UNIT I
PROCESSES AND THREADS
1. INTRODUCTION
An Operating System is a program that manages the computer hardware. It also provides
a basis for application programs and acts as an intermediary between a user of a computer
and the computer hardware.
1.1 What Is An Operating System?
An Operating System is an important part of almost every computer system. A
computer system can be divided roughly into four components:
i.
Hardware
ii.
Operating System
iii.
Application Programs
iv.
Users – that is shown in Figure1.1
User 1
Compiler s
User 2
User 3
Assembler
Text Editor
…….
User n
Database System
System and Application Program
Operating System
Computer Hardware
Figure 1.1: Abstract view of the components of a computer system
The hardware-the central processing unit (CPU), the memory, and the input/output (I/O)
devices-provides the basic computing resources.
The application programs-such as word processors, spreadsheets, compilers, and
web browsers-define the ways in which these resources are used to solve the computing
problems of the users. The operating system controls and coordinates the use of the
hardware among the various application programs for the various users. The operating
system provides the means for the proper use of these resources in the operation of the
computer system.
Operating systems can be explored from two viewpoints: the user and the system.
1.1.1 User View
The user view of the computer varies by the interface being used. For PC’s the
operating system is designed mostly for ease of use, with some attention paid to
performance, and none paid to resource utilization.
For mainframe or minicomputer, users will access the same computer through other
terminals. Hence the OS is designed to maximize resource utilization to assume that all
available CPU time, memory and input/output devices are used efficiently.
For workstations connected to networks, the OS is designed to compromise
between individual usability and resource utilization.
For handheld computers, the OS is designed for individual usability and also
performance per amount of battery life.
1.1.2 System View
From the computer's point of view, the operating system is the program is most
intimate with the hardware. We can view an operating system as a resource allocator. A
slightly different view of an operating system is a control program.
As a resource allocator, it faces numerous and possibly conflicting requests for
resources, the operating system must decide how to allocate them to specific programs and
users so that it can operate the computer system efficiently and fairly.
A control program manages the execution of user programs to prevent errors and
improper use of the computer.
1.2 Mainframe Systems
Mainframe computer systems were the first computers used to tackle many
commercial and scientific applications. The growth of mainframe systems can be
categorized as
Batch Systems  Multiprogrammed systems  Time sharing systems
1.2.1 Batch Systems
The major task of operating system in early computers was to transfer control
automatically from one job to the next. The operating system was always resident in
memory (Figure 1.2).
To speed up processing, operators batched together jobs with similar needs and
ran them through the computer as a group. Thus, the programmers would leave their
programs with the operator. The operator would sort programs into batches with similar
requirements and, as the computer became available, would run each batch. The output
from each job would be sent back to the appropriate programmer.
OS
User Program
Area
Figure 1.2 Memory layout for a simple batch system
1.2.2 Multiprogrammed Systems
Multiprogramming increases CPU utilization by organizing jobs so that the CPU
always has one to execute. The operating system keeps several jobs in memory
simultaneously (Figure 1.3). The operating system picks and begins to execute one of the
jobs in the memory. Eventually, the job may have to wait for some task, such as an I/O
operation, to complete. At that time the operating system switches to execute another job.
When that job needs to wait, the CPU is switched to another job, and so on. Eventually, the
first job finishes waiting and gets the CPU back.
0
Operating System
Job 1
Job 2
Job 3
Job 4
512k
Figure 1.3 Memory layout for a multiprogramming system
If several jobs are ready to be brought into memory, and if there is not enough room
for all of them, then the system must choose among them. Making this decision is called
Job Scheduling.
1.2.3 Time-Sharing Systems
Time sharing (or multitasking) is a logical extension of multiprogramming. Where
the CPU executes multiple jobs by switching among them, but the switches occur so
frequently that the users can interact with each program while it is running. A time-shared
operating system allows many users to share the computer simultaneously.
A time-shared operating system uses CPU scheduling and multiprogramming to
provide each user with a small portion of a time-shared computer. Each user has at least
one separate program in memory. A program loaded into memory and executing is
commonly referred to as a process. Time-sharing operating systems are even more
complex than multiprogrammed operating systems. In both, several jobs must be kept
simultaneously in memory, so the system must have memory management and protection
To obtain a reasonable response time, jobs may have to be swapped in and out of
main memory to the disk that now serves as a backing store for main memory. A common
method for achieving this goal is virtual memory, which is a technique that allows the
execution of a job that may not be completely in memory.
The main advantage of the virtual-memory scheme is that programs can be larger
than physical memory. Further, it abstracts main memory into a large, uniform array of
storage, separating logical memory as viewed by the user from physical memory. This
arrangement frees programmers from concern over memory-storage limitations.
Time sharing systems provide a mechanism for concurrent execution, which
requires sophisticated CPU-scheduling schemes. To ensure orderly execution, the system
must provide mechanisms for job synchronization and communication and it may ensure
that jobs do not get stuck in a deadlock, forever waiting for one another.
Time sharing systems must also provide a file system. The file system resides on a
collection of disks, hence, disk management must be provided.
1.3 Desktop Systems
PC operating systems were neither multiuser nor multitasking. The goals of these
operating systems have changed with time; instead of maximizing CPU and peripheral
utilization, the systems opt for maximizing user convenience and responsiveness.
These systems include PCs running Microsoft Windows and the Apple Macintosh.
The hardware cost for microcomputers are sufficiently low that individuals have sole use
of the computer and the CPU utilization is no longer a prime concern.
