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1. Introduction

Welcome to Pintos. Pintos is a simple operating system framework for the 80x86 architecture. It supports kernel threads, loading and running user programs, and a file system, but it implements all of these in a very simple way. In the Pintos projects, you and your project team will strengthen its support in all three of these areas. You will also add a virtual memory implementation.

Pintos could, theoretically, run on a regular IBM-compatible PC. Unfortunately, it is impractical to supply every student a dedicated PC for use with Pintos. Therefore, we will run Pintos projects in a system simulator, that is, a program that simulates an 80x86 CPU and its peripheral devices accurately enough that unmodified operating systems and software can run under it. In class we will use the Bochs and QEMU simulators. Pintos has also been tested with VMware Player.

These projects are hard. They have a reputation of taking a lot of time, and deservedly so. We will do what we can to reduce the workload, such as providing a lot of support material, but there is plenty of hard work that needs to be done. We welcome your feedback. If you have suggestions on how we can reduce the unnecessary overhead of assignments, cutting them down to the important underlying issues, please let us know.

This chapter explains how to get started working with Pintos. You should read the entire chapter before you start work on any of the projects.

1.1 Getting Started

To get started, you'll have to log into a machine that Pintos can be built on. The CS 212 "officially supported" Pintos development machines are the FarmShare UNIX machines, as described on the FarmShare webpage. We will test your code on these machines, and the instructions given here assume this environment. We cannot provide support for installing and working on Pintos on your own machine, but we provide instructions for doing so nonetheless (see section G. Installing Pintos).

Once you've logged into one of these machines, either locally or remotely, start out by adding our binaries directory to your PATH environment. Under bash, Stanford's default login shell, you can do so with this command:(1)

export PATH=/afs/ir.stanford.edu/class/cs212/`uname -m`/bin:$PATH
Under tcsh, the older default, use:
set path = ( /afs/ir.stanford.edu/class/cs212/`uname -m`/bin $path )
Notice that both ` are left single quotes or "backticks," not apostrophes ('). It is a good idea to add this line to the .profile (for bash) or .login (for tcsh) file in your home directory. Otherwise, you'll have to type it every time you log in.

1.1.1 Source Tree Overview

Now you can clone the git repository of pintos by executing

git clone http://cs212.scs.stanford.edu/pintos.git

Alternatively, fetch http://cs212.stanford.edu/pintos.tar.gz and extract it.

Let's take a look at what's inside. Here's the directory structure that you should see in pintos/src:

Source code for the base kernel, which you will modify starting in project 1.

Source code for the user program loader, which you will modify starting with project 2.

An almost empty directory. You will implement virtual memory here in project 3.

Source code for a basic file system. You will use this file system starting with project 2, but you will not modify it until project 4.

Source code for I/O device interfacing: keyboard, timer, disk, etc. You will modify the timer implementation in project 1. Otherwise you should have no need to change this code.

An implementation of a subset of the standard C library. The code in this directory is compiled into both the Pintos kernel and, starting from project 2, user programs that run under it. In both kernel code and user programs, headers in this directory can be included using the #include <...> notation. You should have little need to modify this code.

Parts of the C library that are included only in the Pintos kernel. This also includes implementations of some data types that you are free to use in your kernel code: bitmaps, doubly linked lists, and hash tables. In the kernel, headers in this directory can be included using the #include <...> notation.

Parts of the C library that are included only in Pintos user programs. In user programs, headers in this directory can be included using the #include <...> notation.

Tests for each project. You can modify this code if it helps you test your submission, but we will replace it with the originals before we run the tests.

Example user programs for use starting with project 2.

These files may come in handy if you decide to try working with Pintos on your own machine. Otherwise, you can ignore them.

1.1.2 Building Pintos

As the next step, build the source code supplied for the first project. First, cd into the threads directory. Then, issue the make command. This will create a build directory under threads, populate it with a Makefile and a few subdirectories, and then build the kernel inside. The entire build should take less than 30 seconds.

Watch the commands executed during the build. On the Linux machines, the ordinary system tools are used.

Following the build, the following are the interesting files in the build directory:

A copy of pintos/src/Makefile.build. It describes how to build the kernel. See Adding Source Files, for more information.

