Transcript Operating Systems I

Operating Systems I
An Introduction
to
Operating System
Concepts
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What’s an Operating System?
“The Boss”
 “The Big Cheese”
 “The Godfather”
 “The Power Behind the Scenes”
 A control program the manages all the
resources of the computer on which it
runs.

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OS Introduction

Most computers multitask (do many
things “at once”)
o read/write disks and tapes
o send/receive network traffic
o run several programs.

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Resources must be shared and
programs coordinated
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OS Introduction

The operating system manages multitasking systems.
o What resources are available.
o Who gets to use them.
--- “Not now Bobby, it’s Sally’s turn.” -- Also provides:
o standard interfaces (ways to use) to
computer hardware
o standard utility programs supporting other
software.
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“McDonalds Model” of an Operating
System
Fries
Customers
(Applications)
Grill
Counter
Drinks
Crew
(Operating System)
Crew Serves Customers. Customers Don’t Cross Counter
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Operating System Evolution
“Batch” Operating Systems

“One-at-a-time” in a “batch”
o Usually single-task
o Execution required significant setup of
equipment and software
o Each program was a job (“batch job”)
o OS designed to simplify setup and
transition between jobs
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Batch Operating Systems (cont’d)

Operators served as “High Priests.”
o Users didn’t touch computers
•User’s “submitted” jobs
•Picked up program output
•Written/punched input and instructions
•Printed/punched output and error reports
o Turn around: minutes to days
“Error 410: Unterminated Do Loop, line 66”
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Operating System Evolution
Interactive Processing

OS for Interactive Processing
o Started with “operator-like” single user
o Users interact directly via remote terminals
(workstations) --- “On-Line processing”
o Users demand timely response
o Machines too expensive for single users
N.B. “real-time” means “must respond in
limited time or fail” --- not on-line system
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Operating System Evolution
Time-Sharing Systems
Really “time-slicing” systems
 Operating system:

o puts jobs in a “circular queue”
o gives each job a predetermined “time slice”
o swaps old job out and new one in at the
end of each slice
o repeats fast enough to give illusion of
simultaneously execution
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Operating System Evolution
Time-Sharing Systems (continued)

“Radar O’Reilly” model of operating
systems
o Read the mail
o Make the coffee
o Check on Hawkeye
o Answer the phone

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Only one job at a time, but all progress
“on the average.”
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Operating System Evolution
Time Sharing (cont’d)

Other Benefits:
o best allocation of user time --- no “dead
time” at the keyboard
o no waiting for peripherals --- other
processes can execute while one waits.

Costs:
o complexity
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Operating System Evolution
Examples

Batch Systems:
o IBM 1620, early MITS

Interactive systems:
o DOS, early MacOS, VIC20, Commodore
64

Time-Sharing systems:
o Unix, Linux, Multics, IBM MVS, DEC VMS
o Windows 95/98/NT, later MacOS
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Different Operating Systems on the
Same Machine ?

Only one at a time
o VAX: VMS or Ultrix
o IBM PCs: Linux or Windows or OS/2

Emulators let one OS act like another
o “Soft-PC”: Mac
Windows
o “Wine” and “Wabi”: Linux
Windows
o Cygnus Windows: Win95/98/NT
Unix
o Not always complete
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Inside the Operating System
The OS Shell

Defines interface between OS and
users
o Windows GUI
o UNIX command line
o UNIX users can choose among a variety of
shells
•csh is the “C shell”
•tcsh is an enhanced “C shell”
o Shell programming
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Inside the Operating System
OS Shell interface
Users
Users
O/S
Users
shell
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Inside the Operating System
Different Shells/Same Results
BASH
Different
Command
Sets
SH
CSH
Operating
System
Same
Results
TCSH
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Inside the Operating System
The OS Kernel
Central part (“hard nugget”) of
Operating System
 Kernel Components

o File Manager
o Device Drivers
o Memory Manager
o Scheduler
o Dispatcher
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Inside the Opeating System
File Manager

Maintains information about the files
that are available on the system
o location
o size, type, and protections
o what storage is still available
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Files usually allowed to be grouped in
directories or folders. Allows
hierarchical organization.
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Inside the Operating System
Device Drivers
Software to communicate with
peripheral devices or controllers
 Each driver is unique
 Translates general requests into specific
steps for that device

“read poker.exe”
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Device
Driver
“read device 0xA3,
cylinder 3, track 13,
sector 43”
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Inside the Operating System
Memory Manager
Controls use of main memory
 Allocates memory for each program and
its data
 Reclaims memory when programs
release it (or finish).
 Is supposed to know what areas are
free.
“Memory leak”
manager loses track

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Inside the Operating System
Memory Manager (cont’d)

Virtual memory (“Imaginary memory”)
o Programs only need their current data in
memory. (“What’s your VIN?”)
o Store remainder on disk & keep location.
o Bring other data back (and put this data
out) when other data is needed.
o Creates the illusion of additional memory
by rotating (swapping) programs and data
between main memory and mass storage
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Inside the Operating System
Virtual Memory for People
Calendar
You
Sticky
notes
Notebooks
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Inside the Operating System
Scheduler
Maintains a record of processes that are
present, adds new processes, removes
completed processes
 Tracks

o memory area(s) assigned
o priority
o state of readiness to execute (ready/wait)
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Inside the Operating System
Dispatcher
Executes processes when scheduled
and ready
 Gives each process a small (50 ms)
time slice in which to execute
 Stops process when time is up.
 Waits for scheduler to update process
record
 Starts next scheduled process.

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What’s a Process
The dynamic activity of executing a
program
 The process state is a “snapshot” of
the machine, including values of the
CPU registers, program counter and
other memory cells
 A single program can be associated
with multiple processes simultaneously

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Inside the Operating System
Types of software

Applications software
Do something “for the outside world.”
 Often transportable


System Software
Performs tasks supporting operation of
computer systems (“not outside world”)
e.g. operating systems
 Usually tightly coupled to hardware

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Inside the Operating System
Types of software (cont’d)

Utility software
o provides fundamental not included OS
(check spelling, count words, calculator)
o “extend” the OS (or “support” applications)
o Distinction between applications and
utilities is often vague
o Distinction between OS and utilities is also
vague
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Inside the Operating System
Utilities
Operating Systems usually come with
some associated utility programs
 UNIX usually has the text editors emacs
and vi (and sometimes pico)
 UNIX has its own sort utility
 UNIX has its own mail utility

(The contraction “it’s” means “it is,” not “belonging to it.”)
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Inside the Operating System
Summary
Shell -- interface to user
 File Manager -- manages mass memory
 Device Drivers -- communicate with
peripherals
 Memory manager -- manages main
memory
 Scheduler & Dispatcher -- manage
processes

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