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Operating Systems: Three Easy Pieces

Credit goes to palladian

Introduction

First, we should be frank: it's really hard to find a good self-contained online course on operating systems. OSTEP is the best course we've found so far. We describe below two approaches to the course, a "Base" approach which is suitable for most students, and an "Extended" approach, which is appropriate for students intending to specialize in systems programming.

The "base" approach covers all of our operating system curriculum requirements and should take about 80 hours of work.

The "extended" approach contains all of the work of the base approach and more. It involves learning very serious C and x86 assembly, and delving deep into kernel programming. It takes significantly more time (over 200 hours) and is much more challenging. For those students interested in this area of computing it is also highly rewarding.

Base Approach

  1. Read through the free online textbook Operating Systems: Three Easy Pieces
  2. Complete the homework questions at the end of each chapter. (There is an associated Github repository for the homeworks.)

This should take about 8 weeks, 10 hours/week. That's all you need to do!

You will need a Unix/Linux system, some basic command line tools, and a C compiler (such as GCC or Clang). On Windows, you can install Ubuntu in a virtual machine, or use WSL (Windows Subsystem for Linux). Mac OS is Unix-like, so it should be OK to use.

Course Links

C

Question: I see some C code in this book. How much C do I need to know?

Answer: You'll need to read and understand some C code in this book. You'll need basic understanding of arrays, pointers and print formatting. You can consult the free book Modern C by Jen Gustadt. The CS50 Manual pages are also helpful to look up functions. You shouldn't spend too much time on learning C.

The code you'll read is fairly simple and presented in short fragments. The book helps you out quite a bit by manually introducing many C APIs such as the Process API, the Thread API, and so on. You can type, compile and run the code fragments, and read the corresponding explanations. The book explains them in great detail in a conversational style that's fun to read.

You will also write a little bit of C code. Only a minority of the chapters (about 10 out of 50) ask you to write some C code (while the other chapters require you to run provided simulation code and answer questions). These are usually simple, short C programs that imitate the code that was presented in that chapter, with small modifications.

If you are getting stuck on these, please don't spend too much time on them. There is a great solution set here. There is no honor code for this, so you are free to use the solutions. If you find yourself spending too much time, feel free to read and understand the solutions instead. Your main priority should be to gain understanding of operating systems concepts, not to master C coding.

Extended Approach

If you've chosen this option, then this is the first course in the OSSU curriculum for which you'll need to learn some prerequisites on your own before starting it, in addition to the courses that come before it in the curriculum. You might also run into some issues running the scripts for homework demos and for testing your solutions to the projects (although we hope we've solved most of those by now).

That said, if you're able to commit the time required for the prerequisites, we believe the reward is well worth the effort: this course is exciting, interesting, and quite useful for other fields of computer science and programming. One big attraction of this course is the opportunity to see a simplified but fully-functional Unix-like operating system in action and understand the concepts and design decisions that went into it as well as the low-level implementation details.

You should either watch all the lecture videos or read chapters 1 through 47 in the textbook (don't worry, the chapters are usually just a few pages long) as well as finish the projects listed below. We also strongly encourage you to do the homework exercises as they're assigned on the course website or in the book chapters; think of these like the "check-your-understanding" questions that pop up in the middle of lecture videos on sites like Coursera or edX.

Prerequisites

This class requires a lot of experience programming in C. You should finish one of the C books listed in the resources below before starting this course; if you try to learn C at the same time as the course material, you're likely to feel overwhelmed. If you haven't used C before, you should expect to spend a lot of time on this; it's hard to predict how long it might take for each person, but a rough estimate might be 8-10 hours per week for 3-5 weeks. You can always learn C alongside another OSSU course or even redo the exercises for other courses in C to gain practice with it.

You should also finish both parts of Nand2Tetris before starting this course. OSTEP focuses on the real-world x86 and x86_64 architectures, so you'll have to fill in some gaps in order to translate the concepts you learned in Nand2Tetris to a new architecture. You can do that with the x86 resources below, but note that they all assume you know C, so learn that first. This should take around 6-8 hours in total.

