CS 471 Operating Systems

Class web site
Fall 2017

Assignment 2: System Calls and Processes

Due December 6, 2017, before midnight (new date)

[OS/161 Instructions]


System call support
Understanding system calls
Read and Write syscalls
Useful info of C Macros

Assignment Organization

Please note: There are not many explicit directions in this assignment as it is very open-ended. If you need help with instructions such as "tag your CVS repo," please see the last assignments for how to do that.

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 a true multi-tasking operating system. After this assignment, it will be capable of running multiple processes (as long as those use the system calls you implement) at once from actual compiled programs stored in your account. These programs will be loaded into OS/161 and executed in user mode by System/161; this will occur under the control of your kernel and the command shell in bin/sh.

First, however, you must implement 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.

You will also be implementing the subsystem that keeps track of the multiple tasks you will have in the future. 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 are written for you. The existing 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 will 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." The system gives you one system call, reboot(), which is implemented in the function sys_reboot() in main.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.

User level programs

The System/161 simulator can run normal programs compiled from C. The programs are compiled with a cross-compiler, cs161-gcc. This compiler runs on the host 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.


Beginning with this assignment, please note that your design documents become an important part of the work you submit. The design documents 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! Work with your partner to plan everything you will do. Don't even think about coding until you can precisely explain to each other 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 (we suggest your partner; roommates seem to have a low tolerance for this sort of thing). In order to reach an understanding, you may have to invent terminology and notation-this is fine (just be sure to explain it to your TA in your design document). 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 divide up the code and have each partner answer questions for different modules. After reading the code and answering questions, get together and exchange summations of the code you each reviewed. Once you have done this, you should be ready to discuss strategy for designing your code for this assignment. WHATEVER YOU DO, DO NOT WORK WITH PEOPLE IN OTHER TEAMS. THAT CONSTITUTES A VIOLATION OF THE HONOR CODE! AND WE TAKE THAT SERIOUSLY...

Code walk-through (20 points)

Include the answers to the code walk-through questions as the first part of your design document.

kern/userprog: This directory contains the files that are responsible for the loading and running of user-level programs. Currently, the only files in the directory are loadelf.c, runprogram.c, and uio.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 implementation of multiprogramming. Note that to answer some of the questions, you will have to look in files outside this directory.

loadelf.c: This file contains the functions responsible for loading an ELF executable from the filesystem and into virtual memory space. (ELF is the name of the executable format produced by cs161-gcc.) 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. We recommend not stressing about this until Assignment 3.

runprogram.c: This file contains only one function, runprogram(), which is responsible for running a program from the kernel menu. It 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 concern itself with. Additionally, once you have designed your process system, runprogram() should be altered to start processes properly within this framework; for example, a program started by runprogram() should have the standard file descriptors available while it's running.

uio.c: 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/copyinout.c.


  1. What are the ELF magic numbers?
  2. What is the difference between UIO_USERISPACE and UIO_USERSPACE? When should one use UIO_SYSSPACE instead?
  3. Why can the struct uio that is used to read in a segment be allocated on the stack in load_segment() (i.e., where does the memory read actually go)?
  4. In runprogram(), why is it important to call vfs_close() before going to usermode?
  5. What function forces the processor to switch into usermode? Is this function machine dependent?
  6. In what file are copyin and copyout defined? memmove? Why can't copyin and copyout be implemented as simply as memmove?
  7. What (briefly) is the purpose of userptr_t?
  8. kern/arch/mips/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.

    trap.c: 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. md_usermode() is the key function for returning control to user programs. 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. Part of this assignment will be to write a useful version of this function.

    syscall.c: mips_syscall() is the function that delegates the actual work of a system call to the kernel function that implements it. Notice that reboot() is the only case currently handled. You will also find a function, md_forkentry(), which is a stub where you will place your code to implement the fork() system call. It should get called from mips_syscall().


  9. What is the numerical value of the exception code for a MIPS system call?
  10. Why does mips_trap() set curspl to SPL_HIGH "manually", instead of using splhigh()?
  11. How many bytes is an instruction in MIPS? (Answer this by reading mips_syscall() carefully, not by looking somewhere else.)
  12. Why do you "probably want to change" the implementation of kill_curthread()?
  13. What would be required to implement a system call that took more than 4 arguments?
  14. lib/crt0: This is the user program 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 the user program's 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.

    lib/libc: This is the user-level C library. There's obviously a lot of code here. We don't expect you to read it all, although it may be instructive in the long run to do so. Some job interviewers have an uncanny habit of asking people to implement standard C library functions on the whiteboard. For present purposes you need only look at the code that implements the user-level side of system calls, which we detail below.

    errno.c: This is where the global variable errno is defined.

    syscalls-mips.S: This file contains the machine-dependent code necessary for implementing the user-level side of MIPS system calls.

    syscalls.S: This file is created from syscalls-mips.S at compile time and is the actual file assembled into the C library. The actual names of the system calls are placed in this file using a script called callno-parse.sh 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.


