Assignment 2: System calls and processes

1. Due Dates and Mark Distribution

Due Date: 8am, Wed May 7th

Marks: Worth 25 marks (of the 100 available for the class mark component of the course)

The 10% bonus for one week early is available for the basic assignment.

The advanced assignment is assessable for COMP3891/COMP9283 students. It is worth 5 marks of the 100 class marks available.

COMP3231/COMP9201 students can do the advanced part with the permission of the lecturer, and only if basic assignment is completed a week prior to the deadline. Marks obtained are added to any shortfall in the participation mark component. The limited bonus marks are also available.

A small number of additional bonus marks are available as described in the advanced section.

The familiarisation questions contained herein are the subject of your week 8 tutorial. Please answer the questions and bring them to your tutorial.

2. Introduction

In this assignment you will be implementing a set of file-related system calls. Upon completion, your operating system will be able to run a single application at user-level and perform some basic file I/O.

A substantial part of this assignment is understanding how OS/161 works and determining what code is required to implement the required functionality. Expect to spend at least as long browsing and digesting OS/161 code as actually writing and debugging your own code.

If you attempt the advanced part, you will add process related system calls and the ability to run multiple applications.

Your current OS/161 system has minimal support for running executables, nothing that could be considered a true process. Assignment 2 starts the transformation of OS/161 into something closer to a true operating system. After this assignment, it will be capable of running a process from actual compiled programs stored in your account. The program will be loaded into OS/161 and executed in user mode by System/161; this will occur under the control of your kernel. First, however, you must implement part of the interface between user-mode programs ("userland") and the kernel. As usual, we provide part of the code you will need. Your job is to design and build the missing pieces.

Our code can run one user-level C program at a time as long as it doesn't want to do anything but shut the system down. We have provided sample user programs that do this (reboot, halt, poweroff), as well as others that make use of features you might be adding in this and future assignments. So far, all the code you have written for OS/161 has only been run within, and only been used by, the operating system kernel. In a real operating system, the kernel's main function is to provide support for user-level programs. Most such support is accessed via "system calls." We give you two system calls: sys_reboot() in main/main.c and sys_time() in syscall/time_syscalls.c. In GDB, if you put a breakpoint on sys_reboot() and run the "reboot" program, you can use "backtrace" to see how it got there.

For those attempting the advanced version of the assignment, you will also be implementing the subsystem that keeps track of multiple tasks. You must decide what data structures you will need to hold the data pertinent to a "process" (hint: look at kernel include files of your favorite operating system for suggestions, specifically the proc structure.) The first step is to read and understand the parts of the system that we have written for you.

User-level programs

Our System/161 simulator can run normal C programs if they are compiled with a cross-compiler, os161-gcc. This runs on a host (e.g., a Linux x86 machine) and produces MIPS executables; it is the same compiler used to compile the OS/161 kernel. To create new user programs, you will need to edit the Makefile in bin, sbin, or testbin (depending on where you put your programs) and then create a directory similar to those that already exist. Use an existing program and its Makefile as a template.


In the beginning, you should tackle this assignment by producing a DESIGN DOCUMENT. The design document should clearly reflect the development of your solution. They should not merely explain what you programmed. If you try to code first and design later, or even if you design hastily and rush into coding, you will most certainly end up in a software "tar pit". Don't do it! Plan everything you will do. Don't even think about coding until you can precisely explain to your partner what problems you need to solve and how the pieces relate to each other. Note that it can often be hard to write (or talk) about new software design, you are facing problems that you have not seen before, and therefore even finding terminology to describe your ideas can be difficult. There is no magic solution to this problem; but it gets easier with practice. The important thing is to go ahead and try. Always try to describe your ideas and designs to someone else. In order to reach an understanding, you may have to invent terminology and notation, this is fine. If you do this, by the time you have completed your design, you will find that you have the ability to efficiently discuss problems that you have never seen before. Why do you think that CS is filled with so much jargon? To help you get started, we have provided the following questions as a guide for reading through the code. We recommend that you answer questions for the different modules and be prepared to discuss them in your Week 8 tutorial. Once you have prepared the answers, you should be ready to develop a strategy for designing your code for this assignment.

3. Code walk-through

Bring your answers to the code walk-through questions to your week 8 tutorial.

