Chapter 2: Operating-System
Structures
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Objectives
• Describing services operating system provides to users and system programs.
• Discussing the various ways of structuring an operating system.
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Chapter 2: Operating-System Structures
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Operating System Services.
User Operating System Interface.
System Calls interface.
System Programs.
Operating System Design and Implementation.
Operating System Structure.
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Operating System Services
• There are two sets of operating-system services one provided for
users and the other set provided for the system programs
– The one set of operating-system services that provides functions helpful to
the user:
• User interface (UI)- Command-Line (CLI) and Graphics User Interface
(GUI), batch interface.
• Program execution - The system must be able to load a program into
memory and to run that program, end execution, either normally or
abnormally
• I/O operations- programs require I/O operations.
• File-system manipulation - (reading, writing , searching , listing file
Information, managing permission, creating and deleting of files and
directories).
• Processes communications (on the same computer or over a network),
via shared memory or by message passing.
• Error detection (in CPU, memory hardware, I/O devices or user program),
and taking the appropriate
action for consistent computing.
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– E.g. Debugging facilities user and programmer to manage errors.
Operating System Services (cont)
– Another set of OS functions exists for ensuring the efficient operation of the
system itself via resource sharing
• Resource allocation - When multiple users or multiple jobs running
concurrently, resources must be allocated to each of them (CPU cycles ,
main memory, and file storage).
• Accounting - To keep track of which users use how much and what kinds of
computer resources.
• Protection (means concurrent processes should not interfere with each
other)
• Security for the system from outsiders requires (authentication,
authorization..)
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User Operating System Interface (CLI, GUI)
• CLI(Command Line Interface) allows direct command entry
– Implementation could be in kernel
– Systems program or
– Multiple flavors implementation (e.g. shells in UNIX).
• GUI (Graphic User Interface) is a user-friendly desktop graphical interface by
using mouse, keyboard, and monitor
– Icons represent directories(folders), files, programs, actions, etc
– GUI invented at Xerox PARC in 1973 then by Apple Macintosh computers in
the 1980s.
• Hybrid CLI and GUI: Many systems now include both CLI and GUI interfaces
– (e.g. Microsoft Windows is GUI with CLI “command” shell,
– Apple Mac OS X as “Aqua” GUI interface with UNIX kernel underneath and
shells available,
– Solaris is CLI with optional GUI interfaces (Java Desktop, KDE).
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Example CLI in windows (DOS)
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Example GUI in windows
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Example GUI in LINUX
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Example Interface of Android
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System Calls
• System calls (for programmers) are programming interface to the
provided by the OS.
• Typically written in a high-level language (C or C++).
• Managed in two ways by programs of high-level
– Application Program Interface (API) or
– Direct system call use.
• Three most common APIs are
– Win32 API for Windows,
– POSIX API for POSIX-based systems
– Java API for the Java virtual machine (JVM).
• Why use APIs rather than system calls?
– Portability on any system that supports the same API.
– To avoid more details and difficulties working with actual system
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call.
API – System Call – OS Relationship(Example)
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Standard C Library (Example)
• C program invoking printf() library call, which calls write()
system call.
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Types of System Calls
• Process control
• File management
• Device management
• Information maintenance
• Communications
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System Programs
• System programs is the users’ view of the operating system, not the actual system
calls.
• Some of them are simply translated from user program to system calls; others are
more complex.
• System programs used for providing convenient environment for program
development and execution.
• System programs they can be divided into:
– File manipulation .
– Status information.
– File modification.
– Programming language support.
– Program loading and execution.
– Communications.
– Application programs.
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System Programs (Examples)
• Status information
– Some systems implement a registry - used to store and retrieve configuration
information
• File management - Create, delete, copy, rename, print, dump, list for manipulating
files and directories.
• status information that ask for information like date, time, amount of available
memory, disk space, number of users etc.
• Others provide detailed performance, logging, and debugging information like
audit and log files.
• The programs format and print the output to the terminal or other output devices.
