” CS 471课程编程 辅导、C++留学生程序 写作CS 471 Operating SystemsFall 2020Programming Assignment 2 (PA2):Due date: Friday, December 4, 11:59 pmTable of contents1. Introduction2. Code walk–through (20 points)3. Design (20 points) and Implementation (60 points)4. Coding Practices, Submission and Grading1. IntroductionYour current OS/161 system has minimal support for running executables – nothing that could beconsidered a true process. This project involves the transformation of OS/161 into a true multi‐taskingoperating system. The project is left open–ended deliberately; there are several design options that youcan consider and Commit to. After this assignment, your OS/161 kernel will be capable of runningmultiple processes (as long as those processes use the system calls you implement) at once from actualcompiled programs stored in your account. These programs will be loaded into OS/161 and executed inuser mode by System/161; this will occur under the control of your kernel. In other words, in this project,you will implement some of the system calls we discussed in the beginning of the semester, includingfork, exec, and waitpid.Before starting, you will need to complete a few steps (as in OS/161 Asst 1) to tag your Git repository,configure OS/161 for ASST 2, and build for ASST 2. Please see the previous assignments (OS161/Asst 1) hand out for instructions on how to do that. When you run your kernel configured for thisassignment, you will again see a menu with various options (including test options).As a first step, you must implement the interface between user-mode programs (userland) and thekernel. As usual, we provide part of the code you will need. Your job is to design and build the missingpieces. You will also be implementing the subsystem that keeps track of the multiple tasks you will havein 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 procstructure.)In order to do that you will have to read and understand the parts of the system that are written for you.This is an essential component of this assignment. The existing code can run one user-level C program ata time as long as it doesnt want to do anything but shut the system down. We have provided sample userprograms that do this (reboot, halt, poweroff), as well as others that make use of features youwill be adding in this assignment.2So far, all the code you have written for OS/161 has only been run within, and only been used by, theoperating system kernel. In a real operating system, the kernels main function is to provide support foruser-‐level programs. Most such support is accessed via system calls. The system gives you one systemcall, reboot(), which is implemented in the function sys_reboot() in main.c. In GDB, if youput a breakpoint on sys_reboot and run the reboot program, you can use backtrace tosee how it got there.User–level programsThe System/161 simulator can run normal programs compiled from C. The programs are compiled with across–‐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 toedit the Makefile in bin, sbin, or testbin (depending on where you put your programs) and thencreate a directory Similar to those that already exist. Use an existing program and its Makefile as atemplate.DesignPlease note that your design documents become an important part of the work you submit in this project.The design documents should clearly reflect the development of your solution. They should not merelyexplain what you programmed.Note that it can often be hard to write (or talk) about new software design – you are facing problems thatyou 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. Now is a really good time towork closely with your partner. As you design this assignment, you may have to invent terminologyand 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 discussproblems that you have never seen before.To help you get started, we have provided the following questions as a guide for reading through thecode. We recommend that you divide up the code and have each partner answer questions for differentmodules. After reading the code and answering questions, get together and exchange summations of thecode you each reviewed. Once you have done this, you should be ready to discuss strategy for designingyour code for this assignment. As always, make sure to comply with the GMU and CS honor codes theonly person you can cooperate with is your partner (if you have one).2. 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 ofuser-level programs. Currently, the only files in the directory are loadelf.c, runprogram.c, anduio.c, although you may want to add more of your own during this assignment. Understanding thesefiles is the key to getting started with the implementation of multiprogramming. Note that to answer someof the questions, you will have to look in files outside this directory.3loadelf.c: This file contains the functions responsible for loading an ELF executable from thefilesystem and into virtual memory space. (ELF is the name of the executable format produced bycs161-gcc.) Of course, at this point this virtual memory space does not provide what is normallymeant by virtual memory — although there is translation between the addresses that executables believethey are using and physical addresses, there is no mechanism for providing more memory than existsphysically.runprogram.c: This file contains only one function, runprogram(), which is responsible forrunning a program from the kernel menu. It