3.1 General system calls

kern/arch/mips/: traps and syscalls

Exceptions are the key to operating systems; they are the mechanism that enables the OS to regain control of execution and therefore do its job. You can think of exceptions as the interface between the processor and the operating system. When the OS boots, it installs an "exception handler" (carefully crafted assembly code) at a specific address in memory. When the processor raises an exception, it invokes this, which sets up a "trap frame" and calls into the operating system. Since "exception" is such an overloaded term in computer science, operating system lingo for an exception is a "trap", when the OS traps execution. Interrupts are exceptions, and more significantly for this assignment, so are system calls. Specifically, syscall.c handles traps that happen to be syscalls. Understanding at least the C code in this directory is key to being a real operating systems junkie, so we highly recommend reading through it carefully.


mips_trap() is the key function for returning control to the operating system. This is the C function that gets called by the assembly exception handler. kill_curthread() is the function for handling broken user programs; when the processor is in usermode and hits something it can't handle (say, a bad instruction), it raises an exception. There's no way to recover from this, so the OS needs to kill off the process. The advance part of this assignment will include writing a useful version of this function.


syscall() is the function that delegates the actual work of a system call off to the kernel function that implements it. Notice that reboot and time are the only cases currently handled.


  1. What is the numerical value of the exception code for a MIPS system call?
  2. How many bytes is an instruction in MIPS? (Answer this by reading syscall() carefully, not by looking somewhere else.)
  3. What is the contents of the struct trapframe? Where is the struct trapframe that is passed into syscall() stored?
  4. What would be required to implement a system call that took more than 4 arguments?
  5. What is the purpose of userptr_t?

3.2 Moving data between kernel and userland.


This file contains functions for moving data between kernel and user space. Knowing when and how to cross this boundary is critical to properly implementing userlevel programs, so this is a good file to read very carefully. You should also examine the code in lib/uio.c.


  1. What is the difference between UIO_USERISPACE and UIO_USERSPACE? When should one use UIO_SYSSPACE instead?
  2. Why can the struct uio that is used to read in an ELF segment be allocated on the stack in load_segment() (i.e., where does the memory read actually go)?
  3. In what file are copyin() and copyout() defined? memmove()? Why can't copyin() and copyout() be implemented as simply as memmove()?

3.3 The VFS interface.


The files vfs.h and vnode.h in this directory contain function declarations and comments that are directly relevant to this assignment.


  1. How are vfs_open, vfs_close used? What other vfs_() calls are relevant?
  2. What are VOP_READ, VOP_WRITE? How are they used?
  3. What does VOP_TRYSEEK do?
  4. Where is the struct thread defined? What does this structure contain?
  5. Where is the struct proc defined? What does this structure contain?

3.4 Loading and running programs.

These questions mostly relate to the advanced assignment, but are worth exploring to understand how a program starts running.

 kern/syscall This directory contains some syscall implementations, and the files that are responsible for the loading and running of userlevel programs. Currently, the only files in the directory are loadelf.c, runprogram.c, and time_syscalls.c, although you may want to add more of your own during this assignment. Understanding these files is the key to getting started with the assignment, especially the advanced part, the implementation of multiprogramming. Note that to answer some of the questions, you will have to look in files outside this directory.


This file contains the functions responsible for loading an ELF executable from the filesystem and into virtual memory space. Of course, at this point this virtual memory space does not provide what is normally meant by virtual memory, although there is translation between the addresses that executables "believe" they are using and physical addresses, there is no mechanism for providing more memory than exists physically.


This file contains only one function, runprogram(), which is the function that is responsible for running a program from the kernel menu. Once you have designed your file system calls, a program started by runprogram() should have the standard file descriptors (stdout, stderr) available while it's running.

In the advanced assignment, runprogram() is a good base for writing the execv() system call, but only a base. When writing your design doc, you should determine what more is required for execv() that runprogram() does not need to worry about. Once you have design your process framework, runprogram() should be altered to start processes properly within this framework.


  1. What are the ELF magic numbers?
  2. In runprogram(), why is it important to call vfs_close before going to usermode?
  3. What function forces the processor to switch into usermode? Is this function machine dependent?


Answer these questions by reading the fork() man page and the sections dealing with fork() in the textbook.


