Download 1.2. OS Introduction Computer System Types

Operating System Principles
AUA CIS331
Albert Minasyan
1.2.2. Desktop Systems
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The goals of the operating systems have changed with time
Instead of maximizing CPU and peripheral utilization, the systems opt for maximizing user
convenience and responsiveness.
With improvement of chip technology the processors and computing power resources
became cheaper.
Then appear real possibilities to create user friendly operating systems.
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Personal computers PCs appeared in the 1970s. During their first decade, the CPUs in PCs lacked the
features needed to protect an operating system from user programs. PC operating systems therefore
were neither multiuser nor multitasking. However, 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 MS-DOS operating system from Microsoft has been superseded by
multiple flavors of Microsoft Windows, and IBM has upgraded MS-DOS to the OS/2 multitasking
system.
The Apple Macintosh operating system has been ported to more advanced hardware, and now includes
new features, such as virtual memory and multitasking.
Old Desktop OS
Intermed. Desktop OS
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No protection
Single User
Single Task
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DOS
Windows <=3.11
 Windows 95,98,ME
(not fully multi user)
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No protection
Multi User(not full)
Multitasking
Microsoft OS improves
New Desktop OS
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Protection and Security
Multi User
Multitasking
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Windows NT
Windows 2000
Windows XP
Vista
Windows 7
With the release of MacOS X, the core of the operating system is now based on Mach and FreeBSD
UNIX for scalability, performance, and features, but it retains the same rich GUI.
Linux, a UNIX-like operating system available for PCs, has also become popular recently.
Operating systems for these computers have benefited in several ways from the development of
operating systems for mainframes. Microcomputers were immediately able to adopt some of the
technology developed for larger operating systems. On the other hand, the hardware costs for
microcomputers are sufficiently low that individuals have sole use of the computer, and CPU
utilization is no longer a prime concern. Thus, some of the design decisions made in operating systems
for mainframes may not be appropriate for smaller systems.
Other design decisions still apply. For example, file protection was, at first, not necessary on a
personal machine. However, these computers are now often tied into other computers over local-area
networks or other Internet connections.
When other computers and other users can access the files on a PC, file protection again becomes a
necessary feature of the operating system. The lack of such protection has made it easy for malicious
programs to destroy data on systems such as MS-DOS and the Macintosh operating system. These
programs may be self-replicating, and may spread rapidly via worm or virus mechanisms and disrupt
entire companies or even worldwide networks. Advanced timesharing features such as protected
memory and file permissions are not enough, on their own, to safeguard a system from attack.
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Operating System Principles
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Albert Minasyan
1.2.3. Single and Multi Processor Systems
1.2.3.1 Single Processor Systems
Until recently, most computer systems used a single processor. On a single processor system, there is
one main CPU capable of executing a general-purpose instruction set, including instructions from user
processes. Almost all single processor systems have other special-purpose processors as well. They
may come in the form of device-specific processors, such as disk, keyboard, and graphics controllers;
or, on mainframes, they may come in the form of more general-purpose processors, such as I/O
processors that move data rapidly among the components of the system.
All of these special-purpose processors run a limited instruction set and do not run user processes.
Sometimes, they are managed by the operating system, in that the operating system sends them
information about their next task and monitors their status. For example, a disk-controller
microprocessor receives a sequence of requests from the main CPU and implements its own disk queue
and scheduling algorithm. This arrangement relieves the main CPU of the overhead of disk scheduling.
PCs contain a microprocessor in the keyboard to convert the keystrokes into codes to be sent to the
CPU. In other systems or circumstances, special-purpose processors are low-level components built
into the hardware. The operating system cannot communicate with these processors; they do their jobs
autonomously. The use of special-purpose microprocessors is common and does not turn a singleprocessor system into a multiprocessor. If there is only one general-purpose CPU, then the system is a
single-processor system.
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It is very effective to add a microprocessor with its own memory to manage a disk system
thus significantly reducing the load of the main CPU.
