Chapter 3: OS and Networks

1
Chapter 3 OS and Networks

• With the number of resources available, we need
  Operating Systems and Networks
• The OS helps us control and use a variety of
  resources
• Network resources
• printers
• file servers
• other computers (through telnet, ftp, etc)
• Local resources
• CPU
• main memory
• disk drive(s) and other storage devices
• sound card, monitor, keyboard, etc
• We will examine the OS and network
• their roles and responsibilities

2
Evolution of the Operating System

• Early computers single process systems
• punch cards, mounting tapes, setting switches
• each job run by itself
• programmers were the only users, acted as system
  monitors
• This led to early OSs where the system was
  responsible for handling batch processing chores
• programmers submitted their jobs remotely or
  through an operator and did not have to
  physically enter the computer room
• jobs waited on secondary storage in a job queue
  (first-in first-out structure)
• see figure 3.1 p. 121

3
Further Evolution

• As hardware evolved, so did the OS
• resources accessed through job control cards
• change from punch cards to terminals
• change from single-user to multi-user systems
• interactive environment through time sharing
  (multitasking)
• see figure 3.2 p. 122
• Today
• system administrator (instead of operator)
• software and hardware installation rather than
  the day-to-day running of the computer system
• PCs replace terminals connected to
  mainframe/minicomputer
• Graphical User Interfaces (GUI)
• Multitasking commonplace

4
Multiprocessor Systems

• May be in a single computer (parallel computer)
  or distributed across a network
• Distributed systems are much more common than
  parallel computers
• By networking PCs, we have power equal to
  mainframe computers but at a much cheaper cost
• The coordination problems of a network are
  essentially the same faced in a multitasking
  operating system and so we can view the control
  of a network as being that of a distributed
  operating system
• Networks allow for load balancing which can be a
  difficult task to perform, but suffer from lack
  of scalability
• We will examine networks in detail in sections
  3.5 and 3.6

5
Uniformity of OSs good or bad?

• There used to be many OSs available for PCs, but
  today, we essentially see Microsoft and Macintosh
  -- is uniformity good or bad?
• Pros once you learn the OS, you can use it on
  any computer application software development is
  simplified
• Cons gives the producer of the OS tremendous
  power in the marketplace that, if misused, could
  potentially harm the marketplace -- is MS
  trustworthy?
• Currently, there is an antitrust suit against MS

6
Types of software

• Applications Software
• Software we run
• word processor
• spreadsheet
• database
• graphics program
• games
• System Software
• Software that the computer uses to maintain its
  own environment
• Utilities include system add-ons like antiviral
  software, screensavers, communication software
  and compression- decompression software

Software
Applications Systems
Utility Operating System
Shell Kernel
7
The Operating System

• Shells user interfaces including GUIs
• Windows 3.1 is a shell on top of MS-DOS
• UNIX has many shells like Borne, C, Korn

• The remainder of the systems software is the OS
  itself
• The Kernel the core programs that make up the
  OS, that are always part of the OS
• memory manager and memory architecture
• virtual memory, page size,
• process manager
• scheduler, dispatcher, process synchronization
• device drivers
• file manager

User
Operating System
User
Shell
User
8
Boot Program

• In order to use modern computers, the OS must be
  loaded and running
• The OS is loaded into RAM
• But when the computer is first turned on, RAM is
  empty!
• How do we get the computer to load any program
  (including the OS) if the OS is not already
  loaded and running?
• Boot strapping (or booting) is the first thing
  that the computer does when turned on
• The boot program is stored in ROM (and possibly a
  part of it resides on hard disk)

• Boot process
• CPU references a preset location in memory (in
  ROM)
• This boot program then tests the system (memory,
  attached peripherals)
• Performs initialization of registers and memory
  values
• loads the kernel of the OS from a predetermined
  location (floppy disk, optical disk, hard disk)
• CPU branches to the first instruction in RAM of
  the OS kernel
• See figure 3.5 p. 131

9
Concept of a Process

• The process is a program in an executing state
• this is the basic unit of the operating system
• as opposed to the instruction which is the basic
  unit for the CPU
• the process state is the current status of the
  process and includes its
• execution status (executing, ready, waiting,
  terminated)
• value of registers (program counter, instruction
  register, general-purpose registers, status
  flags)
• resources (open files, memory)
• other information (accounting information, owner,
  priority)

10
Process Administration

• Multitasking or time-sharing systems are
  responsible for handling several active processes
  
• System maintains a process table that stores the
  process state of each process
• Ready processes wait in a ready queue
• When the CPU is available, the dispatcher selects
  a process to move to the CPU
• The CPU executes some number of cycles on this
  process
• this number is known as the quantum or time
  slices (e.g. 50 milliseconds)
• If quantum elapses (or process requires an OS
  service like I/O), CPU is interrupted and
  performs a process switch
• See figure 3.6 p. 133