  1. What is the purpose of the fork() system call?
  2. What process state is shared between the parent and child?
  3. What process state is copied between the parent and child?

3.4 The userland side of system calls.

This section is mostly related to the advanced version, but is still generally insightful to understand how system calls transition into the kernel.


The userland C startup code.There's only one file in here, mips/crt0.S, which contains the MIPS assembly code that receives control first when a user-level program is started. It calls main(). This is the code that your execv() implementation will be interfacing to, so be sure to check what values it expects to appear in what registers and so forth.


There's obviously a lot of code in the OS/161 C library (and a lot more yet in a real system's C library...) We don't expect you to read it all, although it may be instructive in the long run to do so. Job interviewers have an uncanny habit of asking people to implement simple standard C functions on the whiteboard. For present purposes you need only look at the code that implements the user-level side of system calls.


This is where the global variable errno is defined.


This file contains the machine-dependent code necessary for implementing the userlevel side of MIPS system calls. syscalls.Sis created from this file at compile time and is the actual file assembled to put into the C library. The actual names of the system calls are placed in this file using a script called that reads them from the kernel's header files. This avoids having to make a second list of the system calls. In a real system, typically each system call stub is placed in its own source file, to allow selectively linking them in. OS/161 puts them all together to simplify the makefiles.


  1. What is the purpose of the SYSCALL macro?
  2. What is the MIPS instruction that actually triggers a system call? (Answer this by reading the source in this directory, not looking somewhere else.)

4. Basic Assignment


Remember to use a 3231 subshell (or continue using your modified PATH) for this assignment, as outlined in asst0.

SVN setup

Only one of you needs to do the following. In this section, you will be setting up the svn repository that will contain your code. We suggest your partner sit in on this part of the assignment.

Import the OS/161 sources into your repository as follows:

% cd /home/cs3231/assigns
% svn import asst2/src file:///home/osprjXXX/repo/asst2/trunk -m "Initial import"

Make an immediate branch of this import for easy reference when you generate your diff:

% svn copy -m "Tag initial import" file:///home/osprjXXX/repo/asst2/trunk file:///home/osprjXXX/repo/asst2/initial


The following instructions are now for both partners. You have now completed setting up a shared repository for both partners. We'll assume your cs3231 directory is intact from the previous assignment. Change to your directory:

% cd ~/cs3231

Now checkout a copy of the os161 sources to work on from your shared repository:

% svn checkout file:///home/osprjXXX/repo/asst2/trunk asst2-src
You should now have a asst2-src directory to work on.

Building and Testing Your Assignment

Configure OS/161 for Assignment 2

Before proceeding further, configure your new sources.

% cd ~/cs3231/asst2-src
% ./configure

Unlike previous the previous assignment, you will need to build and install the user-level programs that will be run by your kernel in this assignment.

% cd ~/cs3231/asst2-src
% bmake
% bmake install

For your kernel development, again we have provided you with a framework for you to run your solutions for ASST2.

You have to reconfigure your kernel before you can use this framework. The procedure for configuring a kernel is the same as in ASST0 and ASST1, except you will use the ASST2 configuration file:

% cd ~/cs3231/asst2-src/kern/conf
% ./config ASST2
You should now see an ASST2 directory in the compile directory.

Building for ASST2

When you built OS/161 for ASST1, you ran make from compile/ASST1 . In ASST2, you run make from (you guessed it) compile/ASST2.
% cd ../compile/ASST2
% bmake depend
% bmake
% bmake install

If you are told that the compile/ASST2 directory does not exist, make sure you ran config for ASST2.

Command Line Arguments to OS/161

Your solutions to ASST2 will be tested by running OS/161 with command line arguments that correspond to the menu options in the OS/161 boot menu.

IMPORTANT: Please DO NOT change these menu option strings!

Running "asst2"

For this assignment, we have supplied a user-level OS/161 program that you can use for testing. It is called asst2, and its sources live in src/testbin/asst2.

You can test your assignment by typing p /testbin/asst2 at the OS/161 menu prompt. As a short cut, you can also specify menu arguments on the command line, example: sys161 kernel "p /testbin/asst2".
Note: If you don't have a sys161.conf file, you can use
this one.

Running the program produces output similar to the following prior to starting the assignment.

