2 operating system structures

Operating-System Structures
 Operating System Services
 User Operating System Interface
 System Calls
 Types of System Calls
 System Programs
 Operating System Design and Implementation
 Operating System Structure
 Virtual Machines
 Operating System Generation
 System Boot
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Loganathan R, CSE , HKBKCE

1. Operating System Services
• One set of operating-system services provides functions that are helpful to the user:
– User interface - Almost all operating systems have a user interface (UI)
Command-Line (CLI), Batch Interface & Graphics User Interface (GUI)
– Program execution - Load a program into memory and to run that program, end
execution, either normally or abnormally (indicating error)
– I/O operations - A running program may require I/O, which may involve a file or
an I/O device.
– File-system manipulation - Programs need to read and write files and
directories, create and delete them, search them, list file Information,
permission management.
– Communications – Between process in the same or different computer via
shared memory or message passing
– Error Detection - Errors may occur in the CPU and memory hardware ( memory
error or power failure), in I/O devices (parity error on tape, a connection failure
on a network, or lack of paper in the printer) and in the user program
(arithmetic overflow, illegal memory location access, or a too-great use of CPU
time)

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1. Operating System Services Contd..
• 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
• Many types of resources - Some (such as CPU cycles, main memory, and
file storage) may have special allocation code, others (such as I/O
devices) may have general request and release code.
– Accounting - To keep track of which users use how much and what kinds of
computer resources
– Protection and security - The owners of information stored in a multiuser or
networked computer system may want to control use of that information,
concurrent processes should not interfere with each other
• 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 it. A chain is only as strong as its weakest link.

Loganathan R, CSE , HKBKCE 3

2.User Operating System Interface
• 2.1 Command Interpreter / CLI
• Implemented in kernel, sometimes by systems program
• Function of the command interpreter is to get and execute
the user-specified command.
• Multiple command interpreters to choose from the
interpreters are known as shells
• Example :Bourne shell, C shell, Bourne-Again shell, the Korn
shell
• Command Interpreter Implementation
• Command interpreter itself contains the code to execute the
command
• Through system programs – uses the command to identify a file to
be loaded into memory and executed
• Example rm file.txt


User Operating System Interface Cntd..
 2.2 Graphical User Interfaces
 User-friendly desktop metaphor interface
◦ Usually mouse, keyboard, and monitor
◦ Icons represent files, programs, actions, etc
◦ Various mouse buttons over objects in the interface cause various
actions (provide information, options, execute function, open
directory (known as a folder)
◦ Invented at Xerox PARC
 Many systems now include both CLI and GUI interfaces
◦ 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)


3. System Calls
 Programming interface to the services provided by the OS
 Typically written in a high-level language (C or C++)
 Example of how system calls are used to copy the files


Application Programming Interface(API)
 System calls are mostly accessed by programs via a high-
level Application Program Interface (API) rather than
direct system call use
• API specifies a set of functions that are available to an
application programmer, including the parameters that are
passed
 Three most common APIs are Win32 API for Windows,
POSIX API for POSIX-based systems (including virtually all
versions of UNIX, Linux, and Mac OS X), and Java API for
the Java virtual machine (JVM)
 Why use APIs rather than system calls?
• Program portability
• Actual system calls can be more detailed and difficult to
work with than the API


System Call Implementation
• Programming languages provides a system-call interface that serves as the
link to system calls made available by the OS
• System-call interface intercepts function calls in the API and invokes the
necessary system call within the OS
• System-call interface maintains a table indexed according to these numbers
associated with each system call
• The system call interface invokes intended system call in OS kernel and
returns status of the system call and any return values

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System Call Parameter Passing
• Registers
• Parameters stored in a block, or table, in memory, and address of block
passed as a parameter in a register (Linux and Solaris) as shown
• Parameters placed, or pushed, onto the stack by the program and popped off
the stack by the operating system
– Block and stack methods do not limit the number or length of parameters
being passed


4. Types of System Calls
4.1 Process control
• End, abort • Load, execute
• Create process terminate –process
• get process attributes, set process attributes
• Wait for time • Wait event, signal event
• Allocate and free memory

fork()
exec()
exit()