is a good base for writing the execv() system call, but only abase — when writing your design doc, you should determine what more is required for execv() thatrunprogram() does not concern itself with. Additionally, once you have designed your processsystem, runprogram() should be altered to start processes properly within this framework; forexample, a program started by runprogram() should have the standard file descriptors available whileits running.uio.c: This file contains functions for moving data between kernel and user space. Knowing when andhow to cross this boundary is critical to properly implementing user-level programs, so this is a good fileto read very carefully. You should also examine the code in lib/copyinout.cQuestions1. What are the ELF magic numbers?2. What is the difference between UIO_USERISPACE and UIO_USERSPACE? When should one useUIO_SYSSPACE instead?3. Why can the struct uio that is used to read in a segment be allocated on the stack inload_segment() (i.e., where does the memory read actually go)?4. In runprogram(), Why is it important to call vfs_close() before switching to usermode?5. What function forces the processor to switch into usermode? Is this function machine dependent?6. In what file are copyin, copyout and memmove are defined? Why cant copyin and copyout beimplemented simply as memmove?7. What is the purpose of userptr_t? Explain briefly.kern/arch/mips/mips: traps and syscallsExceptions are the key to operating systems; they are the mechanism that enables the OS to regain controlof execution and therefore do its job. You can think of exceptions as the interface between the processorand the operating system. When the OS boots, it installs an exception handler (carefully craftedassembly 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 overloadedterm in computer science, operating system lingo for an exception is a trap — when the OS trapsexecution. In OS 161 terminology, interrupts are exceptions, and more significantly for this assignment,so are system calls. (The terminology difference between OS/161 and many OS textbooks areunfortunate; but it is not uncommon at all recall our class discussion about interrupts andexceptions). Specifically, syscall.c handles traps that happen to be syscalls. Understanding at least4the C code in this directory is key to being a real operating systems programmer, so we highlyrecommend reading through it carefully.trap.c: mips_trap() is the key function for returning control to the operating system. This is theC function that gets called by the assembly exception handler. md_usermode() is the key function forreturning control to user programs. kill_curthread() is the function for handling broken userprograms; when the processor is in usermode and hits something it cant handle (say, a bad instruction), itraises an exception. Theres no way to recover from this, so the OS needs to kill off the process. Part ofthis 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 thekernel function that implements it. Notice that reboot() is the only case currently handled. You willalso find a function, md_forkentry(), which is a stub where you will place your code to implementthe fork() system call. It should get called from mips_syscall().Questions8. What is the numerical value of the exception code for a MIPS system call?9. Why does mips_trap() set curspl to SPL_HIGH manually, instead of using splhigh()?10. How many Bytes is an instruction in MIPS? (Answer this by reading mips_syscall() carefully,not by looking somewhere else.)11. Why do you probably want to change the implementation of kill_curthread()?12. What would be required to implement a system call that took more than 4 arguments?lib/crt0: This is the user program startup code. Theres 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. Itcalls the user programs main(). This is the code that your execv() implementation will beinterfacing 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. Theres obviously a lot of code here. We dont expect youto read it all, although it may be instructive in the long run to do so. For present purposes you need onlylook 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 theuser-level side of MIPS system calls.syscalls.S: This file is created from syscalls-mips.S at compile time and is the actual fileassembled into the C library. The actual names of the system calls are placed in this file using a scriptcalled callno-parse.sh that reads them from the kernels header files. This avoids having to make asecond list of the system calls. In a real system, typically each system call stub is placed in its own sourcefile, to allow selectively linking them in. OS/161 puts them all together to simplify the makefiles.5Questions13. What is the purpose of the SYSCALL macro?14. What is the MIPS instruction that actually triggers a system call? (Answer this by reading the sourcein this directory, Not looking somewhere else.)3. Design (20 points) and Implementation (60 points) – (80 points total)Download necessary filesAs in the previous ASST1, you need to download two files from Blackboard and put them in theirappropriate directory. You need to update your source code with these two files before you begin yourassignment: callno.h in os161-1.11/kern/include/kern/ unistd.h in os161-1.11/include/After replacing the files accurately, tag your repository as asst2-begin, if you have not already doneso.System calls and exceptionsImplement