Unknown syscall 55
Unknown syscall 55
Unknown syscall 55
Unknown syscall 55
Unknown syscall 55
Unknown syscall 55
Unknown syscall 3
Fatal user mode trap 4 sig 10 (Address error on load, epc 0x400814, vaddr 0xeeeee00f)
asst2 produces the following output on a (maybe partially) working assignment.
OS/161 kernel [? for menu]: p /testbin/asst2
Operation took 0.000212160 seconds
OS/161 kernel [? for menu]:
* File Tester
* write() works for stdout
* write() works for stderr
* opening new file "test.file"
* open() got fd 3
* writing test string
* wrote 45 bytes
* writing test string again
* wrote 45 bytes
* closing file
* opening old file "test.file"
* open() got fd 3
* reading entire file into buffer
* attemping read of 500 bytes
* read 90 bytes
* attemping read of 410 bytes
* read 0 bytes
* reading complete
* file content okay
* testing lseek
* reading 10 bytes of file into buffer
* attemping read of 10 bytes
* read 10 bytes
* reading complete
* file lseek  okay
* closing file
* testing fork
Unknown syscall 0
* fork FAILED.
Unknown syscall 3
Fatal user mode trap 4 sig 10 (Address error on load, epc 0x400814, vaddr 0xeeeee00f)

Note that the final fatal error is expected, and is due to exit() (system call 3) not being implemented by OS/161. If exit()returns to userland (which would not happen in a complete OS implementation), the userland exit library code simply accesses an illegal memory address in order to cause a fault, which subsequently causes the program (and system) to stop. You can distinguish this expected fault from other faults by the address accessed: 0xeeeee00f.

The Assignment: File System Calls

Implement the following file-based system calls. The full range of system calls that is listed in kern/include/kern/syscall.h. For this assignment you should implement: open, read, write, lseek, close, dup2. Note: You are implementing the kernel code that implements the system call functionality within the kernel. The C stubs that user-level applications call to invoke the system calls are already automatically generated when you build OS/161.

Note that the basic assignment does not involve implementing fork() (that's part of the advanced assignment). However, the design and implementation of your system calls should nonetheless not assume a single process.

It's crucial that your syscalls handle all error conditions gracefully (i.e., without crashing OS/161.) You should consult the OS/161 man pages (also included in the distribution) and understand fully the system calls that you must implement. Your system calls must return the correct value (in case of success) or error code (in case of failure) as specified in the man pages. Some of the auto-marking scripts rely on the return of appropriate error codes; adherence to the guidelines is as important as the correctness of the implementation.

The file userland/include/unistd.h contains the user-level interface definition of the system calls that you will be writing for OS/161 (including ones you will implement in later assignments). This interface is different from that of the kernel functions that you will define to implement these calls. You need to design this interface and put it in kern/include/syscall.h. As you discovered (ideally) in Assignment 0, the integer codes for the calls are defined in kern/include/kern/syscall.h. You need to think about a variety of issues associated with implementing system calls. Perhaps, the most obvious one is: can two different user-level processes (or user-level threads, if you choose to implement them) find themselves running a system call at the same time?

open(), read(), write(), lseek(), close(), and dup2()

For any given process, the first file descriptors (0, 1, and 2) are considered to be standard input (stdin), standard output (stdout), and standard error (stderr) respectively. For this basic assignment, the file descriptors 1 (stdout) and 2 (stderr) must start out attached to the console device ("con:"). You will probably modify runprogram() to achieve this. Your implementation must allow programs to use dup2() to change stdin, stdout, stderr to point elsewhere.

Although these system calls may seem to be tied to the filesystem, in fact, these system calls are really about manipulation of file descriptors, or process-specific filesystem state. A large part of this assignment is designing and implementing a system to track this state. Some of this information (such as the cwd) is specific only to the process, but others (such as offset) is specific to the process and file descriptor. Don't rush this design. Think carefully about the state you need to maintain, how to organise it, and when and how it has to change.

While this assignment requires you to implement file-system-related system calls, you actually have to write virtually no low-level file system code in this assignment. You will use the existing VFS layer to do most of the work. Your job is to construct the subsystem that implements the interface expected by userland programs by invoking the appropriate VFS and vnode operations.

