Transcript Part II: Software

Part II: Software
• In part II, we focus on topics associated
with software. In particular, we will
investigate the discovery, representation,
and communication of algorithms.
• Operating systems and networks.
• Algorithms.
• Programming languages.
• Software engineering.
Ch. 3 Operating Systems and
Networks
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The evolution of operating systems.
Operating system architecture.
Coordinating the machine’s activities.
Handling Competition among processes.
Networks.
Network protocols.
Operating Systems
• Why needs an operating system?
• Computer applications often require a single
machine to perform activities that may compete
with one another for the machine’s resources. It
requires a high degree of coordination to ensure
that unrelated activities do not interfere with one
another and that communication between related
activities is efficient and reliable.
• What is an operating system? A software
system which handles such a coordination
task.
The evolution of Operating
Systems
• Single-processor systems.
• Batch processing - the execution of jobs
(programs) by collecting them in a single batch,
then executing them without further interaction
with the user.
• A job queue (FIFO) and a job control language
(JCL).
• The main drawback to batch processing is no
interaction between user and job.
The Evolution of Operating
Systems
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Interactive processing.
Real-time processing.
Time-sharing.
Multitasking - time-sharing for a singleuser system.
• From multiprocessor systems to networks.
Operating System Architecture
Software
Application
Utility
System
Operating
system
Shell
Kernel
Operating System Architecture
• A machine’s software can be divided into two
categories: application software and system
software.
• Application software - the programs for
performing tasks particular to the machine’s
utilization.
• System software - performs tasks which are
common to computer systems in general.
Operating System Architecture
• System software can be divided into two
categories: operating-system software and
utility software.
• Utility software consists of software units
that extend the capabilities of the operating
system. For example, the ability to format
a disk or software for communicating
through a modem over telephone lines.
Operating System Architecture
• Shell - the portion of an operating system
that defines the interface between the
operating system and its users.
• Graphical user interface (GUI).
• Importance of uniformity in the humanmachine interface across a variety of
machines.
• UNIX Vs. MS-DOS and Windows.
Operating System Architecture
• Kernel - the internal part of an operating system,
which contains those software components that
perform the very basic functions required by the
computer installation.
• File manager - file descriptor and directory
(folder).
• Device drivers - communicate with controllers.
Operating System Architecture
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Memory manager - virtual memory.
Scheduler and dispatcher.
Booting (boot strapping).
Bootstrap - a short program placed in ROM and
this program is executed automatically when the
machine is turned on.
– transfer the operating system to memory
– direct the CPU to execute the operating system
Coordinating the Machine
Activities
• Process - is a dynamic activity whose properties
change as time progresses.
• Process state - current status of the activity,
including the current position in the program
being executed and the values in the other CPU
registers and the associated memory cells.
• A program Vs. a process.
• Interprocess communication.
Coordinating the Machine’s
Activities
• Process administration - the tasks
associated with process coordination are
handled by the scheduler and dispatcher
within the operating system’s kernel.
• Process table - keeps information of a
process; when a process is created, it is
assigned a memory area, a priority, and a
status - ready or waiting.
Coordinating the Machine’s
Activities
• The dispatcher is the component of the kernel
that ensures that the scheduled processes are
actually executed.
• In a time-sharing system, the time is divided into
time slices or quanta.
• The dispatcher initiates a timer circuit each time
a process begins its time slice.
• At the end of the time slice, the timer circuit
generates an interrupt.
Coordinating the Machine’s
Activities
• When the CPU receives an interrupt, it
saves the current state of the process in the
process table, and begins executing the
interrupt handler, which is a part of the
dispatcher.
• At this moment, the dispatcher allows the
scheduler to update the process table.
Coordinating the Machine’s
Activities
• The dispatcher then selects the highest priority
process among the ready processes, restarts the
timer circuit, and allows the selected process to
begin its time slice.
• At times a process’s time slice is terminated
before the timer has expired (for example, an I/O
request), the scheduler will update the process
table to reflect the process’s waiting status.
Coordinating the Machine’s
Activities
• Later, when the controller indicates that
the I/O request has been completed, the
scheduler will reclassify the process as
ready.
Coordinating the Machine’s
Activities
• The various units within an operating
system compete for time slices under
control of the dispatcher.