• Some systems programs implement a registry - used to store and retrieve
configuration information.
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System Programs (Examples)
• File modification e.g. Text editors to create and modify files, special commands to
search contents of files or perform transformations of the text.
• Programming-language support - Compilers, assemblers, debuggers and
interpreters.
• Program loading and execution- Absolute loaders, relocatable loaders, linkage
editors, and overlay-loaders, debugging systems for higher-level and machine
language.
• Communications - Provide the mechanism for creating virtual connections among
processes, users, and computer systems.
• Allow users to send messages to one another’s screens, browse web pages, send
electronic-mail messages, log in remotely, transfer files from one machine to
another.
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Operating System Design and Implementation
• Design and Implementation of OS not “solvable”, but some approaches have
proven successful.
• Reasons
– Internal structure of different Operating Systems can vary.
– Start by defining goals and specifications.
– Affected by choice of hardware, type of system, users requirements.
• 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
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Operating System Design and Implementation
• It is important principle for Designers to separate between Policy and Mechanism.
– Policies decide what will be done.
– Mechanisms determine how to do something.
• The separation of policy from mechanism is a very important principle, it allows
maximum flexibility if policy decisions are to be changed later.
• There are several designs for OS
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Simple Structure
Layered Approach
Microkernel System Structure
Modules
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Simple Structure
• E.g. MS-DOS – written to provide the most functionality in the least space
• MS-DOS Not divided into modules, structured but its interfaces and levels of
functionality are not well separated.
• Example: MS-DOS
MS-DOS Layer Structure
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Layered Approach
• The operating system divided into layers, each built on top of lower layers.
• The bottom layer (layer 0), is the hardware; the highest is the user interface.
• With modularity, layers are selected such that each uses functions (operations) and
services of only lower-level layers.
• Example: UNIX systems
Layered Operating System
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UNIX
• UNIX OS is layered structured consists of two separable parts: systems programs
and the kernel.
• Kernel consists of everything below the system-call interface and above the
physical hardware.
• Kernel provides the file system, CPU scheduling, memory management, and other
operating-system functions;
• A large number of functions for one level.
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Microkernel System Structure
• Moved as much from the kernel into “user” space.
• Communication takes place between user modules using message passing.
• Benefits of
– Easier to extend the operating system (new modules added to user space).
– Easier to port the operating system to new hardware design.
– More reliable (less code is running in kernel mode).
– More secure.
• disadvs:
– Performance overhead of user space to kernel space communication
• Example: Tru64 UNIX
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Mac OS X Structure
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Modules
• Most modern operating systems implement kernel modules.
• kernel modules characterized by the use of object-oriented approach.
• Each core component is separate.
• Each modules can call the others over known interfaces.
• Modules are loadable within the kernel as needed .
• Overall, similar to layers but with more flexibility.
• Example: Solaris, Linux, and Mac OS X.
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Solaris Modular Approach
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Virtual Machines
• A virtual machine is a design approach takes the layered approach to its logical
conclusion.
• It treats hardware and the operating system kernel as they were all hardware.
• A virtual machine provides an interface identical to the underlying bare hardware.
• The operating system creates a virtual view of multiple processes, each executing
on its own processor with its own memory.
• The resources of the physical computer are shared to create the virtual machines.
• CPU scheduling can create the appearance that users have their own processor.
• Print spooling(responsible of allocating printer tasks) and a file system can provide
virtual line printers and virtual device readers
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Virtual Machines (Cont.)
• Adv
– The virtual-machine concept provides complete protection of system resources
since each virtual machine is isolated from all other virtual machines.
• this isolation permits no direct sharing of resources.
– 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.
• Disadv
– The virtual machine concept is difficult to implement due to the effort required
to provide an exact duplicate to the underlying machine
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Virtual Machines vs Nonvirtual machine
Non-virtual Machine
Virtual Machine
(a) Nonvirtual machine (b) virtual machine
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Example:The Java Virtual Machine
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End
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