While you are not restricted to only modifying these files, please place most of your implementation in the following files: function prototypes and data types for your file subsystem in kern/include/file.h, and the function implementations and variable instantiations in kern/syscall/file.c.

A note on errors and error handling of system calls

The man pages in the OS/161 distribution contain a description of the error return values that you must return. If there are conditions that can happen that are not listed in the man page, return the most appropriate error code from kern/include/kern/errno.h. If none seem particularly appropriate, consider adding a new one. If you're adding an error code for a condition for which UNIX has a standard error code symbol, use the same symbol if possible. If not, feel free to make up your own, but note that error codes should always begin with E, should not be EOF, etc. Consult UNIX man pages to learn about error codes. Note that if you add an error code to kern/include/kern/errno.h you need to add a corresponding error message to the file user/lib/libc/string/strerror.c.

Design Questions

Here are some additional questions and issues to aid you in developing your design. They are by no means comprehensive, but they are a reasonable place to start developing your solution.

What primitive operations exist to support the transfer of data to and from kernel space? Do you want to implement more on top of these?

You will need to "bullet-proof" the OS/161 kernel from user program errors. There should be nothing a user program can do to crash the operating system when invoking the file system calls. It is okay in the basic assignment for the kernel to panic for an unimplemented system call (e.g. execv()), or a user-level program error.

Decide which functions you need to change and which structures you may need to create to implement the system calls.

How you will keep track of open files? For which system calls is this useful?

For additional background, consult one or more of the following texts for details how similar existing operating systems structure their file system management:

Documenting your solution

This is a compulsory component of this assignment. You must submit a small design document identifying the basic issues in this assignment, and then describe your solution to the problems you have identified. The design document you developed in the planning phase (outlined above) would be an ideal start. The document must be plain ASCII text. We expect such a document to be roughly 5001000 words, i.e. clear and to the point.

The document will be used to guide our markers in their evaluation of your solution to the assignment. In the case of a poor results in the functional testing combined with a poor design document, we will base our assessment on these components alone. If you can't describe your own solution clearly, you can't expect us to reverse engineer the code to a poor and complex solution to the assignment.

Place your design document in design.txt (which we have created for you) at the top of the source tree to OS/161 (i.e. in ~/cs3231/asst2-src/design.txt).

When you later commit your changes into your repository, your design doc will be included in the commit, and later in your submission.

Also, please word wrap you design doc if your have not already done so. You can use the unix fmt command to achieve this if your editor cannot.

5. Basic Assignment Submission

As with the previous assignments, you again will be submitting a diff of your changes to the original tree.

You should first commit your changes back to the repository using the following command. Note: You will have to supply a comment on your changes. You also need to coordinate with your partner that the changes you have (or potentially both have) made are committed consistently by you and your partner, such that the repository contains the work you want from both partners.

% cd ~/cs3231/asst2-src
% svn commit

If the above fails, you may need to run svn update to bring your source tree up to date with commits made by your partner. If you do this, you should double check and test your assignment prior to submission.

Beware! If you have created new files for this assignment, they will not be included in your submission unless you add them, using svn add:

% svn add filename.c
If you add files after running svn commit, you will need to run svn commit again.

Now generate a file containing the diff.

% cd ~
% svn diff file:///home/osprjXXX/repo/asst2/initial file:///home/osprjXXX/repo/asst2/trunk >~/asst2.diff

Testing Your Submission

here for information on testing and resubmitting your assignment.

Submitting Your Assignment

Now submit the diff as your assignment.
% cd ~
% give cs3231 asst2 asst2.diff
You're now done.

Even though the generated diff output should represent all the changes you have made to the supplied code, occasionally students do something "ingenious" and generate non representative diff output.

We strongly suggest keeping your svn repository intact to allow for recovery of your work if need be.

6. Advanced Assignment

The advanced assignment is assessable for COMP3891/COMP9283 students. It is worth 5 marks of the 100 class marks available. Marks are awarded as follows

The advanced assignment is to complete the basic assignment, plus the additional task below. 

Given you're doing the advanced version of the assignment, I'm assuming you're competent with managing your SVN repository and don't need simple directions. You basically need to generate a diff between your final version and the base. There are two ways you can do this: the simpler (but messier) option is to continue developing along your mainline branch and generate the diff in the same way as for the basic assignment. A neater approach is to create a new branch in SVN to work on your advanced solution. Whichever approach you take, make sure you test your diff before you submit it!