Object file for the entire kernel. This is the result of linking object files compiled from each individual kernel source file into a single object file. It contains debug information, so you can run GDB (see section E.5 GDB) or backtrace (see section E.4 Backtraces) on it.

Memory image of the kernel, that is, the exact bytes loaded into memory to run the Pintos kernel. This is just kernel.o with debug information stripped out, which saves a lot of space, which in turn keeps the kernel from bumping up against a 512 kB size limit imposed by the kernel loader's design.

Memory image for the kernel loader, a small chunk of code written in assembly language that reads the kernel from disk into memory and starts it up. It is exactly 512 bytes long, a size fixed by the PC BIOS.

Subdirectories of build contain object files (.o) and dependency files (.d), both produced by the compiler. The dependency files tell make which source files need to be recompiled when other source or header files are changed.

1.1.3 Running Pintos

We've supplied a program for conveniently running Pintos in a simulator, called pintos. In the simplest case, you can invoke pintos as pintos argument.... Each argument is passed to the Pintos kernel for it to act on.

Try it out. First cd into the newly created build directory. Then issue the command pintos run alarm-multiple, which passes the arguments run alarm-multiple to the Pintos kernel. In these arguments, run instructs the kernel to run a test and alarm-multiple is the test to run.

This command creates a bochsrc.txt file, which is needed for running Bochs, and then invoke Bochs. Bochs opens a new window that represents the simulated machine's display, and a BIOS message briefly flashes. Then Pintos boots and runs the alarm-multiple test program, which outputs a few screenfuls of text. When it's done, you can close Bochs by clicking on the "Power" button in the window's top right corner, or rerun the whole process by clicking on the "Reset" button just to its left. The other buttons are not very useful for our purposes.

(If no window appeared at all, then you're probably logged in remotely and X forwarding is not set up correctly. In this case, you can fix your X setup, or you can use the -v option to disable X output: pintos -v -- run alarm-multiple.)

The text printed by Pintos inside Bochs probably went by too quickly to read. However, you've probably noticed by now that the same text was displayed in the terminal you used to run pintos. This is because Pintos sends all output both to the VGA display and to the first serial port, and by default the serial port is connected to Bochs's stdin and stdout. You can log serial output to a file by redirecting at the command line, e.g. pintos run alarm-multiple > logfile.

The pintos program offers several options for configuring the simulator or the virtual hardware. If you specify any options, they must precede the commands passed to the Pintos kernel and be separated from them by --, so that the whole command looks like pintos option... -- argument.... Invoke pintos without any arguments to see a list of available options. Options can select a simulator to use: the default is Bochs, but --qemu selects QEMU. You can run the simulator with a debugger (see section E.5 GDB). You can set the amount of memory to give the VM. Finally, you can select how you want VM output to be displayed: use -v to turn off the VGA display, -t to use your terminal window as the VGA display instead of opening a new window (Bochs only), or -s to suppress serial input from stdin and output to stdout.

The Pintos kernel has commands and options other than run. These are not very interesting for now, but you can see a list of them using -h, e.g. pintos -h.

1.1.4 Debugging versus Testing

When you're debugging code, it's useful to be able to run a program twice and have it do exactly the same thing. On second and later runs, you can make new observations without having to discard or verify your old observations. This property is called "reproducibility." One of the simulators that Pintos supports, Bochs, can be set up for reproducibility, and that's the way that pintos invokes it by default.

Of course, a simulation can only be reproducible from one run to the next if its input is the same each time. For simulating an entire computer, as we do, this means that every part of the computer must be the same. For example, you must use the same command-line argument, the same disks, the same version of Bochs, and you must not hit any keys on the keyboard (because you could not be sure to hit them at exactly the same point each time) during the runs.

While reproducibility is useful for debugging, it is a problem for testing thread synchronization, an important part of most of the projects. In particular, when Bochs is set up for reproducibility, timer interrupts will come at perfectly reproducible points, and therefore so will thread switches. That means that running the same test several times doesn't give you any greater confidence in your code's correctness than does running it only once.

So, to make your code easier to test, we've added a feature, called "jitter," to Bochs, that makes timer interrupts come at random intervals, but in a perfectly predictable way. In particular, if you invoke pintos with the option -j seed, timer interrupts will come at irregularly spaced intervals. Within a single seed value, execution will still be reproducible, but timer behavior will change as seed is varied. Thus, for the highest degree of confidence you should test your code with many seed values.

