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Operating-System Services and Structure
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Operating Systems
Autumn 2013
CS4023
Operating-System Services and Structure
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Outline
1
Operating-System Services and Structure
OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
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CS4023
Operating-System Services and Structure
Something completely different...
OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Operating System Definition
Intermediary between a user of a computer and the
computer hardware
Control program
Controls execution of programs to prevent errors and
improper use of the computer
Resource allocator
Manages all resources
Decides between conflicting requests for efficient and fair
resource use
CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Operating System Design
Design and Implementation of OS not “solvable”, but some
approaches have proven successful
Internal structure of different Operating Systems can vary
widely
Affected by choice of hardware, type of system
User goals and System goals
User goals: operating system should be convenient to use,
easy to learn, reliable, safe, and fast
System goals: operating system should be easy to design,
implement, and maintain, as well as flexible, reliable,
error-free, and efficient
CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
A View of Operating System Services
CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Outline
1
Operating-System Services and Structure
OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
2
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CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
User Interfaces
User interface (UI)
Command-Line (CLI)
Graphics User Interface (GUI)
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Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
User Interface – CLI
CLI allows direct command entry
Sometimes several alternatives implemented – shells
Primarily fetches a command from user and executes it
Sometimes commands built-in, sometimes just names of
programs
CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
User Interface – GUI
User-friendly interface (desktop metaphor)
Usually mouse, keyboard, and monitor
Icons represent folders, files, programs, actions, etc.
Various mouse buttons over objects in the interface cause
various actions
Many systems now include both CLI and GUI interfaces
Linux, like all UNIXs has CLI and has additional GUI
interfaces: KDE, Gnome
Microsoft Windows is GUI with CLI “command” shell
Apple Mac OS X has “Aqua” GUI interface with UNIX kernel
underneath and shells available
Solaris is CLI with optional GUI interfaces (Java Desktop)
CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Further OS Services (Assistance)
Program execution: OS must be able to load a program
into memory, run that program, and end execution (either
normally or abnormally)
I/O operations: A running program (process) may require
I/O, which may involve a file or an I/O device
File-system manipulation:
File system is of particular interest
Processes need to
read and write files and directories
create and delete them
search them
list file information
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Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Further OS Services (Assistance) (contd.)
Multiprogramming
The CPU always has a process to execute; when the
currently running process needs to wait (e.g., for I/O), OS
allocates the CPU to another job
Time-sharing (multitasking)
Logical extension to multiprogramming; the CPU switches
processes so frequently that users can interact with each
process while it is running, creating interactive computing
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Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Further OS Services (Assistance) (contd.)
Communications: Processes may exchange information,
on the same computer or over a network
Communications may be via shared memory or through
message passing
Error detection: OS needs to be constantly aware of
possible errors
May occur in the CPU and memory hardware, in I/O
devices, in user program
For each type of error, OS should take the appropriate
action to ensure correct and consistent computing
CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Outline
1
Operating-System Services and Structure
OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
2
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CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
OS Services to Maintain Efficient Operation
Resource allocation: When multiple users or multiple
jobs running concurrently, resources must be allocated to
each of them
Many types of resources: some (such as CPU cycles, main
memory, and file storage) may have special allocation
protocol (specialist algorithms), others (such as I/O
devices) may have general request and release protocol
Accounting: To keep track of which users use how much
and what kinds of computer resources
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Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
OS Services to Maintain Efficient Operation (contd.)
Protection and security: to control use of shared data (on
a multiuser system or a network)
Protection involves ensuring that all access to system
resources is controlled
Security of the system from outsiders requires user
authentication, extends to defending external I/O devices
from invalid access attempts
If a system is to be protected and secure, precautions must
be instituted throughout – chain is only as strong as its
weakest link
CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Outline
1
Operating-System Services and Structure
OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
2
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CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Supporting Typical OS Operations
The interrupt mechanism:
Supports several of the typical operations an OS must
support:
making a system call
managing a timer
Interrupt driven by hardware
Software error or request creates exception or trap
Division by zero, request for operating system service
Other process problems include infinite loop, processes
modifying each other or the operating system
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Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Interrupts: System calls
Dual-mode operation allows OS to protect itself and other
system components
User mode and kernel mode
Mode bit provided by hardware
Provides ability to distinguish when system is running user
code or kernel code
Some instructions designated as privileged, only executable
in kernel mode
System call changes mode to kernel, return from call resets
it to user
Excellent discussion of dual mode here
CS4023
Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Making a System Call
Transition from User to Kernel Mode
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Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Interrupts: Timer
Timer to prevent infinite loop / process hogging resources
Set interrupt after specific period
Operating system decrements counter
When counter zero generate an interrupt
Set up before scheduling process to regain control or
terminate program that exceeds allotted time
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Operating-System Services and Structure
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OS Services: Assistance to the User
OS Services: Efficient Operation
OS Operations
Hierarchical Protection Domains
Protection Rings
Hierarchical Protection
Domains, also known as
protection rings
Hardware supported rings
were introduced by Multics
However, most
general-purpose UNIX
systems use only two
rings, even if the hardware
it runs on provides more
CPU modes than that
Privilege rings for X86
processor family
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Operating-System Services and Structure
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Funny Friday
Lost in translation
Introducing Rufus T. Firefly, President-to-be(?) of
Freedonia
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