Transcript Command interpreter (or shell)

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CHAPTER 3
OPERATING SYSTEMS STRUCTURES
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User-Operating System Interface
There are several ways for users to interface with the
operating system.
Here, we discuss two fundamental approaches:
1)
2)
Command Interpreter, that allows users to directly enter
commands to be performed by the operating system.
Graphical User Interface (GUI).
Nowadays, there are two genres of operating systems:
Window-based: like Apple Macintosh, Microsoft Windows.
Command line-based: like MS-DOS, UNIX, and Linux.
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(1) Command Interpreter …X
Linux Terminal
Windows Command Prompt
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(1) Command Interpreter
(Cont’d)……..X
For example, almost in all your OS Lab sessions you
are doing the following:
Launching Linux operating system.
Opening the “terminal”.
The following prompt will be displayed in which you can write
your commands.
[student@localhost ~]$
pwd
The terminal is just a visualization of the command
interpreter.
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(1) Command Interpreter (Cont’d)…..X
Many commands are given to the operating system by
control statements.
Command interpreter (or shell) is a program that reads
and interprets control statements.
Its function is to get and execute the next command statement.
Command interpreter is the interface between the user
and the operating system.
 Command interpreter (or shell) is surrounding the
operating system kernel.
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(1) Command Interpreter (Cont’d)….X
Many commands are given to the operating system by
control statements which deal with:
process creation and management
I/O handling
secondary-storage management
main-memory management
file-system access
protection
networking
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(1) Command Interpreter (Cont’d)….X
 There are two ways in which the commands can be
implemented:
1. The command interpreter itself contains the code to
execute the command.
The number of commands that can be given determines the size of
the command interpreter, since each command requires its own
implementing code.
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(1) Command Interpreter (Cont’d)…..X
alternative approach – used by UNIX –
implementing most commands by system programs.
2. An
In this case, the command interpreter does not understand the
command in any way, it merely uses the command to identify a file
to be loaded into memory and executed.
The command interpreter program, which can be small, does not
have to be changed for new commands to be added.
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(2) Graphical User Interface (GUI)
Here, rather than entering commands directly via a
command-line interface, users employ a mouse-based
window-and-menu system characterized by a desktop.
Pictorial interfacing through icons for programs, files,
directories, and system functions.
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System Calls
System calls provide the interface between a running
program ( process ) and the operating system.
 Generally available as assembly-language instructions
 Several languages defined to replace assembly language for
systems programming allow system calls to be made directly
(e.g., C, C++, and Perl)
Windows platforms are part of Win32 API, which is
available for use by all the compilers written for Microsoft
Windows.
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System Calls
Java dose not allow system calls to be made directly,
because a system call is specific to an operating
system.
Three general methods are used to pass parameters
between a running program and the operating system.
1)
2)
3)
Pass parameters in registers.
Store the parameters in a table in memory, and the table
address is passed as a parameter in a register.
Push (store) the parameters onto the stack by the program,
and pop off the stack by operating system.
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Passing of Parameters as a Table
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Types of System Calls
Process control
End, abort
Load, execute
Create process, terminate process
File management
Create file, delete file
Open, close
Device management
Request device, release device
Read, write, reposition
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Types of System Calls (Cont’d)
Information maintenance
Get time or date, set time or date
Get system data, set system data
Communications
Create, delete communication connection
Send, receive messages
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Process Control
If we create a new job or process, we should be able to
control its execution
Terminate a job or process that we created (create process,
and terminate process)
Determine
and reset the attributes of a job or process
(get process attribute, and set process attributes)
A process or job executing one program may want to
load and execute another program
• Where to return control when the loaded program terminates?
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Process Control - MS-DOS Execution
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UNIX Running Multiple Programs…X
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File Management……….X
Several common system calls deal with files
create and delete files
read, write, close, or reposition files
get file attribute, and set file attribute
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Device Management……….X
Files can be thought of as abstract or virtual
devices.
Thus, many of the system calls for files are also needed for
devices.
The similarity between I/O devices and files is so
great that many operating systems, including UNIX
and MS-DOS, merge the two into a combined filedevice structure.
In this case, I/O devices are identified by special file names.
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Information Maintenance……….X
Many system calls exist simply for the purpose of
transferring information between the user program and
the operating system.
Call to return the current time and date.
Calls to return information about the system (number of current
users, the version number of the operating system).
The operating system keeps information about all its
processes, and there are system calls to access this
information (get process attribute).
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Communications…………X
Processes within an operating systems may need to
communicate.
Before communication can take place, a connection must
be opened.
These identifiers
Computer name (each computer in a network has a name).
Process name (each process in an operating system has a name).
are then passed to the general-purpose open and close
calls provided by the file system, or to specific open
connection and close connection system calls.
System
Programs…………X
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System programs provide a convenient environment for
program development and execution.
They can be divided into:
File manipulation
create, delete, copy, rename, …
Status information
date, time, number of users, memory space, …
File modification ( editors )
Programming language support
Program loading and execution
Communications: creating virtual connections among processes,
users, and different computer systems
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System Programs…….X
System utilities or application programs
Web browsers
Word processors and text formatters
Spreadsheets
Database systems
Plotting and statistical-analysis packages
Command interpreter
Dual-Mode
Operation………..X
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Dual-mode has two separate modes of operation:
Monitor mode
User mode - user program
Why dual-mode operation?
The dual-mode of operation provides us with the means for
protecting the operating system from errant users, and errant
users from one another.
A bit, called mode bit, is added to the hardware of the
computer to indicate the current mode: monitor (0), or
user (1).
Dual-Mode Operation
(Cont’d)..X
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At the system boot time, the hardware starts in monitor
mode. The operating system is then loaded, and starts
user processes in user mode.