system calls and exception handling. The full range of system calls that we think you mightwant over the course of the semester is listed in kern /include/kern/callno.h. For thisassignment You should implement:● getpid● getppid● fork● execv● waitpid● _exitIts crucial that your syscalls handle all error conditions gracefully (i.e., without crashing OS/161.) Youshould consult the OS/161 man pages included in the distribution (in ~os161/os161-1.11/man/syscall directory) and understand fully the system calls that you must implement. Youmust return the error codes as described in those man pages.Additionally, your syscalls must return the correct value (in case of success) or error code (in case offailure) as specified in the man pages. Some of the grading scripts rely on the return of appropriate errorcodes; adherence to the guidelines is as important as the correctness of the implementation. The fileinclude/unistd.h contains the user-level interface definition of the system calls that you will bewriting for OS/161 (including ones that are relevant for another potential assignment). This interface isdifferent from that of the kernel functions that you will define to implement these calls. You need todesign this interface and put it in kern/include/syscall.h. As you may have discovered in OS161/Asst0, the integer codes for the calls are defined in kern/include/kern/callno.h. Youneed to think about a variety of issues associated with implementing system calls. Perhaps the most6obvious one is: can two different user-level processes (or user-level threads, if you choose to implementthem) find themselves running a system call at the same time? Be sure to argue for or against this, andexplain your final decision in the design document.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 theconsole device (con:).An important design consideration is to decide where to put the code for these system calls. A good placewould be inside the kern/userprog directory. For the 6 system calls in this assignment, create 6 filesnamed getpid.c, getppid.c, fork.c, execv.c, waitpid.c, and exit.c, inside thekern/userprog directory. Make sure to add the file names to your kern/conf/conf.kern file inan appropriate place, as you have done in Assignment-0 for the hello.c file. By convention, the names ofthe systems call functions will be as follows: sys_getpid, sys_getppid, sys_fork,sys_execv, sys_waitpid, sys_exit. These names must also be added as declarations in thekern/include/syscall.h file.getpid()A pid, or process ID, is a unique number that identifies a process. The implementation of getpid() isnot terribly challenging, but pid allocation and reclamation are the important concepts that you mustimplement. It is not OK for your system to crash because over the lifetime of its execution youve used upall the pids. Design your pid system; implement all the tasks associated with pid maintenance, and onlythen, implement getpid().getppid()This system call works exactly like getpid(), except that it returns the parent process pid. Note that,the getppid() system call is not included in the OS/161 man pages, and you can assume the followingspec:getppid() will return the process ID of the parent of the calling process. This will be either the ID ofthe process that Created this process using fork() (if the parent hasnt terminated yet), or -1 (if theparent has already terminated).fork(), execv(), waitpid(), _exit()These system calls are probably the most challenging 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 andmake sure that the parent and child processes each observe the correct return value (that is, 0 for the childand the newly created pid for the parent). You will want to think carefully through the design of fork()7and consider it together with execv() to make sure that each system call is performing the correctfunctionality.execv(), although only a system call, is really the heart of this assignment. It is responsible for takingnewly created processes and make them execute something useful (i.e., something different than what theparent is executing). Essentially, it must replace the existing address space with a brand new one for thenew executable (created by calling as_create in the current dumbvm system) and then run it. Whilethis is similar to starting a process straight out of the kernel (as runprogram() does), its not quite thatsimple. Remember that this call is coming out of userspace, into the kernel, and then returning back touserspace. You must manage the memory that travels across these boundaries very carefully. (Also,notice that runprogram() doesnt take an argument vector — but this must of course be handledcorrectly in execv()).Although it may seem simple at first, waitpid() requires a fair bit of design. Read the specification inthe OS 161 waitpid() man page carefully to understand the semantics, and consider these semanticsfrom the ground up in your design. You may also wish to consult the general UNIX man page, thoughkeep in mind that you are not required to implement all the things UNIX waitpid() supports, nor is theUNIX parent/child model of waiting the only valid or viable possibility.The implementation of _exit() is intimately connected to the implementation of waitpid(). Theyare essentially two halves of the same mechanism. Most of the time, the code for _exit() will besimple and the code for waitpid() more complicated, but its perfectly viable to design it the