1.4 Multiprocessor Systems
Multiprocessor systems or parallel systems or tightly coupled systems have more
than one processor in close communication, sharing the computer bus, the clock, and
sometimes memory and peripheral devices.
Multiprocessor systems have three main advantages.
Increased throughput: By increasing the number of processors, getting more work done
in less time.
Economy of scale: by sharing peripherals, mass storage, and power supplies among
multiple processors.
Increased reliability: by distributing functions among multiple processors where in
failure of one processor will not halt the system.
The most common multiple-processor systems now use symmetric multiprocessing (SMP), in which each processor runs an identical copy of the operating
system, and these copies communicate with one another as needed. Here all processors
are pears no master-slave relationship between processors.
Some systems use asymmetric multiprocessing, in which each processor is
assigned a specific task. A master processor controls the system; the other processors
either look to the master for instruction or have predefined tasks.
As microprocessors become less expensive and more powerful, additional
Operating-system functions are off-loaded to slave processors (or back-ends).
CPU
CPU
………
CPU
Memory
Figure 1.4 Symmetric multiprocessing architecture
1.5 Distributed Systems
Distributed systems depend on networking for their functionality. By being able to
communicate, distributed systems are able to share computational tasks, and provide a
rich set of features to users.
Networks vary by the protocols used, the distances between nodes, and the
transport media to an operating system, a network protocol simply needs an interface
device-a network adapter, and software to package data in the communications protocol
to send it and unpackaged it to receive it.
Networks are type cast based on the distances between their nodes.
A Local-Area Network(LAN) exist within a room, a floor or a building.
A Wide-Area Network(WAN) usually exits between buildings, cities or countries.
A Metropolitan-Area Network(MAN) could link buildings within a city.
1.5.1 Client-Server Systems
In client server system, terminals connected to centralized system are being
supplanted by PCs. The general structure of a client-server system is depicted in Figure
1.5.
Client
Client
Client
….
Client
network
Figure 1.5 General
Structure of a client-server system
Server
Server systems can be broadly categorized as compute servers and file servers.
Compute-server systems provide an interface to which clients can send requests to
perform an action, in response to which they execute the action and send back results to
the client.
File-server systems provide a file-system interface where clients can create, update, read,
and delete files.
1.5.2 Peer-to-Peer Systems
The system has a collection of processors that do not share memory or a clock.
Instead, each processor has its own local memory. The processors communicate with one
another through various communication lines, such as high-speed buses or telephone
lines. These systems are usually referred to as loosely coupled systems (or distributed
systems).
A network operating system is an operating system that provides features such as file
sharing, across the network and that includes a communication scheme that allows
different processors and different computers to exchange messages.
1.6 Clustered Systems
Clustered systems gather together multiple CPUs to accomplish computational work.
Clustered systems differ from parallel systems, however, in that they are composed of two
or more individual systems coupled together. The clustered computers share storage and
are closely linked via LAN networking.
Clustering is usually performed to provide high availability. A layer of cluster
software runs on the cluster nodes. Each node can monitor one or more of the others (over
the LAN). If the monitored machine fails, the monitoring machine can take ownership of
its storage, and restart the application(s) that were running on the failed machine.
In asymmetric clustering, one machine is in hot standby mode while the other is
running the applications. The hot standby host (machine) does nothing but monitor the
active server. If that server fails, the hot standby host becomes the active server.
In symmetric mode, two or more hosts are running applications, and they are
monitoring each other. This mode is obviously more efficient, as it uses all of the
available hardware. It does require that more than one application be available to run.
1.7 Real-Time Systems
A real-time system is used when rigid time requirements have been placed on the
operation of a processor or the flow of data; thus, it is often used as a control device in a
dedicated application.
Sensors bring data to the computer. The computer must analyze the data and
possibly adjust controls to modify the sensor inputs.
A real-time system has well-defined, fixed time constraints. Processing must be
done within the defined constraints, or the system will fail
Real-time systems come in two flavors: hard and soft.
Hard Real Time System:
A hard real-time system guarantees that critical tasks be completed on time. This
goal requires that all delays in the system be bounded, from the retrieval of stored data to
the time that it takes the operating system to finish any request made of it.
Soft Real Time System:
A less restrictive type of real-time system is a soft real-time system, where a critical realtime task gets priority over other tasks, and retains that priority until it completes.
As in hard real-time systems, the operating-system kernel delays need to be bounded.
A real-time task cannot be kept waiting indefinitely for the kernel to run it.
Soft real time is an achievable goal that can be mixed with other types of systems.
They are useful in several areas, including multimedia, virtual reality, and advanced
scientific projects-such as undersea exploration and planetary rovers.
1.8 Handheld Systems
Handheld systems include personal digital assistants (PDAs), such as palm pilots or
cellular telephones with connectivity to a network such as the Internet. Developers of
handheld systems and applications face many challenges, most of which are due to the
limited size of such devices.
For example, a PDA is typically about 5 inches in height and 3 inches in width, and
it weighs less than one-half pound. Due to this limited size, most handheld devices have a
small amount of memory, include slow processors, and feature small display screens.
A second issue of concern to developers of handheld devices is the speed of the
processor used in the device. Processors for most handheld devices often run at a fraction
of the speed of a processor in Pc. Faster processors require more power. To include a
faster processor in a handheld device would require a larger battery that would have to be
replaced (or recharged) more frequently.