Download Operating Systems

Operating Systems
At its simplest level the operating system does two things:
 It manages the hardware and software resources of the computer system. The
resources include such things as the processor, memory, storage and I/O.
 It provides a stable, consistent way for applications to deal with the hardware
without having to know all the details of the hardware.
User Input
Program Output
Applications Software
Service Request
Service Response
Operating System
Hardware Instructions
Processing Results
Computer Hardware
Operating Systems perform a wide range of functions that can be loosely grouped as
follows:
1.
2.
3.
Resource Allocation
Managing Processes & Files
Providing a User Interface
Operating systems can be divided into two basic types.

Single-process systems

Multiple-process systems
Multiple-process operations can be organised in three different ways
o
multiuser
o
multitasking
o
multiprocessor
Each of the different types of operating systems allows the computer to be utilised in a
number of different modes:

Interactive

Batch

Real-time
1 Allocate hardware resources
CPU Time
Time in the CPU is divided into time slices which are measured in milliseconds.
Each task the CPU does is assigned a certain number of time slices. When time
expires, another task gets a turn. The first task must wait until it has another turn.
Since time slices are so small, you usually can't tell that any sharing is going on.
Tasks can be assigned priorities so that high priority (foreground) tasks get more
time slices than low priority (background) tasks.
Memory
Memory must be managed also by the operating system. All those rotating turns of
CPU use leave data waiting around in buffers. Care must be taken not to lose data!!
One way to help out the traffic jam is to use virtual memory. This includes disk
space as part of main memory. While it is slower to put data on a hard disk, it
increases the amount of data that can be held in memory at one time. When the
memory chips get full, some of the data is paged out to the hard disk. This is called
swapping. Windows uses a swap file for this purpose.
Storage
A more technical task is that of disk management. Under some operating systems
your hard disk can be divided up, or partitioned into several virtual disks. The
operating system treats each virtual disk as though it were a physically separate disk.
Input and Output
Flow control is also part of the operating system's responsibilities. The operating
system must manage all requests to read data from disks or tape and all writes to these
and to printers.
To speed up the output to printers, most operating systems allow for print spooling,
where the data to be printed is first put in a file. This frees up the processor for other
work in between the times data is going to the printer. A printer can only handle so
much data at a time. Without print spooling you'd have to wait for a print job to finish
before you can do anything else. With it you can request several print jobs and go on
working. The print spool will hold all the jobs and process them in turn.
2 File and Process Managagement
Keeping track of what files are where, is a major job. So an operating system comes
with basic file management commands. A user needs to be able to create and delete
directories for storing files. A user needs to copy, move, delete, and rename files.
This is the group of operating system functions that the user utilises the most.
Directories are used to store information about files and to organise files into related
groups. File information stored in a directory includes name, type, location, size,
ownership, access controls and time stamps.
File security
A file-management system provides facilities to prevent the loss, corruption and
unauthorised access to data stored in files. These include ownership and access
controls and multiuser file access controls.
Process Control
Processes (also called tasks) are the basic unit of software to which resources are
allocated. Each process has a process control block (PCB) that contains information
such as the current state of the process. Generally a process can be in one of three
states: running, ready or blocked.
In multiple-process systems, the operating system is designed so that it can appear to
work on several tasks simultaneously. Tasks, in turn, can be made up of several
threads that can be worked on separately.
System Performance
A user or administrator can check to see whether the computer or network is getting
overloaded. System performance would include response time ( how long it takes for
the computer to respond when data is entered) and CPU utilization (comparing the
time the CPU is working to the time it is idle.)
3 User Interface
Interfaces


Application
User
The operating system functions as an interface on two levels. First, the operating
system interfaces software with hardware. Programs call the operating system to
handle most of the details concerned with controlling the computer hardware. An
application program interface (API) allows the operating system to run the same
application on different hardware combinations.
Second, the operating system interfaces users with the computer system. The
interface may be either a command line interface or a graphical user interface (GUI).
An interface between the user and the kernel is called a shell. It acts as an interpreter
or translator. In other words, the shell accepts commands issued by the user,
interprets these commands, and executes the appropriate programs.
System Security
Some system security is part of the operating system, though additional software can
add more security functions. Single-user systems have to prevent the internals of the
operating system from being accidentally or deliberately modified. For multiple users
who are not all allowed access to everything, there must be a logon or login procedure
where the user supplies a user ID and a secret password. An administrator must set
up the permissions list of who can have access to what programs and what data.
Glossary
Multitasking: Only one CPU is involved, but it switches from one program to another
so quickly that it gives the appearance of executing all of the programs at the same
time.
Multiuser: Refers to computer systems that support two or more simultaneous users.
Multiprocessor: A computer system in which two or more processors have common
access to main storage.