User-level Process Management System Calls


Implement a simplified form of the fork() system call. Your implementation of fork should be the same as that described in the man page, except that it should return 1 to the parent (rather than the child's PID).

The amount of code to implement fork is quite small; the main challenge is to understand what needs to be done. We strongly encourage you to implement the file-related system calls first, with fork in mind.

Some hints:


A pid, or process ID, is a unique number that identifies a process. The implementation of getpid() is not terribly challenging, but pid allocation and reclamation are the important concepts that you must implement. It is not OK for your system to crash because over the lifetime of its execution you've used up all the pids. Design your pid system; implement all the tasks associated with pid maintenance, and only then implement getpid(). When your pid system is working correctly, change your fork() implementation to return the child's pid to the parent, rather than 1.

execv(), waitpid(), _exit()

These system calls are probably the most difficult part of the whole assignment, but also the most rewarding. They enable multiprogramming and make OS/161 a much more useful entity. fork() is your mechanism for creating new processes. It should make a copy of the invoking process and make sure that the parent and child processes each observe the correct return value (that is, 0 for the child and the newly created pid for the parent). You will want to think carefully through the design of fork and consider it together with execv to make sure that each system call is performing the correct functionality. execv(), although "only" a system call, is really the heart of this assignment. It is responsible for taking newly created processes and make them execute something useful (i.e., something different than what the parent is executing). Essentially, it must replace the existing address space with a brand new one for the new executable (created by calling as_create in the current dumbvm system) and then run it. While this is similar to starting a process straight out of the kernel (as runprogram() does), it's not quite that simple. Remember that this call is coming out of userspace, into the kernel, and then returning back to userspace. You must manage the memory that travels across these boundaries very carefully. (Also, notice that runprogram() doesn't take an argument vector, but these must of course be handled correctly in execv()).

Although it may seem simple at first, waitpid() requires a fair bit of design. Read the specification carefully to understand the semantics, and consider these semantics from the ground up in your design. You may also wish to consult the UNIX man page, though keep in mind that you are not required to implement all the things UNIX waitpid() supports, nor is the UNIX parent/child model of waiting the only valid or viable possibility. The implementation of _exit() is intimately connected to the implementation of waitpid(). They are essentially two halves of the same mechanism. Most of the time, the code for _exit() will be simple and the code for waitpid() relatively complicated, but it's perfectly viable to design it the other way around as well. If you find both are becoming extremely complicated, it may be a sign that you should rethink your design.


Feel free to write kill_curthread() in as simple a manner as possible. Just keep in mind that essentially nothing about the current thread's userspace state can be trusted if it has suffered a fatal exception: it must be taken off the processor in as judicious a manner as possible, but without returning execution to the user level.

Design Questions

Here are some additional questions and thoughts to aid in writing your design document. They are not, by any means, meant to be a comprehensive list of all the issues you will want to consider. Your system must allow user programs to receive arguments from the command line. By the end of Assignment 2, you should be capable of executing lines (in user programs) such as:
  char *filename = "/bin/cp";
  char *args[4];
  pid_t pid;
  args[0] = "cp";
  args[1] = "file1";
  args[2] = "file2";
  args[3] = NULL;
  pid = fork();
  if (pid == 0)
    execv(filename, argv);
which will load the executable file cp, install it as a new process, and execute it. The new process will then find file1 on the disk and copy it to file2.

You can test your implementation using OS/161's shell, /bin/sh.

Some questions to think about:

Advanced Assignment Submission

Submission for the advanced assignment is similar to the basic assignment, except the advance component is given to a distinguished assignment name: asst2_adv Again, you need to generate a diff based on the original source tree.

% cd ~/cs3231/asst2-src
% svn commit

Now generate a file containing the diff. (NOTE: How this works depends on how you have set up branches in svn.)

% cd ~
% svn diff file:///home/osprjXXX/repo/asst2/initial file:///home/osprjXXX/repo/asst2/adv >~/asst2_adv.diff
Submit your solution
% cd ~
% give cs3231 asst2_adv asst2_adv.diff

FAQ and Gotchas

See for an up to date list of potential issues you might encounter.