下記論文を扱った研究室内輪読用の資料です This is slides for group reading in Lab. Ritchie, O. M., and Ken Thompson. "The UNIX time-sharing system." The Bell System Technical Journal 57.6 (1978): 1905-1929.

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KUMO B4 shuya
Ritchie, O. M., and Ken Thompson. “The UNIX time-sharing system.”
The Bell System Technical Journal 57.6 (1978): 1905-1929.

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UNIX is general-purpose, multi-user, interactive OS
A hierarchical file system incorporating demountable volumes
Compatible file, device, and inter-process I/O
The ability to initiate asynchro- nous processes
System command language select- able on a per-user basis
Over 100 subsystems including a dozen languages

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UNIX the system are simple, elegance, ease of use
First UNIX is installed on DEC PDP-11/40 computer
Powerful operating system for interactive use
Not expensive either in equipment ($4,000) or in human effort
These programs are available under UNIX
- assembler, text editor, link loader, compiler…
All UNIX software is maintained under UNIX PDP-11/40
http://webpages.charter.net/thecomputercollection/pdp11/pdp1140.jpg

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PDP-11/45 UNIX installed Computer
- 16-bit word (8-bit byte) computer, Core memory 144K bytes
- UNIX occupies 42K bytes
UNIX occupies 42K bytes
The minimum configuration require 50K bytes of core
The minimal system capable of running the software

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The size of UNIX written in C is one third greater than old one
which written in assembly.
New system is not only easier to understand and to modify
but also many functional improvements
- multiprogramming, can share reentrant code
UNIX software is written in C language

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Ordinary Files
- A file contains whatever information the user places on it
Directories
- Files for managing regular files and special files
Special Files
- Treat peripheral devices as virtual files
UNIX has three file system

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Ordinary Files
A file contains whatever information the user places on it
Ex. -> symbolic or binary programs
System don’t expect file structure.
Structure of files is controlled by programs not by system

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Directories
It provide mapping between file names and themselves
Each user has a directory of his own files and
can create subdirectories
Several directories is maintained by system for its own use
- root - all files can be found by tracing a path through
Files are named by sequence of 14 fewer characters

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Directories
Directory names are separated by slash “/”
- If the sequence begin with slash, search begin in “root”
- Not starting with slash, search begin in current directory
The name “.” in each directory refers to directory itself
The name “..” in each directory refers to its parent
Same nondirectory file may appear in several directories
which called linking

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“root” is stored on same device but it is not necessary that
the entire file system hierarchy reside on this device
Mount replaces a leaf of the hierarchy tree by new subtree
After the mount, there is virtually no distinction between files
on the removable volume and parent file system
No link may exist between one file system hierarchy and
another

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Access control scheme in UNIX is quite simple
Each user is assigned a unique user ID number
When a file is created, it is marked with ID of its owner
Also given for new files is set of seven protection bits
Privileged programs are not accessed by other users
The system recognizes “super-user” as except from the usual
constraints on file access

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System calls todo I/O of UNIX is quite simple
filep = open (name, flag)
- filep is called file descriptor which is used to identify to
read, write, manipulate
- name indicates the name of the file
- flag indicates file status such as read, write
location = seek (filep, base, offset)
- offset indicates starting points and base indicates
destination

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System calls todo I/O of UNIX is quite simple
n = read (filep, buffer, count)
n = write (filep, buffer, count)
- n is the number of bytes transmitted
- buffer is is specified byte array
- count is number of bytes to receive / send
write case, n is the same as count
read case, n is less than count when read call returns equal to
zero it indicates the end of the file

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Data structure of UNIX file system
Directory entry contains only a name for the associated file
and pointer which integer is called “i-number” (index number)
“i-list” stored the device on which the directory resides
when the file is accessed
“i-node” contains the description of the file as follow
- owner, protection, address, size, last modification …

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An image is a computer excitation environment
- An image is current state of pseudo computer
- core image, general register values, status of files ...
An process is the execution of an image
- During processing, image resides in core
An image is divided into [text, data, stack] segments
- Machine instruction is placed in text segment
- Data (variables on heap) is placed in data segment
- Stack segment is place for stack, auto variables placed in