Additional processor can function as: HDD Controller, Keyboard Controller . . . other
controllers.
In fact, this use of microprocessors has become so common that it is no longer considered
multiprocessing.
1.2.3.2. Multiprocessor Systems
(tightly coupled systems)
Multiprocessing is implemented connecting the computers to each other on system bus or
other local bus levels.
Many systems to date are single-processor systems; that is, they have only one main CPU. However,
multiprocessor systems (also known as parallel systems or tightly coupled systems) are growing in
importance. Such systems have more than one processor in close communication, sharing the
computer bus, the clock, and sometimes memory and peripheral devices.
Multiprocessing is implemented connecting the computers to each other on system bus or other
local bus levels.
Multiprocessor systems have three main advantages.
Multiprocessor Systems (parallel systems or tightly coupled systems) 3 main
advantages:
 Increased throughput (Many processors, many tasks more volume of calculations)
 Economy of scale (Many tasks on the same data do not need many computers but one

multiprocessor computer)
Increased reliability (graceful degradation, fault tolerant) (failure does not halt the system)
1. Increased throughput. By increasing the number of processors, we hope to get more work done
in less time. The speed-up ratio with N processors is not N; rather, it is less than N. When multiple
processors cooperate on a task, a certain amount of overhead is incurred in keeping all the parts
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working correctly. This overhead, plus contention for shared resources, lowers the expected gain from
additional processors. Similarly, a group of N programmers working closely together does not result
in N times the amount of work being accomplished.
2. Economy of scale (Reduction in cost per unit resulting from increased production).
Multiprocessor systems can save more money than multiple single-processor systems, because they
can share peripherals, mass storage, and power supplies. If several programs operate on the same set
of data, it is cheaper to store those data on one disk and to have all the processors share them, than to
have many computers with local disks and many copies of the data.
3. Increased reliability. If functions can be distributed properly among several processors, then the
failure of one processor will not halt the system, only slow it down. If we have ten processors and one
fails, then each of the remaining nine processors must pick up a share of the work of the failed
processor. Thus, the entire system runs only 10 percent slower, rather than failing altogether. This
ability to continue providing service proportional to the level of surviving hardware is called graceful
degradation. Systems designed for graceful degradation are also called fault tolerant.
Tandem Multiprocessor System (Full duplication).
Continued operation in the presence of failures requires a mechanism to allow the failure to be
detected, diagnosed, and, if possible, corrected. The Tandem system uses both hardware and
software duplication to ensure continued operation despite faults. The system consists of two
identical processors, each with its own local memory. The processors are connected by a bus. One
processor is the primary and the other is the backup. Two copies are kept of each process: one on the
primary processor and the other on the backup. At fixed checkpoints in the execution of the system,
the state information of each job including a copy of the memory image-is copied from the primary
machine to the backup. If a failure is detected, the backup copy is activated and is restarted from
the most recent checkpoint.
Tandem Multiprocessor System Specificities:
 Both hardware and software duplication
 2 identical processors Main and Backup
 Periodical backup (at time checkpoints) of Main CPU state to the Backup CPU.
 In case of Main CPU failure the Backup CPU is activated from the recent checkpoint.
Main (active)
Backup (passive)
CPU
Backup the state at CPU
checkpoints
System BUS
Disks
Memory
Memory
Tandem Multiprocessor System (Full duplication).
Disadvantage: This solution is expensive, since it involves considerable hardware duplication.
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Symmetric Multiprocessing (SMP)
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.
Specificities:
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Parallel running of copies of the same Operating System, with different tasks
Equal rights to control the system
Communication each with other
CPU
CPU
System Bus
Memory
Memory
Symmetric Multiprocessing (SMP)
Advantage:
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The benefit of this model is that many processes can run simultaneously-N processes can run
if there are N CPUs-without causing a significant deterioration of performance.
No forever idle backup processors
SMP means that all processors are peers; no master-slave relationship exists between processors. Each
processor concurrently runs a copy of the operating system.