11
Interrupts

• An Interrupt is when the CPU is stopped from what
  it is currently doing and forced to do something
  else
• Interrupts can be initiated by hardware or
  software
• Hardware timer elapses for a context switch,
  input from keyboard, I/O device completes
• Software process requests I/O, interprocess
  communication occurs
• When the interrupt occurs
• the CPU branches to the proper interrupt handler
  (code stored in a reserved section of main
  memory) that tells it what to do to handle the
  interrupt
• Once done
• the CPU resumes what it was doing before
  interruption

12
Interprocess Communication

• Most software communicates with other software
• e.g. an application communicates a request to the
  OS to save the current file from memory to disk
• All of these software components (applications,
  systems software) compete for time slices in the
  CPU
• The dispatcher (another software component)
  decides which component to execute next
• In the meantime, the OS must also facilitate IPC
• One way to do this is to use a client/server
  model
• client -- software that makes requests of other
  units
• server -- software that waits for requests and
  then fulfills them

13
Process Competition

• The CPU and the time slices allocated to a
  process are just one form of resource
• The OS has many resources
• main memory, servers, disk drives, individual
  files, etc
• In a multitasking environment, managing resources
  is critical
• Without proper management
• processes would be able to hold onto resources
  indefinitely forcing other processes to wait
• resources could be misused leading to, among
  other things, corrupt data

14
Example Bank Account

• Consider the following situation
• You and your wife have a checking account at a
  bank with a balance of 500
• You and your wife enter the bank and wait in line
• You and your wife are both served at the same
  time by two different tellers
• You tell your teller that you want to withdraw
  the 500 leaving a balance of 0
• Your wife tells her teller that she wishes to
  withdraw the 500 leaving a balance of 0
• Each teller performs this task simultaneously
• You and your wife each walk out with 500,
  meaning that you now have 1000 instead of 500
• How can this happen?

15
At the machine level

• Process 1 Process 2 Load R1,
  balance Load R10, balance Sub R2, R1,
  500 Sub R11, R10, 500 JLEZ
  overdraw JLEZ R11, overdraw Store R2,
  balance Store R11, balance ...
• Notice that only if the balance is less than the
  withdrawal amount do we need to worry about it
  because we skip the Store instruction
• In a multitasking system both processes are may
  be executed concurrently
• What happens if the first process executes up to
  the Sub instruction before being interrupted and
  the second process then executes up to or past
  the Sub instruction?
• Both processes will not perform the Jump
  instruction since R1 and R10 will both store 500

16
Safeguarding against this problem

• We must protect multiple accesses made to the
  variable balance
• This memory location becomes a critical section
  which must not be accessed by two (or more)
  processes in an overlapped fashion
• We must disallow interrupts so that one access
  can be made in its entirety before another access
  is permitted
• This idea is known as mutual exclusion --
  allowing at most 1 process to access a resource
  at a time
• How do we implement mutual exclusion to critical
  sections?

17
The Semaphore

• A very simple idea in which a semaphore is used
  to protect access to a critical section
• processes requests the semaphore
• at most, one process is able to have the
  semaphore
• access to the critical section requires first
  obtaining the semaphore
• once done accessing the critical section, the
  process releases the semaphore for another
  process
• the semaphore itself is implemented as an integer
  variable with the value of
• 1 (if available) or 0 (if currently held)
• machine languages include uninterruptable
  instructions to manipulate the semaphore (e.g.,
  test-and-set)

18
Deadlock

• Mutual exclusion leads to its own problem
  deadlock
• If a process is allowed to hold onto a resource
  until it is done with it, then we could have the
  following situation
• P0 currently holds R0
• P1 currently holds R1
• to continue, P0 needs access to R1
• to continue, P1 needs access to R0
• P0 will not be given R1 until P1 no longer needs
  it, but P1 wont give up R1 until it gets R0 and
  P1 cant get R0 until P0 is done with it but P0
  wont give up R0 until it gets R1
• In this case, P0 and P1 are deadlocked even
  though R0 and R1 are not currently being used!