On the other hand, when Bochs runs in reproducible mode, timings are not realistic, meaning that a "one-second" delay may be much shorter or even much longer than one second. You can invoke pintos with a different option, -r, to set up Bochs for realistic timings, in which a one-second delay should take approximately one second of real time. Simulation in real-time mode is not reproducible, and options -j and -r are mutually exclusive.

The QEMU simulator is available as an alternative to Bochs (use --qemu when invoking pintos). The QEMU simulator is much faster than Bochs, but it only supports real-time simulation and does not have a reproducible mode.

1.2 Grading

We will grade your assignments based on test results and design quality, each of which comprises 50% of your grade.

1.2.1 Testing

Your test result grade will be based on our tests. Each project has several tests, each of which has a name beginning with tests. To completely test your submission, invoke make check from the project build directory. This will build and run each test and print a "pass" or "fail" message for each one. When a test fails, make check also prints some details of the reason for failure. After running all the tests, make check also prints a summary of the test results.

For project 1, the tests will probably run faster in Bochs. For the rest of the projects, they will run much faster in QEMU. make check will select the faster simulator by default, but you can override its choice by specifying SIMULATOR=--bochs or SIMULATOR=--qemu on the make command line.

You can also run individual tests one at a time. A given test t writes its output to t.output, then a script scores the output as "pass" or "fail" and writes the verdict to t.result. To run and grade a single test, make the .result file explicitly from the build directory, e.g. make tests/threads/alarm-multiple.result. If make says that the test result is up-to-date, but you want to re-run it anyway, either run make clean or delete the .output file by hand.

By default, each test provides feedback only at completion, not during its run. If you prefer, you can observe the progress of each test by specifying VERBOSE=1 on the make command line, as in make check VERBOSE=1. You can also provide arbitrary options to the pintos run by the tests with PINTOSOPTS='...', e.g. make check PINTOSOPTS='-j 1' to select a jitter value of 1 (see section 1.1.4 Debugging versus Testing).

All of the tests and related files are in pintos/src/tests. Before we test your submission, we will replace the contents of that directory by a pristine, unmodified copy, to ensure that the correct tests are used. Thus, you can modify some of the tests if that helps in debugging, but we will run the originals.

All software has bugs, so some of our tests may be flawed. If you think a test failure is a bug in the test, not a bug in your code, please point it out. We will look at it and fix it if necessary.

Please don't try to take advantage of our generosity in giving out our test suite. Your code has to work properly in the general case, not just for the test cases we supply. For example, it would be unacceptable to explicitly base the kernel's behavior on the name of the running test case. Such attempts to side-step the test cases will receive no credit. If you think your solution may be in a gray area here, please ask us about it.

1.2.2 Design

We will judge your design based on the design document and the source code that you submit. We will read your entire design document and much of your source code.

Don't forget that design quality, including the design document, is 50% of your project grade. It is better to spend one or two hours writing a good design document than it is to spend that time getting the last 5% of the points for tests and then trying to rush through writing the design document in the last 15 minutes. Design Document

We provide a design document template for each project. For each significant part of a project, the template asks questions in four areas:

Data Structures

The instructions for this section are always the same:

Copy here the declaration of each new or changed struct or struct member, global or static variable, typedef, or enumeration. Identify the purpose of each in 25 words or less.

The first part is mechanical. Just copy new or modified declarations into the design document, to highlight for us the actual changes to data structures. Each declaration should include the comment that should accompany it in the source code (see below).

We also ask for a very brief description of the purpose of each new or changed data structure. The limit of 25 words or less is a guideline intended to save your time and avoid duplication with later areas.


This is where you tell us how your code works, through questions that probe your understanding of your code. We might not be able to easily figure it out from the code, because many creative solutions exist for most OS problems. Help us out a little.

Your answers should be at a level below the high level description of requirements given in the assignment. We have read the assignment too, so it is unnecessary to repeat or rephrase what is stated there. On the other hand, your answers should be at a level above the low level of the code itself. Don't give a line-by-line run-down of what your code does. Instead, use your answers to explain how your code works to implement the requirements.


An operating system kernel is a complex, multithreaded program, in which synchronizing multiple threads can be difficult. This section asks about how you chose to synchronize this particular type of activity.