Single Tasking: MS-DOS execution, Multitasking: FreeBSD
(a) At system startup, (b) Running a running multiple programs
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program Loganathan R, CSE , HKBKCE

4. Types of System Calls Contd
4.2 File management
• File – create, delete, open, close, read, write, reposition
• Get / set file attributes
4.3 Device management
The various resources controlled by the OS can be thought of as devices
• Device – request, release, read, write, reposition
• get / set device attributes • Logically attach or detach devices
4.4 Information maintenance
Transferring information between the user program and the OS
and keeps information about all its processes and system calls
• get / set – time or date • get / set system data
• get / set – process, file or device attributes
4.5 Communications
Inter-process communication Models :
Message passing model & Shared-memory model
• Create, delete communication connection • Send, receive messages
• Transfer status information • Attach or detach remote devices
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5. System Programs
 System programs provide a convenient environment for
program development and execution.
 Categories
◦ File management - create, delete, copy, rename, print, dump, list,
and manipulate files and directories
◦ Status information - date, time, available memory or disk space, no
of users, performance, logging, and debugging information
◦ File modification - text editors to create and modify the content of
files, search contents of files.
◦ Programming language support - Compilers, assemblers, debuggers
and interpreters
◦ Program loading and execution - absolute loaders, relocatable
loaders, linkage editors, overlay loaders and debuggers
◦ Communications - Creating virtual connections among processes,
users, and computer systems
◦ System utilities Application programs - web browsers, word
processors , text formatters, spreadsheets, database systems,
compilers, plotting and statistical-analysis packages and games
 Most users’ view of the operation system is defined by
system programs, not the actual system calls
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6. OS Design and Implementation
 Design and Implementation of OS not “solvable”,
but some approaches have proven successful
 6.1 Design Goals
– The design of the system is affected by the choice of
hardware and the type of system: batch, time shared,
single user, multiuser, distributed, real time, or general
purpose
– The requirement can be divided to 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


6. OS Design and Implementation Contd..
6.2 Mechanisms and Policies
• Mechanisms determine how to do something, policies decide
what will be done
– Example : timer construct is a mechanism for ensuring CPU
protection(loop), but deciding how long the timer is to be set for
a particular user is a policy decision
– The separation of policy from mechanism is a very important
principle, it allows maximum flexibility if policy decisions are to
be changed later
6.3 Implementation
• Traditionally, written in assembly language, now C or C++
• Advantages of using a higher-level language
– Faster Coding , more compact, easier to understand and debug
– Easier to port—to move to some other hardware
• Disadvantages of using a higher-level language
– Reduced speed and increased storage requirements

7. Operating-System Structure
• How OS components are
interconnected and melded into a
kernel.
7.1 Simple Structure
• Started as small, simple, and limited
systems and then grown beyond their
original scope
• Example : 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
MS-DOS layer structure
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7. Operating-System Structure Contd…
• 7.2 Layered Approach
– 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 H/W The highest (layer N) is the UI.
– A layer is an implementation of an abstract object consists of data
structures and a set of routines that can be invoked by higher-level layers
• Advantage
• Simplicity of construction and
debugging
• Each layer is implemented with only
those operations provided by lower
level layers
• The first layer can be debugged
without any concern for the rest of the
system
• Correct functioning of first layer can
be assumed while the second layer is
debugged, and so on
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Example: UNIX
 UNIX – limited by hardware functionality, the original UNIX operating system
had limited structuring. 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, and other
operating-system functions; a large number of functions for one level

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7.3 Microkernels
• Structures the OS by removing all nonessential
components from the kernel and implementing
them as system and user-level programs.
 Communication takes place between user modules
using message passing
 Benefits:
◦ Easier to extend a microkernel
◦ Easier to port the operating system to new architectures
◦ More reliable (less code is running in kernel mode)
◦ More secure
 Detriments:
◦ Performance overhead of user space to kernel space
communication


7.4 Modules
– Uses object-oriented approach
– Each core component is separate
– Each talks to the others over known interfaces
– Each is loadable as needed within the kernel
– a core kernel with seven types of loadable kernel modules