P0 P1 R0 R1
See figure 3.9 on page 139 for a
real-life deadlock
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More on Deadlock

• To handle deadlock
• deadlock prevention
• force all processes to request all resources up
  front, only allow a process to proceed if we are
  guaranteed no deadlock can arise
• deadlock avoidance
• only grant a resource request if it will not lead
  to deadlock
• deadlock detection
• grant any resource request and look for
  deadlocks, if one arises kill off the processes
  that caused the deadlock and randomly restart
  them
• We can also permit processes to continue rather
  than waiting for a resource by moving some of the
  action off-line through spooling and buffering

• For deadlock to occur, 3 conditions must be true
• There is competition for non-shareable resources
• The resources are requested on a partial basis
• a process that has received some resources may at
  a later point request more resources
• Resources are not pre-emptable
• a resource cannot be forcibly retrieved from a
  process
• Notice that some resources are non-shareable
• printer, memory location
• Some resources are not pre-emptable
• printer, open disk file

20
Networks

• Classification LAN and WAN
• Distinction is usually by location
• LAN within 1 building or building complex
• But also by the technology used to link the
  computers together
• LAN direct cable WAN long distance
  communications

• Another classification open network or
  proprietary (closed) network
• Another classification network topology
• Ring, Bus, Star, Hybrid/irregular

21
The Internet

• Any network of networks is an internet
• The Internet is the largest such network
• a global network that encompasses more than 30
  million computers and tens of thousands of local
  area networks
• Started out as the ARPAnet in the late 60s
  connecting four local area networks together
• by the early 70s, more networks were added
• by the 80s, commercial networks were added, ARPA
  gave control to independent units

22
How does the Internet work?

• Four key components to the Internet
• all computers on the Internet speak the same
  language TCP/IP, a network protocol
• routers are machines that route messages from one
  location to another
• each local area network on the Internet requires
  a router
• all computers have a unique Internet address
• called an IP address
• domain name servers provide information across
  the Internet so that one machine can learn
  another machines Internet address

23
Internet Domains and Addresses

• An Internet address is a 32-bit number
• This number must be unique
• The number is subdivided into 4 groups of 8 bits
  that determine the domain, local area network and
  machine within the network
• First byte denotes the domain name (such as edu,
  gov, com, etc)
• Second byte denotes the specific domain address
  (such as panam or ohio-state)
• Third byte denotes local area network within the
  domain (such as cs department or tech-res)
• Fourth byte denotes the computer (host) within
  that LAN
• For convenience, these 32-bit numbers can be
  represented using aliases (symbolic names) with
  network identifiers used to translate from alias
  to number representation

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Sending/Receiving messages

• You want to send a message
• e.g., http or ftp request, E-mail
• Your computer packages up the message
• Local name server looks up in its table the
  destination computers IP address
• if found, IP address is placed into the message
  and the message is sent off
• else your name server contacts a domain name
  server on the Internet for the information
• The message is placed on the Internet
• sent across the network to all machines in the
  given domain

• Routers on the Internet listen for messages and
  route the message to the proper subnetwork
• Within a given subnetwork, all machines listen to
  the network for all messages
• When a message comes across the subnetwork with
  the given machines address, that machine copies
  it into memory and alerts the OS that a message
  has arrived, all other machines ignore the
  message
• OS interrupts CPU and handles the message

25
More on the Internet

• How does your message get to the Internet and how
  do you receive messages?
• Your computer must connect to an Internet server
  of some kind
• Panam has a connection through T1 lines that
  connect UTPA to UTSA and through UTSA to the UT
  System which connects to the Internet through
  microwave and satellite communications
• At home, you can connect through the phone lines
  to an Internet Service Provider
• AOL, Compuserve, Prodigy, Hiline, Juno, Hotmail,
  etc

• Some Internet Services
• Mail service
• WWW
• hypertext transfer via http using browsers to
  display text and activate html and Java commands
  
• see fig 3.12 for URL example
• FTP (file transfer protocol)
• older version of http, files transferred but not
  displayed
• user must know exact address
• Telnet (remote login to servers on the Internet)

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Network Protocols

• Protocol - set of rules governing communication
• Communication across a network uses a protocol
• The protocol is composed of layers
• Application/usr requests message to be sent
• OS takes message, wraps it up
• OS includes error correcting codes and
  destination addresses
• Name Server finds actual address which is added
  to wrapper
• message sent...

For two computers to communicate, they must speak
the same protocol Examples include TCP/IP,
Ethernet and ISO
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Security

• When dealing with a multi-user system or
  networks, security becomes an issue
• How do we ensure that the user has the right to
  access a given resource
• such as access to a given file, a region of
  memory or an E-mail message?
• The OS maintains security through user rights
  (such as file protection information)
• User must prove who he/she is through a login
  process using a publicly known login name and a
  secret password

• Security Violations
• Notice that passwords may not be secure. Do you
• use a word that might be guessed (such as your
  name)?
• share passwords with others?
• change your password often?
• write your password down?
• Network eavesdropping is possible
• One solution is to encrypt messages
• Other security problems include
• Trapdoors, computer viruses, network worms, and
  program trojan horses