Chapter 3: OS and Networks

1. Chapter 3: OS and Networks • With the number of resources available, we need Operating Systems and Networks • Network resources: • printers • file servers • other computers • 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 processor 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 OS’s 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 • 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 OS’s: good or bad? • There used to be many OS’s 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 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 Applications Systems Utility Operating System Shell Kernel

7. 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 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 Operating System User Operating System User Shell User

8. 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 Boot Program

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, keyboard entry, 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 we do 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 P0 P1 R0 R1 • 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 won’t give up R1 until it gets R0 and P1 can’t get R0 until P0 is done with it but P0 won’t 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! See figure 3.9 on page 139 for a real-life deadlock

19. 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) 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 More on Deadlock

20. 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 Networks

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 60’s connecting four local area networks together • by the early 70’s, more networks were added • by the 80’s, 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 • network identifiers (often called domain name servers) provide information across the Internet so that one machine can learn another machine’s 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

24. 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 computer’s 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 machine’s 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 Sending/Receiving messages

25. 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) More on the Internet

26. 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

27. 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 Security