a time
Non-interactive
Early examples of time-sharing OSes include
MIT CTSS (Compatible Time-Sharing System)
Berkeley Timesharing System
Dartmouth Time-Sharing System (DTSS)
Most of the OSes were machine-specific.
9/12/16
EECS 582 – F16

4. Multics Multiplexed Information and Computing Service
Started in 1964 at MIT; led by Fernando Corbató w/ GE and Bell Labs
Time-sharing operating system
Developed initially for GE-645 mainframe, a 36-bit machine w/ support for virtual memory
Hierarchical file system
Protection ring-based security
Access Control Lists (ACL) …
Bell Labs pulled out in 1969
Honeywell maintained it until 2000
9/12/16
EECS 582 – F16

5. UNIX Started by Ken Thompson and Dennis Ritchie in 1969
“UNIX was never a ‘‘project;’’ it was not designed to meet any specific need except that felt by its major author, Ken Thompson, and soon after its origin by the author of this paper, for a pleasant environment in which to write and use programs.”
- Dennis Ritchie, “The UNIX Time-sharing System – A Retrospective”
9/12/16
EECS 582 – F16

6. UNIX First version on DEC PDP-7 and PDP-9 (circa 1969-70)
Second version on unprotected DEC PDP-11/20
Third version w/ multiprogramming on PDP-11/40, 45 etc.
Described in today’s paper
February, 1971: PDP-11 Unix first operational
PDP = Programmed Data Processor
16-bit precursor to 32-bit VAX (Virtual Address eXtension)
9/12/16
EECS 582 – F16

7. PDP-11/40 16-bit word (8-bit byte) 144K bytes core memory Storage
UNIX took 42K bytes
Minimal system 50K bytes
Storage
1M byte fixed-head disk
4X 2.5M byte moving-head disk cartridge
1X 40M byte moving-head disk pack
14 variable speed communication interfaces
Many other attached devices
9/12/16
EECS 582 – F16

8. Simplicity Simple abstractions Simple file system design
Everything is a file
Simple file system design
Hierarchical for programming simplicity
Simple connectors
Pipes that look like files
Simple maintenance
Self-maintained
“It is hoped, however, the users of UNIX will find that the most important characteristics of the system are its simplicity, elegance, and ease of use.”
- DR & KT, “The UNIX Time-Sharing System”
9/12/16
EECS 582 – F16

9. Influences Berkeley Timesharing System
Process forks
Multics
I/O system calls
Multics and TENEX
Shell and related
CTSS
“…in essence a modern implementation of MIT’s CTSS system…” – D. Ritchie
“The success of UNIX lies not so much in new inventions but rather in the full exploitation of a carefully selected set of fertile ideas, and especially in showing that they can be keys to the implementation of a small yet powerful operating system.”
- DR & KT, “The UNIX Time-Sharing System”
9/12/16
EECS 582 – F16

10. Key Concepts Names and namespaces Process management User interface
Directories, paths, and links
Root, home, and current directory
Rooted tree and mounting sub-trees on leaves
Special files and access control
Process management
System calls, address space, signals/interrupts
User interface
Shell, I/O redirection, and pipes
9/12/16
EECS 582 – F16

11. Hierarchical Namespace
Easy to traverse
Easy to modify/add/delete
Access control is simpler
Hierarchical as well
Can be arbitrarily increased
Mount new drives!
9/12/16
EECS 582 – F16

12. Everything is a File Regular files Special files Directories Devices
Networks
Directories
Same as files, except when not
Keeps track of pointers to files, not actual content
Simplifies linking!
9/12/16
EECS 582 – F16

13. I/O Calls Inspired by Multics Primary functions
open, close, read, write, seek
Interface that has passed the test of time: haven’t effectively changed yet!
BSD socket API is similar
open, close, send, recv
9/12/16
EECS 582 – F16

14. File System Implementation
Thompson and Ritchie took special pride in their design of the file system
My opinion from the reading
Given the long-term impact, I think it’s justified!
We’ll skip the details, and wait for next week when we discuss file systems
9/12/16
EECS 582 – F16

15. Execution Image is an execution environment
Container is the current parallel
Process is the execution of the image
Program text is write-protected and shared between all running processes of the same program
Created using fork
Inspired by Berkeley Timesharing System
9/12/16
EECS 582 – F16

16. Processes and the Kernel
Separate virtual address space
Only one thread!
Kernel
Mediator for accessing shared resources and kernel services P1 P2 P3
Kernel
Interactions via system calls and signals/interrupts
9/12/16
EECS 582 – F16

17. Worse is Better Simplicity Ease of Use and Maintenance
Consistency Across Interfaces
Completeness of Design
Correctness of Specification and Implementation
Placement of Functionality
9/12/16
EECS 582 – F16

18. Next Class…
System R
9/12/16
EECS 582 – F16