The UNIX Time-Sharing System Mosharaf Chowdhury EECS 582 – W1611/11/16

Time Sharing As opposed to batch processing Submit jobs to queues to be processed one at a time Non-interactive Divide a shared resource between multiple programs and users to lower the cost of computing John McCarthy wrote a memorandum in 1959 Early examples include MIT CTSS (Compatible Time-Sharing System) Berkeley Timesharing System Dartmouth Time-Sharing System (DTSS) 1/11/16EECS 582 – W162

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 /11/16EECS 582 – W163

UNIX Started by Ken Thompson and Dennis Ritchie in /11/16EECS 582 – W164 “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”

UNIX First version on DEC PDP-7 and PDP-9 (circa ) 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 1/11/16EECS 582 – W165

PDP-11/40 1/11/16EECS 582 – W bit word (8-bit byte) 144K bytes core memory 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 pg

Simplicity Simple abstractions Everything is a file Simple file system design Hierarchical for programming simplicity Simple connectors Pipes that look like files Simple maintenance Self-maintained 1/11/16EECS 582 – W167 “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”

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 1/11/16EECS 582 – W168 “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”

Key Concepts Names and namespaces 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 1/11/16EECS 582 – W169

Hierarchical Namespace Easy to traverse Easy to modify/add/delete Access control is simpler Hierarchical as well Can be arbitrarily increased Mount new drives! 1/11/16EECS 582 – W1610

Everything is a File Regular files Special files Devices Networks Directories Same as files, except when not Keeps track of pointers to files, not actual content Simplifies linking! 1/11/16EECS 582 – W1611

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 1/11/16EECS 582 – W1612

No Locks 1/11/16EECS 582 – W1613 “We take the view that locks are neither necessary nor sufficient, in our environment, to prevent interference between users of the same file. They are unnecessary because we are not faced with large, single-file data bases maintained by independent processes. They are insufficient because locks in the ordinary sense, whereby one user is pre- vented from writing on a file which another user is reading, cannot prevent confusion when, for example, both users are editing a file with an editor which makes a copy of the file being edited.” - DR & KT, “The UNIX Time-Sharing System”

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 read FFS 1/11/16EECS 582 – W1614

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 1/11/16EECS 582 – W1615

Processes and the Kernel Process Separate virtual address space Only one thread! Kernel Mediator for accessing shared resources and kernel services 1/11/16EECS 582 – W1616 P1P2P3 Kernel Interactions via system calls and signals/interrupts

Pipes This is the ‘|’ when writing shell commands Mechanism for inter-process communication Using file descriptor Again files! Allows to build large workflows by connecting separate programs We do the same today 1/11/16EECS 582 – W1617

Worse is Better & End-to-End Arguments 1.Simplicity 2.Ease of Use and Maintenance 3.Consistency Across Interfaces 4.Completeness of Design 5.Correctness of Specification and Implementation 6.Placement of Functionality 1/11/16EECS 582 – W1618

Next Class… System R 1/11/16EECS 582 – W1619