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New process is presented by only by fork system call
processid = fork (label)
When fork is executed by a process, it spills into two process
- parent, control returns directly from fork
- child, control is passed to location label
The processid is the identification of the other program
fork may determine whether process is the parent or child

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pipe is used for interprocess communication
filep = pipe ( )
- It returns filep and creates interprocess channel called pipe
- pipe is passed from parent to child process in the image
by the fork call
- One pipe returns two file descriptor (for read and write)

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Requests the system to read in and execute program
execute (file, arg1, arg2, …, argn)
- arg1 is the same string as file, so that program may
determine the name by which it was invoked
- All the code and data in the process using excuse is
replaced from file

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processid = wait ( )
- wait suspend execution until its children has completed
execution
- It returns the processid of the terminated process
exit (status)
- exit terminates a process, destroy its image, close its files
- Process may also terminate as result of various illegal
actions or user-granted signals

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When communicate UNIX, the Shell is helpful
The shell is command line interpreter
command arg1, arg2, …, argn
- Command line consist of command name and arguments
- The command name and arguments separated by spaces
The site of command
- Directory /bin contains all the commands generally used
- Path of command may be including “/” character to specify

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No special coding is needed handling I/O redirection
The shell start with two files which have file descriptor 0, 1
- 0 is for reading, programs read massages typed by users
- 1 is for writing, understood as the standard output
The shell can change standard assignments of file descriptor
The file name following “<” and “>” are called redirect
- “<” means place out put
- “>” take input followed arguments

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Example of Standard I/O use
$ ls >there
- Create a file called “there” and places the listing
$ ed <script
- It interprets script as a file of editor command
Execution Example

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Filter copies standard input to its standard output
A sequence of commands separated by vertical vars causes
to execute simultaneously and standard out put of each
commands is delivered to the next commands.
$ cat /var/log/secure | grep sudo > sudo_result
- $ cat /var/log/secure > temp1
- $ grep sudo < temp1 > temp2
- $ sudo_result < temp2
Example
Divided…

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The shell is itself a command
$ ls; ed
- With semicolon when ls ends, ed is executed
$ as source >output &
- Assembling source and diagnostic output going to output
$ as source >output & ls >files &
- Both assembly and listing in the background
$ (date; ls) >x &
- Print current date followed by listing output going to x

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Sequential use of commands
$ as source
$ mv a.out testprog
$ testprog
as assembles code and output a.out (binary)
mv causes a.out to be renamed testprog
testprog executes itself
$ sh <tryout
sh to excute the commands sequentially

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Waiting
read return
Analyze
command
line
fork called
How to work the Shell
The new-line character
ending the line is typed
Putting the arguments
in a form for execute
fork childfork parent
wait
exec
exit
Die process

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When a process is created by the fork primitive
- It inherits the core image, all open files in parents with file
descriptor
- Its child process automatically inherits file descriptor value
When a n argument with “<” or “>” is given
- Offspring process, before it execute, makes the standard
I/O file descriptor 0 or 1 respectively refer to the named file
Terminate point
- Discovering an end of file, the Shell terminates

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init creates one process for each typewriter channel
The last step in the initialization of UNIX is the creation of a
single process and invocation of a program called init.
When init was invoked there were no files open, in each
process I/O devices will receive 0 or 1 file descriptor.
When user typed correct login name and password, init will
wake up and change the user’s default current directory.
Logout need only type the end of the file sequence.

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Faults cause the processor to trap to a system routine
Faults are such as..
- Referring nonsexist memory, unimplemented instruction…
When an illegal action is caught, the system terminates the
process and writes the user’s image on file core
A debugger can be used to determine faults
“deletes” is interrupt signal which simply cease execution
quit signal is also used to force a core image to be produced

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Three consideration influenced the design of UNIX
❶ From programmers perspective…
- Interactive system is much more productive and satisfying to
use than a “batch” system
❷ Severe regulation of system an d its software
- The size constraint made economy and elegance of design.
❸ UNIX is able to maintain themselves
- Programs are always available and modified online, we were
willing to rewrite the system when new idea were invented