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feastcanadianSoftware and s/w Development

Dec 14, 2013 (3 years and 7 months ago)

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Functions of an Operating System
































The system to perform automatic management of secondary storage.

A File System


The most basic properties of file storage devices need only:






involves



also known as









involves

























is divided into






















constitutes




is made up of



















functioning is


Functions of an Operating System

Memory Management

System

Device
Management

Process management

file management

Process
synchronisation


Inter
-
process

communication


process scheduling

routines


methods


relocation

protection

logical organization
of program in
memory

swapping
and
loading

partition

paging and segmentation

Input/Output (I/O)
Request

Interrupt
s:

Traps and
Supervisory
calls

Buffering

SPOOLING

-

To create and
delete files

-

To control access to his/her files

-

Reference his file using symbolic names

-

His/her files protected against system failures

-

Automate control of file access to prevent corruption of data

-


The system to perform automatic management of secondary
storage

-

Map access requests from virtual to real file space, and

-

Transmit records between file

storage and main storage


However many diverse operations must be performed in order to service a file request. Suppose one wants a particular record i
n a given file. Then the file system must locate the named file. This entails
searching a dir
ectory to find its unique file identifier. The volume on which this file must be located. If it is not found, appropriate mes
sages must be sent to the operator so that he mounts it. The address of the
desired record is obtained and Input/Output commands to

the hardware is generated in order to effect its transfer to main memory. Buffering is necessary, especially if more than one

record is requested.


The various functions of a file system from logical to physical operations can be described by a se
ries of levels of software. The levels and their functions are:

1)

Symbolic file System: File directory is searched to find the entry for the file name.

2)

Basic file System: file entry information is brought into main memory.

3)

Protection: Schemes to protect file

against unauthorized access.

4)

Logical file system: logical address of requested record is obtained.

5)

Physical file system: logical address transformed into physical address.

6)

Device strategies: Appropriate Input/Output commands are set up for device.



IV) N
etwork Operating Systems:


A Network is a group of two or more computers linked together so that they can share resources ( hardware , software and dat
a) and can communicate with one another. These networks are managed by
operating systems.(
We shall see more on networks later)

NOTE:

An operating system is not the only system software (system software do not meet the end user’s need):

1)

Programming languages such as compilers, interpreters, assemblers are system software i.e

language translators are system software

2)

Utility programs are system software. Utility programs perform routine functions and operations e.g sorting names in alphabet
ical order

3)

Device drivers are system software.






i)

Process Management

b)

Process s
ynchronisation

c)

Inter
-
process communication

d)

CPU/process scheduling



Objectives
-
fairness


-
policy enforcement


-
efficiency


-

response time


-
turnaround


-
throughput


-
waiting time



2 categories of scheduling


-
non preemptive scheduling


-
preemptive scheduling



scheduling policies or deciplines


-
FCFS or FiFo


-
SJF


-
SRT


-
RR



ii)
Memory Management



routines


-
relocation


-
protection


-

logical organization of program in memory


-
main and secondary memory storage and control



methods


-
swapping and loading


-
partition



linked list of free block


-
first fit method


-
best fit method


-
worst fit method



buddy systems


-
paging and segmentation


-
introduction to virtual memory management

iii) Device Management :


It is the process of activating and controlling the pe
ripheral devices in a computer by the OS using drivers

1)

Input/Output (I/O) Request:


This involves management of I/O devices by the OS. A process uses an I/O stream and the characteristics of a device are held
in a device descriptor. W
hen a process wishes to perform I/O, it
makes an I/O request by constructing an input
-
output request block(IORB) that is placed on a device request queue of the desired device. The IORB are taken from the device

request queue by the device
handler which pe
rforms the request and then signals when the request is completed.

The device descriptor must contain:

-

An identification

-

Instructions to operate the device

-

Pointers to character translation tables

-

Current status of device

-

User process currently using
device.


The IORB contains information such as:

-

Destination of I/O

-

Quantity of I/O(e.g character, block)

-

Mode of I/O (e.g read, write)

-

Pointer to originating process

-

Pointer to error location (to handle any error)

-

Pointer to next
IORB


The stream descriptor contains information such as:

-

The stream number

-

Type of device descriptor (e.g file device, printer device)

-

Pointer to the appropriate device driver ( i.e program that determine the elementary functions

of a peripheral device)

-

Pointer to matching device descriptor

-

Information on buffers used

2)

Interrupts: ( An interrupt is a signal generated by hardware or software which may cause a break in the execution of the CPU
when I/O is required)


This invo
lves identifying the source of the interrupt and responding to it accordingly. This must be done within the “crisis time” , i
f any, of the device, else there will be loss of information.


The Interrupt Handler(IH) first saves the values of the regis
ters it could need. Besides, Interrupt Handler(IH) operates in a dedicated area of memory and uses very few registers. Identi
fying the source
of an interrupt depends on the hardware.


Interrupts may be disabled by software i.e. they are held pending u
ntil when they are enabled. Interrupts routine may be selectively disarmed. This may be done through a mask register whose bi
ts are
set of particular interrupts to be ignored; control is not passed to the appropriate routine.