Problems:
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However, we must carefully control I/O to ensure that the data reach the appropriate
processor (if the memories are separate they could be written not in the correct memory).
Also, since the CPUs are separate, one may be sitting idle while another is overloaded,
resulting in inefficiencies. (as the data is in other memory).
These inefficiencies can be avoided if the processors share memory dynamically. Then the idle
processor can take some part of the task and run on the same data as the other processor.
These inefficiencies can be avoided if the processors share certain data structures. A multiprocessor
system of this form will allow processes and resources—such as memory—to be shared dynamically
among the various processors and can lower the variance among the processors. Such a system must
be written carefully.
System Bus
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The Figure illustrates one more SMP architecture.
With this shared memory SMP system the memory is shared and processors can access the same data
without difficulties. The processors are quite autonomous because of own internal caches.
Problems:
• Shared bus problem remains to take care. Caches partially resolve it.
Virtually all modern operating systems-including Windows, UNIX, Linux - now provide
support for SMP.
Asymmetric Multiprocessing
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. This scheme defines a master-slave relationship. The master processor
schedules and allocates work to the slave processors.
Specificities:
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Parallel running of the same or other Operating Systems, with different tasks (belonging
to user processes.
Not Equal rights to control the system
Communication each with other
Master
CPU
Slave CPU(s)
System Bus
Memory
Memory
Asymmetric Multiprocessing
The difference between symmetric and asymmetric multiprocessing may result from either hardware
or software. Special hardware can differentiate the multiple processors, or the software can be written
to allow only one boss and multiple workers. For instance, Sun Microsystems’ operating system
SunOS Version 4 provided asymmetric multiprocessing, whereas Version 5 (Solaris) is symmetric on
the same hardware.
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MultiCore
A recent trend in CPU design is to include multiple computing cores on a single chip. Such
multiprocessor systems are termed multicore. They can be more efficient than multiple chips with
single cores because on-chip communication is faster than between-chip communication. In
addition, one chip with multiple cores uses significantly less power than multiple single-core chips.
It is important to note that while multicore systems are multiprocessor systems, not all multiprocessor
systems are multicore. In our coverage of multiprocessor systems throughout this text, unless we state
otherwise, we generally use the more contemporary term multicore, which excludes some
multiprocessor systems.
In Figure 1.7, we show a dual-core design with two cores on the same chip. In this design, each core
has its own register set as well as its own local cache. Other designs might use a shared cache or a
combination of local and shared caches.
Aside from architectural considerations, such as cache memory, and bus contention, these
multicore CPUs appear to the operating system as N standard processors.
This characteristic puts pressure on operating system designers—and application programmers—to
make use of those processing cores.
Hyper-threading
[From WikiPedia] Hyper-threading technology (HTT) works by duplicating certain sections of the
processor— those that store the architectural state— but not duplicating the main execution
resources. This allows a hyper-threading processor to appear as the usual "physical" processor and
an extra "logical" processor to the host operating system (HTT-unaware operating systems see two
"physical" processors), allowing the operating system to schedule two threads or processes
simultaneously and appropriately. When execution resources would not be used by the current task in
a processor without hyper-threading, and especially when the processor is stalled, a hyper-threading
equipped processor can use those execution resources to execute another scheduled task.
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Operating System Principles
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Albert Minasyan
This technology is transparent to operating systems and programs. The minimum that is required to
take advantage of hyper-threading is symmetric multiprocessing (SMP) support in the operating
system, as the logical processors appear as standard separate processors.
The architectural state is the part of the CPU which holds the state of a process, this includes:
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Control registers
o Instruction Flag Registers (such as EFLAGS in x86)
o Interrupt Mask Registers
o Memory management unit Registers
o Status registers
General purpose registers (such as AX,BX,CX,DX,etc. in x86)
o Adder Registers
o Address Registers
o Counter Registers
o Index Registers
o Stack Registers
o String Registers
This does not include actual computation units as an ALU for instance.