Course Links

Roadmap

This course was originally taught as CS 537 at the University of Wisconsin by the author of the OSTEP textbook, so the projects are assigned in the course according to the best times to give UWisconsin students access to on-campus resources like recitation sections and office hours. That means they don't match up perfectly with the material being covered at that time in the lectures or textbook chapters. We recommend doing the course in the following order instead.

Reading order

  • Read chapters 1 and 2 of the OSTEP textbook and watch the first half (the introduction) of lecture 1.
  • Do the initial-utilities project; it's intended as a litmus test for you to make sure you're comfortable enough with C before taking this class. You can watch discussion 1 for help. If it takes you more than 2 hours to write the code (not counting the discussion time and any time spent debugging), you should consider spending more time learning C before moving on in the course. (If you want more practice, you can do initial-reverse too, but it's not required.)
  • Watch lectures 1 through 5 and read chapters 3 through 24 of the OSTEP textbook. We recommend doing the homework assignments as they come up in the course calendar or book chapters.
  • Watch discussion 3 and reread chapter 5, then do the processes-shell project.
  • Read the annotated guide to xv6 linked in the resources section below, starting from the beginning and stopping after the System Calls: Processes section.
  • Watch discussion 2, then do the initial-xv6 project.
  • Watch discussion 5, then do the scheduling-xv6-lottery project.
  • Watch discussion 7, then do the vm-xv6-intro project.
  • Watch lectures 6 through 9 (and optionally, the review lecture) and read chapters 25 through 34; again, you're encouraged to do the homework.
  • Watch discussion 10, then do the concurrency-xv6-threads project.
  • Watch discussions 11 and 12, then do the concurrency-mapreduce project.
  • Watch lectures 10 through 14 (and optionally, the second review lecture) and read chapters 35 through 47; remember to do the homework along with the lectures or chapters.
  • Do the filesystems-checker project.

Running the Projects

This course was originally taught as CS 537 at the University of Wisconsin by the author of the OSTEP textbook, which means that the homework and projects were written with those students as a target audience and designed to be run on UWisconsin lab computers with specific software versions pre-installed for students. We hope this section fixes that so you can run them on other computers, but we haven't tested this on every computer, so if you run into other issues, let us know on the Discord channel and we'll try to help out.

In order to run the homework and projects on Linux or macOS, you'll need to have all of the following programs installed:

  • gcc
  • gas
  • ld
  • gdb
  • make
  • objcopy
  • objdump
  • dd
  • python
  • perl
  • gawk
  • expect
  • git

You will also need to install qemu, but we recommend using the patched version provided by the xv6 authors; see this link for details.

On macOS, you'll need to install a cross-compiler gcc suite capable of producing x86 ELF binaries; see the link above for details as well.

On Windows, you can use a Linux virtual machine for the homework and projects. Some of these packages are not yet supported on Apple M1 computers, and virtual machine software has not yet been ported to the new processor architecture; some students have used a VPS to do the homework and projects instead.

Next, clone the ostep-homework and ostep-projects repositories:

git clone https://github.com/remzi-arpacidusseau/ostep-homework/
git clone https://github.com/remzi-arpacidusseau/ostep-projects/
cd ostep-projects

You'll have to clone the xv6-public repository into the directory for each xv6-related OSTEP project. You could use the same copy for all the projects, but we recommend using separate copies to avoid previous projects causing bugs for later ones. Run the following commands in each of the initial-xv6, scheduling-xv6-lottery, vm-xv6-intro, concurrency-xv6-threads, and filesystems-checker directories.

mkdir src
git clone https://github.com/mit-pdos/xv6-public src

Hints and tips for Projects

Resources

C

Please don't try to learn C from sites like GeeksforGeeks, TutorialsPoint, or Hackr.io (we're not even gonna link to them here). Those are great resources for other languages, but C has way too many pitfalls, and C tutorials online are often filled with dangerous errors and bad coding practices. We looked at many C resources for the recommendations below and unfortunately found many bad or unsafe ones; we'll only include the best ones here, so look no further!

We recommend learning C by working through (the entirety of) Jens Gustedt's Modern C, which is freely available online. This book is relatively short and will bring you up to speed on the C language itself as well as modern coding practices for it. Make sure to do all the exercises in the footnotes!