Mac OS X Structure(Darwin)
• Uses a hybrid structure – layered technique with one layer consists of the Mach
microkernel.
• Top layers include application environments and a set of services providing a
graphical interface to applications
• Below these layers is the kernel consists primarily of the Mach microkernel and
the BSD kernel
• Mach provides m/y mgmt, support RPC and IPC facilities, including message
passing and thread scheduling
• BSD component provides a BSD CLI, support for networking and file systems, and
an implementation of POSIX APIs, including Pthreads


8. Virtual Machines
• A virtual machine takes the layered approach to its logical
conclusion.
• It abstracts hardware of a single computer into several different
execution environments i.e. creates illusion of separate
execution environment is running its own private computer.
• Provides an interface identical to the underlying bare hardware
• The operating system creates the illusion of multiple processes,
each executing on its own processor with its own (virtual)
memory
• The resources of the physical computer are shared to create the
virtual machines
• A major difficulty is disk systems, solved using Virtual disks
(minidisks in IBM)
• Users are given their own virtual machines and they can run any
of the operating systems or software packages that are available


8. Virtual Machines Cont…

Non-virtual Machine Virtual Machine

• 8.1 Implementation
– Much work is required to provide an exact duplicate of
the underlying machine
– A virtual user mode and a virtual kernel mode, both of
which run in a physical user mode.
– Transfer from user mode to kernel mode on a real
machine also cause a transfer from virtual user mode to
virtual kernel mode on a virtual machine
– A system call in virtual user mode will cause a transfer to
the virtual-machine monitor, it can simulate the effect of
the system call by changing the register and program
counter for the virtual machine
– The real I/O may take100 milliseconds, the virtual I/O
might take less time(spooled) or more time(interpreted).

• 8.2 Benefits
– Complete protection of the various system resources
– No direct sharing of resources and implementation
through
• Share a minidisk and thus to share files through
software
• Network of virtual machines over the virtual
communications network
– VM system is a perfect vehicle for OS research and
development
– A virtual-machine system eliminates system development
time
• System must be stopped and taken out of use while
changes are made and tested , this period is called
system development time

• 8.3 Examples
– 8.3.1 Vmware
• Abstracts Intel 80X86 hardware into isolated virtual machines.
• Runs as an application on host OS and allows concurrently
running several different guest operating systems as
independent virtual machines

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• 8.3 Examples
– 8.3. 2 The Java Virtual Machine
– Java program consists of one or more classes, for each, the compiler
produces an architecture-neutral bytecode output (.class) file that will
run on any implementation of the JVM.
– JVMis a specification for an abstract computer
– consists of a class loader and a Java interpreter (or a fast just-in-
time (JIT) compiler ) that executes the architecture-neutral bytecodes

Java Program Java API
Class loader
.class files .class files

Java
Interpreter

Host systems
(Windows, Linux, etc)
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9. Operating-System Generation
• It is possible to design, code, and implement an operating system
specifically for one machine at one site
• A processes of configuring or generating system for each specific
computer site, is known as system generation (SYSGEN)
• The SYSGEN program reads from a given file, or asks the operator for
the specific configuration of the hardware system, or probes the
hardware directly
• Once information is determined , a system administrator can use it
to modify the source code of OS , then compile and the output
object version is tailored to the system
• A slightly less tailored level modules are selected from a
precompiled library and linked together to form OS
• Other extreme, all the code is always part of the system, and
selection occurs at execution time, rather than at compile or link
time.
• The major differences among these approaches are the size and
generality 27

10. System Boot
• Operating system must be made available to hardware so
hardware can start it
• When power initialized on system, execution starts at a
predefined memory location (in ROM) where initial bootstrap
program stored
• Small piece of code – bootstrap program or bootstrap
loader, locates the kernel, loads it into memory, and starts its
execution
• Sometimes two-step process where simple boot block /
bootstrap loader fetches a complex boot program, which
loads kernel
• A disk that has a boot partition is called a boot disk or system
disk.
• Problem with ROM is that changing the bootstrap code
requires changing the ROM hardware chips which is resolved
using erasable programmable read-only memory (EPROM)
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2 operating system structures

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