•
•
Thread Context switch time is reduced
Almost parallel running of threads
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Operating System Principles
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Albert Minasyan
1.2.4. Distributed Systems
(loosely coupled systems)
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Unlike Multiprocessing Systems the Loosely Coupled systems are connected to each
other through device controllers and devices.
Thus these connections are slower than tightly coupled connections.
In contrast to the tightly coupled systems the computer networks consist of 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, telephone lines, wireless connections. These systems are usually referred to as loosely coupled
systems (or distributed systems).
A distributed system is a collection of loosely coupled processors interconnected by a
communication network.
From the point of view of a specific processor in a distributed system, the rest of the processors
and their respective resources are remote, whereas its own resources are local.
The processors in a distributed system may vary in size and function. They may include small
microprocessors, workstations, minicomputers, and large general-purpose computer systems.
These processors are referred to by a number of names, such as sites, nodes, computers,
machines, or hosts, depending on the context in which they are mentioned.
 We mainly use site to indicate the location of a machine
 and host to refer to a specific system at a site.
 Node – an individual system (host or sometimes group of hosts, sometime site).
 Generally, one host at one site, the server, has a resource that another host at another site, the
client (or user), would like to use.
The purpose of the distributed system is to provide an efficient and convenient environment for such
sharing of resources.
A distributed system is a collection of physically separate, possibly heterogeneous,
computer systems that are networked to provide the users with access to the various
resources that the system maintains.
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A network, in the simplest terms, is a communication path between two or more 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.
A network is a communication path between two or more systems.
LAN1
City1
City2
WAN
LAN3
Room1, Bulding1
LAN1
MAN
LAN2
Room in Bulding2
Host
Room in Bulding3
Site
Room2, Bulding1
Networks vary by the protocols used, the distances between nodes, and the transport media. TCP/IP is
the most common network protocol, although ATM (Asynchronous Transfer Mode – transport layer)
and other protocols are in widespread use. Likewise, operating-system support of protocols varies.
Most operating systems support TCP/IP, including the Windows and UNIX operating systems. Some
systems support proprietary protocols to suit their needs. To an operating system, a network protocol
simply needs an interface device-a network adapter, for example-with a device driver to manage it,
and software to package data in the communications protocol to send it and to unpackage it to receive
it.
Networks are classified based on the distances between their nodes.
 A local-area network (LAN), exists within a room, a floor, or a building.
A wide-area
network
), usually
exists
betweenworldwide.
buildings, cities,
countries.
Aglobal
company may
have (aWAN
WAN
to connect
its offices,
These or
networks
could run one

A
metropolitan-area
network
(MAN),
could
link
buildings
within
a
city.
protocol or several protocols.
The continuing advent of new technologies brings about new forms of networks. For example, a
metropolitan-area network (MAN), could link buildings within a city. BlueTooth devices
communicate over a short distance of several feet, in essence creating a small-area network.
The media to carry networks are equally varied. They include copper wires, fiber strands, and wireless
transmissions between satellites, microwave dishes, and radios. When computing devices are
connected to cellular phones, they create a network. Even very short-range infrared communication
can be used for networking. At a rudimentary level, whenever computers communicate they use or
create a network. These networks also vary by their performance and reliability.
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To an operating system, a network protocol simply needs an interface device
 a network adapter
 with a device driver - to manage it
 software - to package data in the communications protocol to send it and to unpackage to
receive it.
API Interface to OS for User Applications
Network
Application
Programs
Operating System
Device drivers
Network Adapter
Control API to Hardware
LAN card, Wireless card
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1.2.4.1. Client Server Systems
As PCs have become faster, more powerful, and cheaper, designers have shifted away from the
centralized system architecture. Terminals connected to centralized systems are now being supplanted
by PCs. Correspondingly,
The user-interface functionality that used to be handled directly by the centralized systems is
increasingly being handled by the PCs.