• To coordinate their activities, these
processes must communicate with one
another.
– For example, to schedule a new process, the
scheduler must obtain memory space from the
memory manager for that process
Coordinating the Machine’s
Activities
• The client/server model for inter-process
communication.
• A client makes requests to servers and waits for
replies.
• A server satisfies the requests and sends replies
back to the clients.
• The client/server model in the design of software
leads to uniformity among the types of
communication taking place in the system.
Handling Competition Among
Processes
• Competing resources among processes.
• The file manager allocates both access to current
files and disk space for the construction of new
files.
• The memory manager allocates memory space.
• The scheduler allocates space in the process
table.
• The dispatcher allocates time slices.
Handling Competition Among
Processes
• To control access to the printer, the operating
system must keep track of whether the printer has
been allocated.
– By a flag: check the flag then set the flag
– Problem: check the flag and find it clear; the process
is then interrupted; the new process also requests the
use of the printer; the operating system allows it to use
the printer; later, the first process resumes and the
operating system allows it to use the printer too
Handling Competition Among
Processes
• Interrupt disable and interrupt enable.
• Test-and-set instruction.
• A properly implemented flag is called a
semaphore.
• Critical region - a sequence of instructions
which can be executed by only one process.
– Mutual exclusion
– Guard the critical region with a semaphore
Handling Competition Among
Processes
• Deadlock - when two or more processes
are blocked from processing because each
is waiting for access to resources allocated
to another.
• Three necessary conditions for deadlock:
– 1. There is competition for non-shareable
resources
Handling Competition Among
Processes
– 2. The resources are requested on a partial
basis; that is, having received some resources,
a process will return later to request more
– 3. Once a resource has been allocated, it
cannot be forcibly retrieved
• Deadlock detection and correction to
attack the third condition.
• Deadlock avoidance to attack the first two
conditions.
Handling Competition Among
Processes
– Require each process to request all its
resources at one time
– Make nonshareable resources appear shareable
• holding data for output at a later but more
convenient time
• spooling
Networks
• Local area networks (LAN) Vs wide area
networks (WAN).
• Open networks Vs proprietary networks.
• Network configurations - ring, bus, star,
and irregular.
Networks
• Internet - initiated in 1973 by the Defense
Advanced Research Projects Agency (DARPA)
to develop the ability to connect a variety of
computer networks so that they can function as a
single network.
– A collection of network clusters (domains)
• Internet addressing - network identifier, host
address; ex., 192.207.177.133.
– ssenterprise.awl.com
– alpchen.cs.nthu.edu.tw
Networks
• Email and name server.
– [email protected]
– [email protected]
• The world wide web - hypertext and
hypermedia documents.
• A browser - a client.
Networks
• Uniform resource locator (URL) - a browser can
contact the proper server and request the desired
document.
– Figure 3.12, homepage: http://www.awl.com, portal
• Hypertext Markup Language (HTML).
– Special markers that describe how the document
should appear on the computer screen and which items
within the document are to be linked to other
documents for a browser to perform its task
Network Protocols
• Protocols - the rules that govern the
communication between different
components within a computer system.
• Token ring protocol for networks with the
ring topology (Figure 3.13).
• CSMA/CD (carrier sense, multiple access
with collision detection) in an Ethernet
(Figure 3.14).
Network Protocols
Customer
You
Package
Shipper
Package
Shipper
Container
Airline
Container
Airline
Aircraft
Network Protocols: The Internet
Software Layer
Message source
Message destination
Application layer
Application layer
Transport layer
Transport layer
Network layer
Network layer
Link layer
Link layer
Network Protocols: The Internet
Software Layer
• Application layer consists of software units that
must communicate with each other across the
Internet.
– FTP, telnet, requires name server services
• Transport layer divides long messages into
packets.
• Network layer wraps the packets it receives with
the intermediate address.
• Link layer deals with the communication details
(whether token ring or bus is used).
• Figure 3.17.
Network Protocols
• Open system interconnection (OSI)
reference model produced by International
standards organization (ISO).
– Seven-level hierarchy
• TCP/IP protocol suite defines the fourlevel hierarchy.
Networks
• Unauthorized access to information and
vandalism.
• Passwords and data encryption.
• Virus and worm.