OS Labs

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OS Labs

• 2/25/08
• Frans Kaashoek
• MIT
• kaashoek_at_mit.edu

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New labs for Tsinghua OS course

• Operating systems can be misleadingly simple
• Clean simple abstractions, easily understandable
  in isolation
• Complexity is in how their implementations
  interact
• Learn by doing, focus on interactions
• How do hardware interrupts interact with kernel
  and user-level processes?
• How to use virtual memory to solve many problems?
• Labs build a complete OS from the ground up

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Why might you care?

• Perhaps you will build an OS
• Perhaps you will design hardware
• Or write a device driver, network stack, or a
  file system
• Or need to diagnose application bugs or
  performance
• Or you just want to know how computers work
• Or you just want to have fun programming

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Lab OS JOS

• Labs based on MITs OS class (6.828)

sh
ls
fork
LibOS w. Unix API
LibOS w. Unix API
exofork
Small kernel with low-level API

• JOS has an exokernel/hypervisor structure
• Different than Linuxs organization
• Forces you to see something else and think

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The labs bottom up

• Booting
• Memory management
• User-level environments
• Preemptive multitasking
• File system and spawn
• A shell

required
optional
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Development environment

• Tools gcc, make, etc.
• Target hardware platform x86 PC
• For debugging bochs and qemu
• Lab assigments come with tests
• Details at course web pages

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sh shell

• Interactive command interpreter
• Interface (the shell) to the operating system
• Examples of shell commands
• ls create process
• ls gt tmp1 write output to file
• sh lt script gt tmp1 run sh script
• sort tmp uniq wc process communicate
  with pipe
• compute-pi run program in background
• .

OS ideas isolation, concurrency, communication,
synchronization
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shell implementation

• while (1)
• printf()
• readcommand(command, args)
• pid fork() // new process
  concurrency
• if (pid 0) // child?
• exec (command, args, 0) // run command
• else if (pid gt 0) // parent?
• r wait (0) // wait until child is done
• else
• perror(Failed to fork\n)

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Input/Output (I/O)

• I/O through file descriptors
• File descriptor may be for a file, terminal,
• Example calls
• read(fd, buf, sizeof(buf))
• write(fd, buf, sizeof(buf))
• Convention
• 0 input
• 1 output
• 2 error
• Child inherits open file descriptors from parents

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I/O redirection

• Example ls gt tmp1
• Modify sh to insert before exec
• close(1) // release fd 1
• fd create(tmp1, 0666) // fd will be 1
• No modifications to ls!
• ls could be writing to file, terminal, etc.,
  but programmer of ls doesnt need to know

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Pipe one-way communication

• int fdarray2
• char buf512
• int n
• pipe(fdarray) // returns 2 fds
• write(fdarray1, hello, 5)
• read(fdarray0, buf, sizeof(buf))
• buf contains h, e, l, l, o

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Pipe between parent child

• int fdarray2
• char buf512
• int n, pid
• pipe(fdarray)
• pid fork()
• if(pid gt 0)
• write(fdarray1, "hello", 5)
• else
• n read(fdarray0, buf, sizeof(buf))
• Synchronization between parent and child
• read blocks until there is data
• How does the shell implement a b?

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Implementing shell pipelines

• int fdarray2
• if (pipe(fdarray) lt 0) panic ("error")
• if ((pid fork ()) 0) // child (left end
  of pipe)
• close (1)
• tmp dup (fdarray1) // fdarray1 is the
  write end, tmp will be 1
• close (fdarray0) // close read end
• close (fdarray1) // close fdarray1
• exec (command1, args1, 0)
• else if (pid gt 0) // parent (right end
  of pipe)
• close (0)
• tmp dup (fdarray0) // fdarray0 is the
  read end, tmp will be 0
• close (fdarray0)
• close (fdarray1) // close write end
• exec (command2, args2, 0)
• else
• printf ("Unable to fork\n")

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OS abstractions and ideas

• Processes (fork exec wait)
• Files (open, create, read, write, close)
• File descriptor (dup, ..)
• Communication (pipe)
• Also a number of OS ideas
• Isolation between processes
• Concurrency
• Coordination/Synchronization
• Your job implement abstractions and understand
  ideas

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What will you know at the end?

• Understand OS abstractions in detail
• Intel x86
• The PC platform
• The C programming language
• Unix abstractions
• Experience with building system software
• Handle complexity, concurrency, etc.

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Lab lectures

• Monday 3/3
• Intel x86 introduction, assembly, stack
• Helps with lab 1
• Monday 3/17
• Intel x86 memory management interrupts
• Helps with lab 2 3

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Bonus lectures special topics

• OS architecture
• Virtual machines
• File systems
• Security
• Multicore
• Finding errors in OSes
• Some Wednesdays see schedule

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Have fun!