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

Design (20 points) and Implementation (60 points)
(80 points total + 20 EXTRA points)

Before you begin this assignment, tag your repository as asst2-begin.

System calls and exceptions

Implement system calls and exception handling. The full range of system calls that we think you might want over the course of the semester is listed in kern/include/kern/callno.h. For this assignment you should implement:

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 included in the distribution and understand fully the system calls that you must implement. You must return the error codes as decribed in the man pages.

Additionally, your syscalls 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 grading scripts rely on the return of appropriate error codes; adherence to the guidelines is as important as the correctness of the implementation.

The file 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/callno.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? Be sure to argue for or against this, and explain your final decision in the design document.

FOR EXTRA CREDIT ONLY: open(), read(), write(), lseek(), close(), dup2(), chdir(), and __getcwd()

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). These file descriptors should start out attached to the console device ("con:"), but your implementation must allow programs to use dup2() to change them 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 current working directory) is specific only to the process, but others (such as file 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 organize it, and when and how it has to change.

Note that there is a system call __getcwd() and then a library routine getcwd(). Once you've written the system call, the library routine should function correctly.


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().

fork(), execv(), waitpid(), _exit()

These system calls are probably the most difficult part of the assignment, but also the most rewarding. They enable multiprogramming and make OS/161 a much more useful entity.

fork() is the 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 theme 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 this 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.

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/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 Unix error codes; on Linux systems "man errno" will do the trick.

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 lib/libc/strerror.c.


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.


Right now, the OS/161 scheduler implements a simple round-robin queue. As we learned in class, this is probably not the best method for achieving optimal processor throughput. For this assignment, you should extend this scheduling algorithm, by adding another queue, converting it into a multi-level (2) feedback queuing system. Both queues will use RR, and the quantum of the second queue should be two times the quantum of the first. You want the scheduler to be configurable by making it possible to specify the quantum of the first queue.

It is OK to have to recompile to change these settings, as with the HZ parameter of the default scheduler. And it is OK to require a recompile to switch schedulers. But it shouldn't require editing more than a couple #defines or the kernel config file to make these changes.

In any event, OS/161 should display at bootup which scheduler is in use.

Test your scheduler by running several of the test programs from testbin (e.g., add.c, hog.c, farm.c, sink.c, kitchen.c, ps.c) using the default time slice and scheduling algorithm. Experiment with it. Write down what you expect to happen. Then compare what actually happened to what you predicted and explain the difference.

Design Considerations

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. You do not need to explicit answer or discuss these questions in your design document, but you should at least think about them.

Your system must allow user programs to receive arguments from the command line. For example, you should be able to run the following program:

        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.

Some questions to think about:

Passing arguments from one user program, through the kernel, into another user program, is a bit of a chore. What form does this take in C? This is rather tricky, and there are many ways to be led astray. You will probably find that very detailed pictures and several walk-throughs will be most helpful.

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?

How will you determine: (a) the stack pointer initial value; (b) the initial register contents; (c) the return value; (d) whether you can exec the program at all?

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 (with the exception of explicitly asking the system to halt).

What new data structures will you need to manage multiple processes?

What relationships do these new structures have with the rest of the system?

If you want to do the extra credit: How will you manage file accesses? When the shell invokes the cat command, and the cat command starts to read file1, what will happen if another program also tries to read file1? What would you like to happen?

Submission requirements

Your final submission should be a .tar.gz file. Please name this file cs471_gid1_gid2-asst2.tar.gz, where gid1 and gid2 are you and your partner's email ID. It should contain the following:

Note: Your design document is worth 20% of the grade for this assignment. It should contain:

How to proceed

Before the assignment is due:

  1. Carefully divide up the work. execv() might be the single most demanding part of the assignment, but waitpid() is non-trivial as well. We suggest that one of you should do the basic system calls and the other focus on process support.
  2. For the parts you're assigned, verify that the collaborated design will really work. If something needs to be redesigned, do it now, and run it by your partner.
  3. Implement.
  4. Test, test, test. Test your partner's code especially.
  5. Fix. Perhaps you won't need this step. (We all need to dream, right?)

On the assignment due date:

  1. Commit all your final changes to the system. Make sure your partner has committed everything as well. Make sure you have the most up-to-date version of everything. Re-run make on both the kernel and userland to make sure all the last-minute changes get incorporated.
  2. Run the tests and generate scripts. If anything breaks, curse (politely of course) and repeat step 1.
  3. Tag your CVS repository; prepare the diff and submission source tree.
  4. Submit everything. It doesn't matter which partner submits, but please make sure only one does. Please make sure your final submission is contained in file named cs471_gid1_gid2_asst2.tar.gz