As a result, centralized systems today act as server systems to satisfy requests generated
by client systems.
PC
PC
PC
PC
The general structure of a client-server system.
Server systems can be broadly categorized as compute servers and file servers.
File-Server systems - usual file sharing
Compute-Server systems - Remote Terminal, Application Servers.
The compute-server system provides an interface to which a client can send a request to
perform an action (for example, read data). In response to which, the server executes the action
and sends the results to the client. A server running a database that responds to client requests for
data is an example of such a system.
The file-server system provides a file-system interface where clients can create, update, read,
and delete files. An example of such a system is a web server that delivers files to clients running
web browsers.
Figure 1.18 General structure of a client–server system.
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1.2.4.2. Peer-to-Peer Systems
Another structure for a distributed system is the peer-to-peer (P2P) system model. In this model,
clients and servers are not distinguished from one another; instead, all nodes within the system are
considered peers, and each may act as either a client or a server, depending on whether it is requesting
or providing a service.
Peer-to-peer systems offer an advantage over traditional client-server systems. In a client-server
system, the server is a bottleneck; but in a peer-to-peer system, services can be provided by several
nodes distributed throughout the network.
To participate in a peer-to-peer system, a node must first join the network of peers. Once a node has
joined the network, it can begin providing services to-and requesting services from -other nodes in the
network.
Determining what services are available is accomplished in one of two general ways:

When a node joins a network, it registers its service with a centralized lookup service on the
network. Any node desiring a specific service first contacts this centralized lookup service to
determine which node provides the service. The remainder of the communication takes place
between the client and the service provider.

A peer acting as a client must first discover what node provides a desired service by
broadcasting a request for the service to all other nodes in the network. The node (or nodes)
providing that service responds to the peer making the request. To support this approach, a
discovery protocol must be provided that allows peers to discover services provided by other
peers in the network.
Peer-to-peer networks gained widespread popularity in the late 1990s with several file-sharing
services, such as Napster (later also the Torrent) and Gnutella, that enable peers to exchange files with
one another.
The Napster or Torrent system uses an approach similar to the first type described above: a centralized
server maintains an index of all files stored on peer nodes in the Napster network, and the actual
exchanging of files takes place between the peer nodes.
The Gnutella system uses a technique similar to the second type: a client broadcasts file requests to
other nodes in the system, and nodes that can service the request respond directly to the client.
The future of exchanging files remains uncertain because many of the files are copyrighted (music, for
example), and there are laws governing the distribution of copyrighted material. In any case, though,
peer-to-peer technology undoubtedly will play a role in the future of many services, such as searching,
file exchange, and e-mail.
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•
•
•
•
Albert Minasyan
Peer to Peer system is a distributed network with autonomous hosts.
Each processor has its own local memory
Hosts are interconnected by a communication network.
Generally, one host at one site- the server, has a resource that another host at another
site, the client (or user), would like to use and vice versa is also true.
Purpose: To provide resource sharing via communication network (with
equal rights)
Host1
Host3
Network
Host2
Host4
Peer to peer system
•
•
•
All nodes within the Peer to Peer system are considered peers
Clients and servers are not distinguished from one another
Each may act as either a client or a server, depending on whether it is
requesting or providing a service.
Figure 1.19 Peer-to-peer system with no centralized service.
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1.2.5. Clustered Systems
This is multiprocessing through LAN.
Like multiprocessor systems, clustered systems gather together multiple CPUs to accomplish
computational work. Usually they are composed of two or more individual systems - or nodes coupled together. Each node may be a single processor system or a multicore system.
And they have all the advantages that Multiprocessor systems have:
• Increased throughput (Many processors, many tasks more volume of calculations)
• Economy of scale (Many tasks on the same data do not need many computers but one
multiprocessor computer)
• Increased reliability (graceful degradation, fault tolerant- failure does not halt the system)
Clustered systems differ from multiprocessor systems, however
 The connection between nodes is done by LAN connection but not by system bus.
