6.828 2016 L1: O/S overview

Overview

• 6.828 goals

  • Understand operating systems in detail by designing and implementing a small O/S
  • Hands-on experience with building systems ("Applying 6.033")

• What do applications want from an O/S?

  • Abstract the hardware for convenience and portability
  • Multiplex the hardware among multiple applications
  • Isolate applications to contain bugs
  • Allow sharing among applications

• What is an OS?

  • e.g. OSX, Windows, Linux
  • the small view: a h/w management library
  • the big view: physical machine -> abstract one w/ better properties

• Organization: layered picture h/w: CPU, mem, disk kernel: [various services] user: applications, e.g. vi and gcc

  • we care a lot about the interfaces and internal kernel structure

• What services does an O/S kernel typically provide?

  • processes
  • memory
  • file contents
  • directories and file names
  • security
  • many others: users, IPC, network, time, terminals

• What does an O/S abstraction look like?

  • Applications see them only via system calls

  • Examples, from UNIX / Linux:

    fd = open("out", 1);
    write(fd, "hello\n", 6);
    pid = fork();
    

• Why is O/S design/implementation hard/interesting?

  • the environment is unforgiving: weird h/w, no debugger
  • it must be efficient (thus low-level?) ...but abstract/portable (thus high-level?)
  • powerful (thus many features?) ...but simple (thus a few composable building blocks?)
  • features interact: fd = open(); ...; fork()
  • behaviors interact: CPU priority vs memory allocator.
  • open problems: security, multi-core

• You'll be glad you learned about operating systems if you...

  • want to work on the above problems
  • care about what's going on under the hood
  • have to build high-performance systems
  • need to diagnose bugs or security problems

Class structure

• See web site: https://pdos.csail.mit.edu/6.828

• Lectures

  • basic O/S ideas
  • extended inspection of xv6, a traditional O/S
  • several more recent topics
  • xv6 programming to re-inforce xv6 understanding

• Lab: JOS, a small O/S for x86 in an exokernel style

  • you build it, 5 labs + final lab of your choice
  • kernel interface: expose hardware, but protect -- no abstractions!
  • unprivileged library: fork, exec, pipe, ...
  • applications: file system, shell, ..
  • development environment: gcc, qemu
  • lab 1 is out
  • make grade

• Code review

• Two quizzes: one in class hours, one in final's week

Example system calls

• 6.828 is largely about design and implementation of system call interface. let's start by looking at how programs use that interface.

• a simple example: what system calls does "ls" call?

  • Trace system calls:
  • On OSX: sudo dtruss /bin/ls
  • On Linux: strace /bin/ls

• simpler program: echo

  • what are file descriptors? (0, 1, 2, etc. in read/write) echo.c

  • what if want to redirect i/o to a file? echo.c

• a more interesting program: the Unix shell.

  • shell is the Unix command UI
  • typically handles login session, runs other processes
  • you saw it in 6.033: http://web.mit.edu/6.033/www/assignments/handson-unix.html
  • the shell is also a programming/scripting language
  • look at some simple examples of shell operations, how they use different O/S abstractions, and how those abstractions fit together. See Unix paper if you are unfamiliar with the shell.

• Simplified xv6 sh.c

  • See chapter 0 of xv6 book
  • Basic organization: parsing and executing commands (e.g., ls, ls | wc, ls > out)
  • Shell implemented using system calls (e.g., read, write, fork, exec, wait) conventions: -1 return value signals error, error code stored in errno, perror prints out a descriptive error message based on errno.
  • Many systems calls are encapsulated in libc calls (e.g., fgets vs read)

• what does fork() do? copies user memory copies process kernel state (e.g. user id) child gets a different PID child state contains parent PID returns twice, with different values
• parent and child may run concurrently (e.g., on different processors).

• what does wait() do? waits for any child to exit what if child exits before parent calls wait?

• how to execute echo?

   if (execv(ecmd->argv[0], ecmd->argv) == -1) {
     perror("exec failed");
     exit(-1);
   }
  

• how to do I/O redirection (i.e., >)

   close(rcmd->fd);
   if(open(rcmd->file, rcmd->mode, S_IRWXU) < 0) {
     perror("open failed\n");
     exit(-1);
   }
  

  cool: echo doesn't need to be changed at all!

• Homework assignment for shell