Transcript The Unix Time-Sharing System

The Unix Time-sharing System
Chuck Davin
Software Design & Engineering
CSE 350
Spring 2001
CSE 350, Spring 2001
JMS 1
The Unix Time-sharing System
• D. M. Ritchie and K. Thompson.
The Unix time-sharing system.
BSTJ, 57:6 (July-August, 1978), 1905-1929.
• K. Thompson.
Unix implementation.
BSTJ, 57:6 (July-August, 1978), 1931-1946.
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Features
• A hierarchical filesystem incorporating
demountable volumes
• Compatible file, device, interprocess
communication
• The ability to initiate asynchronous
processes
• System command language selectable on a
per-user basis
• Over 100 subsystems including a dozen
languages
• High degree of portability
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Unix History
• First version (ca. 1969-70) on DEC PDP-7 and
PDP-9.
• Second version on unprotected DEC PDP11/20.
• Third version with multiprogramming on PDP11/34, 40, 45, 60, 70.
• Fourth version on DEC PDP-11/70 and
Interdata 8/32.
February, 1971: PDP-11 Unix first operational.
Familiar epoch: midnight, January 1, 1970.
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Economics of the 1970s
• Hardware cost: $40,000
• Software cost: two man-years
• Cost recovery for disk space
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PDP-11/70 Platform
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16-bit word (8-bit byte)
768 KBytes core memory
System kernel 90 KBytes
Minimal system 96 Kbytes
Two 200 MByte moving-head disks
20 variable speed (300 to 1200 baud) communication
interfaces
12 communication lines (9600 baud)
Synchronous interfaces (2400 and 4800 baud) for intermachine file transfer
Nine-track tape, line printer, phototypesetter
Voice synthesizer, digital switching network, chess
machine
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A Unix System in 1978
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User population: 125
Maximum simultaneous users: 33
Directories: 1630
Files: 28300
Disk blocks (512-byte) used: 301,700
Daily command invocations: 13500
Daily (non-idle) CPU hours: 9.6
Daily connect hours: 230
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The C Programming Language
• System re-coded in C in summer, 1973
• 1/3 bigger than assembler language version
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Observation
"The most important role of the
system is to provide a file
system."
(Page 1907)
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Filesystem Properties
• Internal file structure controlled by
application programs
• Internal file structure not imposed by system
• Hierarchy
• Directories as files
• "." and ".." convention
• Links
• Restrictions on namespace topology
• Mountable volumes
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File Types
• Ordinary
• Directory
• Special
– Character
– Block
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Filesystem Representation
• i-Number
• i-List
• i-Node
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File Properties
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Owner user-id
Owner group-id
Protection bits
Physical disk (or tape) address of contents
Size
Time of creation
Time of last use
Time of last modification
Number of links
File type
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File Protection
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User (owner) read, write, execute
Group read, write, execute
Others read, write, execute
Set-user-id
--drwxwrxrwx
• "Execute" permission on directories
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Filesystem API
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filep = open (name, flag)
filep = create (name)
n = read (filep, buffer, count)
n = write (filep, buffer, count)
location = lseek (filep, offset, base)
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File Space Management
• Blocks 0 through 9 indicated in i-Node
• Blocks 10 through 137 indicated indirectly
• Blocks 138 through 16521 indicated doubly
indirectly
• Blocks 16522 and higher indicated triply
indirectly
Performance Techniques: caching and readahead
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Special File Naming
• Example Name: /dev/foo
• Major Device Number: selects driver code
• Minor Device Number: selects device
instance within class
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File System Data Structure
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Process Management API
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processid = fork ()
execute (file, arg1, ..., argN)
processid = wait (& status)
exit (status)
filep = pipe ()
• Traps, Signals
• Minimalist, integrated process
synchronization
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Process Control Data Structure
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The Shell and Reusability
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Redirection
Stdin, stdout, stderr
Pipes and filters
Argument parsing and globbing
Multitasking
Basic control structures
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Perspective
"The success of the Unix
system is largely due to the
fact that it was not designed
to meet any predefined
objectives.“
(Page 1926)
Motivation: dissatisfaction with existing
facilities.
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Retrospective Design
Considerations
• Easy to write, test, run programs
– Interactive use
• Constraints on size
• Self-maintenance
– Available source code
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Easy Programming
• Device-independent file abstraction
• No "access methods"
• Few system constraints on program
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Influences
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fork () from GENIE time-sharing system
I/O API from Multics
Shell concept from Multics
Implementing "system" code as user
primitives from Multics
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Unix Implementation
• 10,000 lines of C code
• 1,000 lines of assembler code
– 800 lines not possible in C
– 200 lines for efficiency
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Observation
"Throughout, simplicity has
been substituted for
efficiency."
(Page 1932)
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Observation
"The UNIX kernel is an I/O
multiplexer more than a
complete operating system.
This is as it should be."
(Page 1945)
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Unsupported Features
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File access methods
File disposition
File formats
File maximum size
Spooling
Command language
Logical records
Physical records
Logical file names
Multiple character sets
Operator and operator console
Login and logout
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