1
Module 1
OS Services –OS Operations - Services System Calls
OS Structure - System Boot
Dr. Abdul Quadir Md, SCOPE
VIT Chennai
2
Operating System Services
Operating system services 3
OS provides an environment for the execution of
programs
It provides set of services to programs and to users of
those programs
Operating system services 4
One set of services to the user
– User interface
– Program execution
– I/O operations
– File system manipulations
– Communications
– Error detection
Operating system services 5
Set of services ensuring the efficient operation of the
system
– Resource allocation
– Accounting
– Protection and security
6
OS - User services
User interface
– Nearly all OS have a user interface
– These can CLI, Batch and GUI
Program execution
– The system must be able to load the
program in memory and execute
7
OS - User services
I/O operation
– a running program may require I/O
– users cannot control I/O devices directly
– OS must provides means to do I/O
File system manipulations
–read and write files and directories
– create, delete, search and list files
– Permission management
8
OS - User services
Communication
- tied together by computer networks
– shared memory or through message passing
- packets of information moves between processes
by the OS
9
OS - User services
Error detection
In CPU - power failure
Memory hardware - memory error
In I/O devices - lack of paper in printer
In User programs - arithmetic over flow
10
OS - System related services
Resource allocation
–multiple users running at the same time
-resources must be allocated to all
–types of resources managed are
• Main memory
• File storage
• CPU cycles
11
OS - System related services
Accounting
Keeps track of
-users activity
-resource allocation
- utilization
– Usage statistics may be valuable tool for
researchers to improve computing services
12
OS - System related services
Protection and security
– When several processes runs concurrently, it not be possible
for one process to interfere
–It ensuring that all access to system resources is controlled
– Security of the system from the outsiders is also important
–Each user to authenticate by password to gain access
13
Operating-System Operations
Operating-System Operations 14
Interrupt driven (hardware and software)
Hardware interrupt by one of the devices
Software interrupt (exception or trap):
Software error (e.g., division by zero)
Request for operating system service
Other process problems include infinite loop, processes modifying
each other or the operating system
Operating-System Operations (cont.) 15
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
Increasingly CPUs support multi-mode operations
i.e. virtual machine manager (VMM) mode for guest VMs
Transition from User to Kernel Mode 16
Timer to prevent infinite loop / process hogging resources
Timer is set to interrupt the computer after some time period
Keep a counter that is decremented by the physical clock.
Operating system set the counter (privileged instruction)
When counter zero generate an interrupt
Set up before scheduling process to regain control or terminate program that
exceeds allotted time
Kernal 17
Source: WIkipedia
18
Systems Calls
System calls
19
OS - mechanism to request services from the kernel
Service request points are called “system calls”
In Linux system - interface layer between the user space and
kernel space.
This layer consists of library made up of functions that
communicate between normal user applications and the Kernel.
System calls
20
C library is called libc/glibc
This provides wrapper functions for the system calls
Implemented using software interrupt or trap
Sets the
-System call number
-C arguments into the general registers
-Executes some machine instruction that generates a software
interrupt in the kernel
21
Example on how system calls are used
Simple program to read data from one file and copy them to another file
Program inputs
-Name of two files
-opens the input file and creates output file and open it
Each of these operations require another system calls
Now that both files are setup, enter into the loop that
-read from input file (system call) and
-writes into the output file(another system call)
Example of System Calls 22
System call sequence to copy the contents of one file to another
file
23
Interfacing between user and kernel
space
C library interface
24
the ‘printf’ statement
The C library interprets this call and invokes the
necessary system call(s) in OS
the write() system call
The C library takes the value returned by write()
and passes it back to the user program.
system call and library function
25
Examples of systems calls
• Read() / write ()
• Fork()/ exec ()
Example of library functions
• Date/time
• Strcpy/atoi/printf
System call Categories /Types 26
Five major Categories/ Types:
– Process management
– File management
– Device management
– Information/Maintenance
– Communication
Process management 27
– create process the process
– Terminate process
– Load the process
– Execute the process
– get/set process attributes
– wait for time, wait event, signal event
File management 28
–create file, delete file
– open, close
– read, write, reposition
– get/set file attributes
Device Management 29
– request device, release device
– read, write, reposition
– get/set device attributes
– logically attach or detach devices
Information Maintenance 30
– get/set time or date
– get/set system data
– get/set process, file, or device attributes
Communication 31
– create, delete communication connection
– send, receive messages
– transfer status information
– attach or detach remote devices
32
Operating System Structure
Operating System Structure 33
General-purpose OS is very large program
Various ways to structure ones
Simple structure – MS-DOS
More complex -- UNIX
Layered – an abstrcation
Microkernel -Mach
Simple Structure -- MS-DOS 34
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
Non Simple Structure -- UNIX 35
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
Traditional UNIX System Structure 36
Beyond simple but not fully layered
Layered Approach 37
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.
With modularity, layers are
selected such that each uses
functions (operations) and services
of only lower-level layers
Microkernel System Structure 38
Moves as much from the kernel into user space
Mach example of microkernel
Mac OS X kernel (Darwin) partly based on Mach
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
Microkernel System Structure 39
Modules
40
Many modern operating systems implement loadable kernel
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
Overall, similar to layers but with more flexible
Linux, Solaris, etc
41
Solaris Modular Approach
Hybrid Systems 42
Most modern operating systems are actually not one pure model
Hybrid combines multiple approaches to address performance, security,
usability needs
Linux and Solaris kernels in kernel address space, so monolithic, plus
modular for dynamic loading of functionality
Windows mostly monolithic, plus microkernel for different subsystem
personalities
Apple Mac OS X hybrid, layered, Aqua UI plus Cocoa programming
environment
Below is kernel consisting of Mach microkernel and BSD Unix parts, plus
I/O kit and dynamically loadable modules (called kernel extensions)
Mac OS X Structure
43
iOS 44
Apple mobile OS for iPhone, iPad
Structured on Mac OS X, added functionality
Does not run OS X applications natively
Also runs on different CPU architecture (ARM vs. Intel)
Cocoa Touch Objective-C API for developing apps
Media services layer for graphics, audio, video
Core services provides cloud computing, databases
Core operating system, based on Mac OS X kernel
Android 45
Developed by Open Handset Alliance (mostly Google)
Open Source
Similar stack to IOS
Based on Linux kernel but modified
Provides process, memory, device-driver management
Adds power management
Runtime environment includes core set of libraries and Dalvik virtual machine
Apps developed in Java plus Android API
Java class files compiled to Java bytecode then translated to executable than runs in Dalvik VM
Libraries include frameworks for web browser (webkit), database (SQLite),
multimedia, smaller libc
Android Architecture 46
47
System Boot
System Boot 48
When power initialized on system, execution starts at a fixed memory
location
Firmware ROM used to hold initial boot code
Operating system must be made available to hardware so hardware can
start it
Small piece of code – bootstrap loader, stored in ROM or EEPROM locates the
kernel, loads it into memory, and starts it
Sometimes two-step process where boot block at fixed location loaded by
ROM code, which loads bootstrap loader from disk
Common bootstrap loader, GRUB, allows selection of kernel from multiple
disks, versions, kernel options
Kernel loads and system is then running
49
ns ?
e st io
y Q u
A n
50
References
http://www.digilife.be/quickreferences/qrc/linux%20system%20call%
20quick%
Silberschatz, Gagne, Galvin: Operating System Concepts, 6th Edition
http://docs.cs.up.ac.za/programming/asm/derick_tut/syscalls.html
51
You!
hank
T