While the book above is our default recommendation, we also recommend K.N. King's C Programming: A Modern Approach as a second, more beginner-friendly option. It has some disadvantages: it's much longer (almost 850 pages), it's not available for free (and copies can be hard to find), and it's not quite as recent as Modern C (but still relevant nonetheless). That said, it has more exercises if you want extra practice, and the Q&A sections at the end of each chapter are filled with pearls of C wisdom and answers to C FAQs. It also covers almost the entirety of the C language and standard library, so it doubles as a reference book.

CS 50 doesn't quite cover enough C for OSTEP, but if you've already taken CS 50, you can supplement it with the books above.

Additional (optional) resources include:

  • CS 50 Manual Pages: a great reference for looking up C library functions; most functions include both the usual manual as well as a beginner-friendly "less comfortable" option (just note that the "less comfortable" version uses string as an alias for char *.)
  • cdecl: a tool to translate C gibberish into English.
  • C track on exercism.org: additional practice exercises.
  • Secure Coding Practices in C and C++: if you want to understand why other C resources are so unsafe.
  • The C Programming Language: the original book on C by its creators. Too outdated for OSTEP, but a good read if you manage to find a copy.

x86 Architecture and Assembly Language

Nand2Tetris has already introduced most of the concepts you'll need to understand systems and computer architectures, so now you just need to port that knowledge to the real-world (32-bit) x86 architecture.

The easiest way to do that is by watching a subset of the lectures from the Computer Systems: A Programmer's Perspective course (or reading the matching chapters in the textbook of the same name). The lectures you'll need are:

Further more, it is recommened to do the following labs. These labs are meant to teach you how to work with assembly:

  • Bomb Lab: Writeup, self-study handout.

    A "binary bomb" is a program provided to students as an object code file. When run, it prompts the user to type in 6 different strings. If any of these is incorrect, the bomb "explodes," printing an error message and logging the event on a grading server. Students must "defuse" their own unique bomb by disassembling and reverse engineering the program to determine what the 6 strings should be. The lab teaches students to understand assembly language, and also forces them to learn how to use a debugger. It's also great fun. A legendary lab among the CMU undergrads.

  • Attack Lab: Writeup, self-study handout.

    Students are given a pair of unique custom-generated x86-64 binary executables, called targets, that have buffer overflow bugs. One target is vulnerable to code injection attacks. The other is vulnerable to return-oriented programming attacks. Students are asked to modify the behavior of the targets by developing exploits based on either code injection or return-oriented programming. This lab teaches the students about the stack discipline and teaches them about the danger of writing code that is vulnerable to buffer overflow attacks. Note: run the targets with the -q flag to prevent them from trying to contact a non-existent grading server.

Additional (optional) resources include:

xv6

You don't need to read anything about xv6 until after you start OSTEP; in fact, we recommend holding off on the xv6-related projects until you've finished the entire section on virtualization. After that, you'll need a guide to walk you through the source code.

The xv6 authors provide a book that you can read alongside the source code. There's also a handy line-numbered PDF version of the code with an index to see exactly where each function or constant gets used.

However, that book glosses over a lot of the details in the code that you might find challenging, including the advanced C features used, the x86 architecture- specific instructions, and the concurrency aspects (if you haven't finished that section of OSTEP before starting the xv6 projects). To solve this problem, we provide an annotated guide to xv6 that goes over the entire xv6 code and analyzes it line-by-line with explanations of the C features, hardware specs, and x86 conventions used. That means it's longer than the official xv6 book, so you don't have to read all of it (and you can probably skip the optional sections unless you care about device drivers), but you can use it as a reference if you're scratching your head about some part of the code.

Here is another in-depth explanation of the xv6 code.

Also here is an excellent video series walking through much of the xv6 code.

Miscellaneous

You'll need a general sense of how Makefiles work in order to use the Makefile for xv6. This tutorial covers much more than you need; just read the "Getting Started" and "Targets" sections and come back to the rest later if you need to look something up (but you shouldn't have to).

Additional (optional) resources include:

  • GCC Command Options: a guide to command-line flags for the GNU C compiler gcc.
  • Linker Scripts: a guide to writing scripts for the GNU linker ld.
  • OSDev Wiki: a great resource for all kinds of OS concepts and implementation details.
  • Linux Kernel Development: if you want to apply your xv6 knowledge toward contributing to the Linux kernel, this is a great read after OSTEP.