CS120: Lecture 6

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CS120 Lecture 6

• MP Johnson
• Hunter
• mpjohnson_at_gmail.com

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Agenda

• Review
• OSs
• History/motiv
• Duties
• Multiple processes
• Modern history - Stephenson

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Program exec add
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Machines ? OS

• Can now write programs
• Complex programs still very difficult
• I/O, mem management, etc.
• Need an operating system

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OSs

• Stephenson An OS was a very long string of
  ones and zeroes that, when properly installed and
  coddled, gave you the ability to manipulate other
  very long strings of ones and zeroes.
• OS prog / set or progs that
• Manages computer, files
• Helps you run programs

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Software taxonomy
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OS parts, tasks

• Shell provide access
• Command-line, GUI (window manager)
• Give access to files
• Kernel system housekeeping
• Booting
• Mem manage
• Security
• Comm with device drivers
• Process management

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Booting

• Starting up the comp, so programs can run
• Last time saw
• Can run prog, once instructions are in RAM
• Another prog could write them to RAM
• but who runs that prog?
• Prob for every running prog, some other prog
  must have run it

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Booting

• Short for bootstrapping
• Legend German nobleman Baron Münchhausen lifted
  himself out of a swamp, by pulling his own hair
  
• Later version pulled seft out of the sea, but
  pulling his boot straps

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Booting

• Soln small prog (bootstrap) stored in ROM
  memory
  
• At system startup, PC points to bootstrap
• When it runs, it copies OS into RAM
• And points PC there (JUMP)
• Now OS can take over
• And load/run other programs
• Another analogy throw bundle of roap

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Booting

• Depending on ROM/BIOS, many machines look for
  floppy/CD/USB
  
• Try to boot from there
• People install Linux on USB flash or iPod
• Boot from that
• Can also dual-boot
• Bootstrap asks users which OS

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Mem manage

• Saw that PC points to mem location
• Each program has own space
• OS prevents from reading/writing other programs
  data
  
• Prob may not have enough RAM to hold all
  programs user runs
  
• Soln virtual memory
• Redirect RAM request to HD
• Slow

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Security

• Commonest idea login
• Prompt for user/pass
• In Win, press ctr-alt-del to login (why?)
• Finer-grain
• Read/write permissions for each file/dir
• Some programs cant write to HD
• Java applets, ActiveX
• Should they be able to listen on mic? Slashdot

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Device drivers

• Main OS talks to device drivers
• They talk to device controllers,
• Which talk to device
• General msgs sent from OS become more explicit
• OS doesnt need to know about your partic
  printer
  
• It just needs the right device driver
• Abstraction Win x65

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OS motiv

• In old days each program run indy
• User writes prog, waits in line, runs, leaves
• First change separate user, machine
• All programs given to computer operator, run by
  him, results given back
  
• Next change get all jobs, run as group
• batch processing
• Job described in JCL (job control lang)
• Fast, automatic
• But if some bug, must start over from start

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OS motiv

• ? interactive programming/apps
• Command-line or GUI
• Convenient for user
• But slow wasted time while waiting for user
  input
  
• Not everyone gets a mainframe
• So other programs wait longer in line
• Soln time-sharing (user jobs) / multitasking
  (processes)
  
• one of major tasks of modern OS

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Multiple processes

• Process/job/thread single program running
• Process state
• Vals regs
• Vals in (its) RAM
• Program counter val
• Process ! program
• Could have two instances of one prog running
• E.g., dbl-click on Notepad twice

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Multiple processes

• Very common
• Clock prog ticks
• Doc prints
• Browser loads
• Playing game
• NT has 100 processes, before you run anything
• Some machines have mult processors
• dual-core
• But usually processes running
• Cant simply assign 1 to each processor
• ? time-sharing

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Time-sharing

• Keep track of running processes in process table
• Vars for each process
• Where its at
• Divide time into time-slices
• E.g., 50 ms
• At end of timeslice, have interrupt
• Store info for curr proc in P.T.
• Restart another process from where left off

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Time-sharing

• Has some overhead
• Memory of P.T.
• Time to switch
• But overall saves time
• Better uses time when waiting for user
• Modern OSs have preemtive multitasking
• In Win 95/3.1, OS less strict about interrupting
• ? often hung

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Mult. procs ? compet for resources

• What if 2 progs want to print/draw/read at same
  time?
  
• Only 1 can have access at once, or else conflict
• ? OS must control access how?
• Simple idea use a flag
• 1/0, set/clear, says whether printer in use
• On req, if clear, allow and set o.w., make
  wait
  
• When done, set clear, or give to waiting prog

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Flags

• Countereg
• Prog1 requests prn
• Check clear, soINTERUPTset and given
• Prog2 requests prn
• Check clear soINTERUPTset and given X
• Soln make check-and-set a single op
• semaphore

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Spooling

• Related is spooling
• Wont discuss strategies, but idea is
• When app wants to send job to device, take it,
  keep in buffer until device is ready
  
• Common e.g. print spooler

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Deadlock

• Very serious problem
• Two processes both waiting for resources,
• Both dependent on other
• Both block each other
• P1 wants to upload from HD to netw card
• Has HD access, waiting for netw access
• P2 wants to download from new to HD
• Has netw access, waiting for HD access
• ? deadlock!

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Dining philosophers (Dijkstra)

• 5 philosophers eating sushi, around table
• 5 chopsticks between them
• To eat, phil must pick up two adjacent
  chopsticks, one at time
  
• Sets them down after each bite
• Goal give strategy avoiding
• Deadlock and
• Starvation (of any phil)

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Modern OS history

• Unix ATT early 1970s
• Apple II CLI late 70s
• QDOS Tim Patterson early 80s
• Purchase by Bill Gates for 50k ?
• MS-DOS 1982
• MacOS Apple 1984
• Win3.0 MS 1990
• Linux Linus Torvalds 1991
• NT MS 1993
• Win95 MS 1995
• x64 MS 2005

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• Stephenson
• hw
• For next time read ch 4