The definition of the term clustered is not concrete; many commercial packages wrestle with what a
clustered system is, and why one form is better than another. The generally accepted definition is that
clustered computers share storage and are closely linked via LAN networking or a faster
interconnect, such as InfiniBand
Definition: clustered computers are autonomous hosts sharing some resources (like
storage) and are closely linked via LAN networking.
Host1
Host3
LAN
Host2
Host4
Memory
Clustered Network
Purpose: provide high availability (hosts monitor the other hosts and continue the tasks of other
hosts in case of failure).
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.
Disadvantage: One of the hosts all the time is idle.
Symmetric clustering: Two or more hosts are running applications, and they are monitoring each
other.
Advantage: Both are loaded.
Problem: It does require that more than one application be available to run to keep busy both
hosts.
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
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machine fails, the monitoring machine can take ownership of its storage, and restart the application(s)
that were running on the failed machine. The failed machine can remain down, but the users and
clients of the application would only see a brief interruption of service.
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.
Other forms of clusters include parallel clusters and clustering over a WAN.
Parallel clusters allow multiple hosts to access the same data on the shared storage.
Because most operating systems lack support for simultaneous data access by multiple hosts, parallel
clusters usually require the use of special versions of software and special releases of applications. For
example, Oracle Real Application Cluster is a version of Oracle’s database that has been designed to
run on a parallel cluster. Each machine runs Oracle, and a layer of software tracks access to the shared
disk. Each machine has full access to all data in the database. To provide this shared access, the
system must also supply access control and locking to ensure that no conflicting operations occur.
This function, commonly known as a distributed lock manager (DLM), is included in some cluster
technology.
Cluster technology is rapidly changing. Cluster directions include global clusters, in which the
machines could be anywhere in the world (or anywhere a WAN reaches). Such projects are still the
subject of research and development.
Clustered system use and features should expand greatly as storage-area networks (SAN)
become prevalent. SANs allow easy attachment of multiple hosts to multiple storage units thus
increasing the reliability of system. Current clusters are usually limited to two or four hosts due to
the complexity of connecting the hosts to shared storage.
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1.2.6. Cloud Computing
Cloud computing is a type of computing that delivers computing, storage, and even applications as
a service across a network. In some ways, it’s a logical extension of virtualization, because it uses
virtualization as a base for its functionality. For example, the Amazon Elastic Compute Cloud (EC2)
facility has thousands of servers, millions of virtual machines, and petabytes of storage available for
use by anyone on the Internet. Users pay per month based on how much of those resources they use.
There are actually many types of cloud computing, including the following:
• Public cloud—a cloud available via the Internet to anyone willing to pay for the services
• Private cloud—a cloud run by a company for that company’s own use
• Hybrid cloud—a cloud that includes both public and private cloud components
• Software as a service (SaaS)—one or more applications (such as word processors or spreadsheets)
available via the Internet
• Platform as a service (PaaS)—a software stack ready for application use via the Internet (for
example, a database server)
• Infrastructure as a service (IaaS)—servers or storage available over the Internet (for example,
storage available for making backup copies of production data)
These cloud-computing types are not discrete, as a cloud computing environment may provide a
combination of several types. For example, an organization may provide both SaaS and IaaS as a
publicly available service.
Certainly, there are traditional operating systems within many of the types of cloud infrastructure.
Beyond those are the VMMs that manage the virtual machines in which the user processes run. At a
higher level, the VMMs themselves are managed by cloud management tools, such as Vware vCloud
Director and the open-source Eucalyptus toolset. These tools manage the resources within a given
cloud and provide interfaces to the cloud components, making a good argument for considering them
a new type of operating system.
Figure 1.21 illustrates a public cloud providing IaaS. Notice that both the cloud services and the cloud
user interface are protected by a firewall.
Figure 1.21 Cloud computing.