Whereas the other sections primarily ask "what" and "how," the rationale section concentrates on "why." This is where we ask you to justify some design decisions, by explaining why the choices you made are better than alternatives. You may be able to state these in terms of time and space complexity, which can be made as rough or informal arguments (formal language or proofs are unnecessary).

An incomplete, evasive, or non-responsive design document or one that strays from the template without good reason may be penalized. Incorrect capitalization, punctuation, spelling, or grammar can also cost points. See section D. Project Documentation, for a sample design document for a fictitious project. Source Code

Your design will also be judged by looking at your source code. We will typically look at the differences between the original Pintos source tree and your submission, based on the output of a command like diff -urpb pintos.orig pintos.submitted. We will try to match up your description of the design with the code submitted. Important discrepancies between the description and the actual code will be penalized, as will be any bugs we find by spot checks.

The most important aspects of source code design are those that specifically relate to the operating system issues at stake in the project. For example, the organization of an inode is an important part of file system design, so in the file system project a poorly designed inode would lose points. Other issues are much less important. For example, multiple Pintos design problems call for a "priority queue," that is, a dynamic collection from which the minimum (or maximum) item can quickly be extracted. Fast priority queues can be implemented many ways, but we do not expect you to build a fancy data structure even if it might improve performance. Instead, you are welcome to use a linked list (and Pintos even provides one with convenient functions for sorting and finding minimums and maximums).

Pintos is written in a consistent style. Make your additions and modifications in existing Pintos source files blend in, not stick out. In new source files, adopt the existing Pintos style by preference, but make your code self-consistent at the very least. There should not be a patchwork of different styles that makes it obvious that three different people wrote the code. Use horizontal and vertical white space to make code readable. Add a brief comment on every structure, structure member, global or static variable, typedef, enumeration, and function definition. Update existing comments as you modify code. Don't comment out or use the preprocessor to ignore blocks of code (instead, remove it entirely). Use assertions to document key invariants. Decompose code into functions for clarity. Code that is difficult to understand because it violates these or other "common sense" software engineering practices will be penalized.

In the end, remember your audience. Code is written primarily to be read by humans. It has to be acceptable to the compiler too, but the compiler doesn't care about how it looks or how well it is written.

1.3 Legal and Ethical Issues

Pintos is distributed under a liberal license that allows free use, modification, and distribution. Students and others who work on Pintos own the code that they write and may use it for any purpose. Pintos comes with NO WARRANTY, not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See section License, for details of the license and lack of warranty.

In the context of Stanford's CS 212 course, please respect the spirit and the letter of the honor code by refraining from reading any homework solutions available online or elsewhere. Reading the source code for other operating system kernels, such as Linux or FreeBSD, is allowed, but do not copy code from them literally. Please cite the code that inspired your own in your design documentation.

1.4 Acknowledgements

The Pintos core and this documentation were originally written by Ben Pfaff blp@cs.stanford.edu.

Additional features were contributed by Anthony Romano chz@vt.edu.

The GDB macros supplied with Pintos were written by Godmar Back gback@cs.vt.edu, and their documentation is adapted from his work.

The original structure and form of Pintos was inspired by the Nachos instructional operating system from the University of California, Berkeley ([ Christopher]).

The Pintos projects and documentation originated with those designed for Nachos by current and former CS 212 (previously CS 140) teaching assistants at Stanford University, including at least Yu Ping, Greg Hutchins, Kelly Shaw, Paul Twohey, Sameer Qureshi, and John Rector.

Example code for monitors (see section A.3.4 Monitors) is from classroom slides originally by Dawson Engler and updated by Mendel Rosenblum.

1.5 Trivia

Pintos originated as a replacement for Nachos with a similar design. Since then Pintos has greatly diverged from the Nachos design. Pintos differs from Nachos in two important ways. First, Pintos runs on real or simulated 80x86 hardware, but Nachos runs as a process on a host operating system. Second, Pintos is written in C like most real-world operating systems, but Nachos is written in C++.

Why the name "Pintos"? First, like nachos, pinto beans are a common Mexican food. Second, Pintos is small and a "pint" is a small amount. Third, like drivers of the eponymous car, students are likely to have trouble with blow-ups.

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