Whenever a trap or interrupt occurs, the hardware
switches from user mode to monitor mode.
Whenever the operating system gains control of the
computer, it is in monitor mode.
The system always switches to user mode before
passing control to a user program.
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Operating System Structures
A system as large and complex as a modern operating
system must be engineered carefully if it is to function
properly and be modified easily.
A common approach is to partition the task into small
components rather than have one monolithic system.
Each of these modules should be a well-defined portion
of the system, with carefully defined inputs, outputs, and
functions.
There are three common OS structures:
1. Simple Structure
2. Layered Structure
3. Microkernel Structure
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(1) Simple Structure
There are numerous commercial systems that do not
have well-defined structure.
 Frequently, such operating systems started as small,
simple, and limited systems, and then grew beyond their
original scope.
Two examples of this approach
MS-DOS
Original UNIX OS
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MS-DOS Layer Structure….X
MS-DOS – written to provide the
most functionality in the least
space



Not divided into modules.
Although MS-DOS has some
structure, its interfaces and levels of
functionality are not well separated.
Application programs are able to
access the basic I/O routines to
write directly to the display and disk
drives.
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UNIX OS Structure……X
–
limited
by
hardware
functionality,
the
original
UNIX
operating
system
had
limited
structuring.
UNIX
The
UNIX OS consists of two
separable parts Systems programs
The kernel
 Consists of everything below the system-
call interface and above the physical
hardware
 Provides the file system, CPU scheduling,
memory
management,
operating-system
and
functions;
a
number of functions for one level.
other
large
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(2) Layered Structure
The operating system is divided into a number of
layers (levels), each built on top of lower layers. The
bottom layer (layer 0), is the hardware; the highest
(layer N) is the user interface.
An operating-system layer is an implementation of
abstract object that is the encapsulation of data, and of
the operations that can manipulate those data.
With modularity, layers are selected such that each
uses functions (operations) and services of only lowerlevel layers.
This approach simplifies debugging and system
verification.
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(2) Layered Structure (Cont’d)
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(2) Layered Structure (Cont’d)
The major difficulty involves the appropriate definition of
the various layers
They tend to be less efficient than the other types.
These limitations have caused a small backlash against
layering in recent years.
Fewer layers with more functionality are being
designed, providing most of the advantages of
modularized code while avoiding the difficult problems of
layer definition and interaction.
Example, IBM, OS/2 Structure
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OS/2 Layered Structure….X
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(3) Microkernel Structure
Moves as much from the kernel into “user” space.
Moves all nonessential components from the kernel, and
implementing them as system and user-level programs.
In general microkernels typically provide minimal process
and memory management, in addition to communication
facility.
The main function of the microkernel is to provide a
communication facility between the client program and
the various services that are also running in user space.
Communication is provided by message passing.
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(3) Microkernel Structure (Cont’d)
Benefits:
1) Easier to extend the operating system. All new services are
added to user space and consequently do not require
modification of the kernel
2)
Easier to port the operating system to new architectures
3)
More reliable most services are running as user – rather than
kernel processes. Less code is running in kernel mode
4)
More secure
Several contemporary operating systems have used the microkernel
approach like:
o Digital Unix, Apple MacOS X Server. Windows NT uses a hybrid
structure
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Monolithic Kernel vs Microkernel
Architecture………………..X
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Virtual Machines…………….
A virtual machine takes the layered approach to its
logical conclusion.
It treats hardware and the operating system kernel as
though they were all hardware.
By using CPU scheduling and virtual-memory
techniques, an operating system can create the
illusion that a process has its own processor with its
own (virtual) memory.
The virtual-machine approach does not provide any
additional functionality, but rather provides an
interface identical to the underlying bare hardware.
Each process is provided with a (virtual) copy of the
underlying computer.
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Virtual Machines (Cont’d)……X
The resources of the physical computer are shared to
create the virtual machines. Some of what happens is as
follows:
 CPU scheduling can create the appearance that users have
their own processor.
 Spooling and a file system can provide virtual card readers
and virtual line printers.
 A normal user time-sharing terminal serves as the virtual
machine operator’s console.
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Virtual Machines (Cont’d)……X
Advantages/Disadvantages of Virtual
Machines………X
The
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virtual-machine concept provides complete
protection of system resources since each virtual
machine is isolated from all other virtual machines.
This isolation, however, permits no direct sharing of
resources. (Advantage)
A virtual-machine system is a perfect vehicle for
operating-systems research and development. System
development is done on the virtual machine, instead of
on a physical machine and so does not disrupt normal
system operation. (Advantage)
The virtual machine concept is difficult to implement
due to the effort required to provide an exact duplicate
to the underlying machine. (Disadvantage)
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Java Virtual Machine (JVM)…….
Compiled
Java programs are platform-neutral
bytecodes executed by a Java Virtual Machine (JVM).
JVM consists of:
1)
2)
3)
Class loader
Class verifier
Runtime interpreter
Just-In-Time (JIT) compilers are used to compile the
bytecode generated by JVM to increase program
performance.
Java Virtual Machine
(Cont’d)………
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System Implementation……X
Traditionally written in assembly language, operating
systems can now be written in higher-level languages.
Code written in a high-level language:
can be written faster
is more compact
is easier to understand and debug
An operating system is far easier to port (move to some
other hardware) if it is written in a high-level language
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System Generation (SYSGEN)……X
Operating systems are designed to run on any of a
class of machines; the system must be configured for
each specific computer site.
System Generation (SYSGEN) program obtains
information concerning the specific configuration of the
hardware system.
Booting – starting a computer by loading the kernel.
Bootstrap program or Bootstrap loader – code stored in
ROM that is able to locate the kernel, load it into
memory, and start its execution.