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1.2.7. Real-Time Systems
Embedded computers are the most prevalent form of computers in existence. These devices are found
everywhere, from car engines and manufacturing robots to DVDs and microwave ovens. They tend to
have very specific tasks.
The systems they run on are usually primitive, and so the operating systems provide limited features.
Usually, they have little or no user interface, preferring to spend their time monitoring and managing
hardware devices, such as automobile engines and robotic arms.
These embedded systems vary considerably. Some are general-purpose computers, running standard
operating systems-such as UNIX-with special-purpose applications to implement the functionality.
Others are hardware devices with a special-purpose embedded operating system providing just the
functionality desired. Yet others are hardware devices with application-specific integrated circuits that
perform their tasks without an operating system.
Embedded systems almost always run real-time operating 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. Systems that control scientific experiments, medical imaging systems,
industrial control systems, and certain display systems are real-time systems. Some automobile-engine
fuel-injection systems, home-appliance controllers, and weapon systems are also real-time systems.
Real-Time Systems usage areas:
 scientific experiments
 medical imaging systems
 industrial control systems (nuclear power plant)
 automobile-engine fuel-injection systems
 home-appliance controllers
 weapon systems
 encryption / decryption systems
Requirements: A real-time system has well-defined, fixed time constraints. Processing must be
done within the defined constraints, or the system will fail.
Flavors: hard and soft
Hard: A hard real-time system guarantees that critical tasks be completed on time.
Soft: critical real-time task gets priority over other tasks, and retains that priority until it completes
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. Such time constraints dictate
the facilities that are available in hard real-time systems.
Secondary storage of any sort is usually limited or missing, with data instead being stored in shortterm memory or in read-only memory (ROM). ROM is located on nonvolatile storage devices that
retain their contents even in the case of electric outage; most other types of memory are volatile. Most
advanced operating-system features are absent too, since they tend to separate the user from the
hardware, and that separation results in uncertainty about the amount of time an operation will take.
For instance, virtual memory is almost never found on real-time systems. Therefore, hard real-time
systems conflict with the operation of time-sharing systems, and the two cannot be mixed. Since none
of the existing general-purpose operating systems support hard real-time functionality, we do not
concern ourselves with this type of system in this text.
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Operating System Principles
AUA CIS331
Albert Minasyan
A less restrictive type of real-time system is a soft real-time system, where a critical real-time 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. Soft real-time systems, however, have more limited utility than hard real-time
systems. Given their lack of deadline support, they are risky to use for industrial control and robotics.
They are useful, however in several areas, including multimedia, virtual reality, and advanced
scientific projects-such as undersea exploration and planetary rovers. These systems need advanced
operating-system features that cannot be supported by hard real-time systems. Because of the
expanded uses for soft real-time functionality, it is finding its way into most current operating
systems, including major versions of UNIX.
1.2.8. Handheld Systems
Handheld systems include personal digital assistants (PDAs), such as Palm and Pocket-PCs, and cellular
telephones, many of which use special-purpose embedded operating systems.
Developers of handheld systems and applications face many challenges, most of which are due to the
limited size of such devices.
Due to this limited size, most handheld devices have a small amount of memory, include slow
processors and small display screens.
The amount of physical memory in a handheld depends on the device, but typically it is much less than on
PCs. As a result, the operating system and applications must manage memory efficiently.
Processors for most handheld devices often run at a fraction of the speed of a processor in a 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. To minimize the size of
most handheld devices, smaller, slower processors which consume less power are typically used.
Therefore, the operating system and applications must be designed not to tax the processor.
The last issue confronting program designers for handheld devices is the small display screens
typically available. Whereas a monitor for a home computer may measure up to 21 inches, the display
for a handheld device is often no more than 3 inches square. Familiar tasks, such as reading e-mail or
browsing web pages, must be condensed onto smaller displays.
Handheld Systems usage areas:
 In very small size devices
 Palm Pilots
 Tablets
 Cellular phones
Requirements: small size dictates
 small amount of memory
 small display
 less powerful processor to consume less
energy
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