other wayaround as well. If you find both are becoming extremely complicated, it may be a sign that you shouldrethink 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 mustreturn. If there are conditions that can happen that are not listed in the man page, return the mostappropriate error code from kern/errno.h. If none seems particularly appropriate, consider adding anew one. If youre 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 shouldalways begin with E, should not be EOF, etc. Consult Unix man pages to learn about Unix error codes; onLinux systems man errno will do the trick. Note that if you add an error code tokern/include/kern/errno.h, you need to add a corresponding error message to the filelib/libc/strerror.ckill_curthread()Feel free to write kill_curthread() in as simple a manner as possible. Just keep in mind thatessentially nothing about the current threads userspace state can be trusted if it has suffered a fatalexception — it must be taken off the processor in as judicious a manner as possible, but without returningexecution to the user level.8Design ConsiderationsHere are some additional questions and thoughts to aid in writing your design document. They are not, byany means, meant to be a comprehensive list of all the issues you will want to consider. You do not needto explicitly answer or discuss these questions in your design document, but you should at least thinkabout them.Your system must allow user programs to receive arguments from the command line. In addition, it mustalso work with the p menu option (e.g., it must work for p testbin/add 2 5.) To accomplishthat, you need to edit the file menu.c. See Testing Your System Call Implementation section belowas well.For instance, you should be able to run the following program:char *filename = /testbin/add;char *args[4];pid_t pid;args[0] = add;args[1] = 3;args[2] = 4;args[3] = NULL;pid = fork();if (pid == 0) execv(filename, argv);which will load the executable file testbin/add, install it as a new process, and execute it bycomputing the sum (7).Some questions to think about:- Passing arguments from one user program, through the kernel, into another user program, is a bitof a chore. What form does this take in C? You will probably need several iterations to get thisright.- What primitive operations exist to support the transfer of data to and from kernel space? Do youwant to implement More on top of these?- How will you determine: (a) the stack pointer initial value; (b) the initial register contents; (c) thereturn 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 benothing a user program can do to crash the operating system (with the exception of explicitlyasking 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?9Testing your system call implementationsTo test your system call implementations, the best way would be to write small programs that make use ofthose system calls (fork, exec,waitpid, etc.), compile with cs161-gcc compiler, and run it underos161.Specifically, when you build and invoke your kernel, on the sys161 menu, choose first the OperationsMenu, and then use the [p] option to invoke the executable. For example, typing:p testbin/forktestwould invoke the forktest executable which is already provided in the testbin directory. Thetestbin directory has other executables; and some of them may be useful for debugging. By readingthe first few lines of C programs there, you may be able to see whether the test program is suitable for thisproject or not.But more importantly, do not restrict your tests with the programs in the testbin directory. In fact itmay be wiser to first use very small programs (like the program with fork() and exec() whose codeis given on the previous page). For example you can store that program in a separate directory undertestbin and compile (by preparing and running a Makefile similar to that available forforktest in testbin ). Those makefiles call cs161-gcc compiler with proper flags. And thenyou can again use p testbin/myprogram if the name of your new executable is myprogram. Inyour submission, include in Your asst2 directory the scripts for the tests that you designed so that we canevaluate them as well.In this project, it is NOT necessary to implement the filesystem related system calls (such as read() andwrite()). You can certainly do that if you wish; but it is not required. However, you will still need ashortcut in order to execute library calls like putchar(), printf() to print to the screen; becausethose calls issue a call to write(). If you dont want to implement write() separately, you cando the following:Go to the kern/arch/mips/mips directory, and open for editing the file syscall.c. Within themips_syscall() function, you will see a switch statement. To handle the calls to write(),add the following to the switch statement:case SYS_write:for (i = 0; i (size_t) tf-tf_a2; ++i) {kprintf(%c, ((char *) tf-tf_a1)[i]);}break;10You would also need to add a declaration for the variable i as an unsigned int, and put it in thebeginning of the mips_syscall()function. Save syscall.c, build your kernel again, and thisshould be sufficient for simply printing to the screen. In fact, even the forktest program will needthis patch in addition to the correct implementation of fork(), and exec(), for proper printing. Butas we mentioned, it is best to test your implementations first with very short programs, before moving toforktest which is more complicated. You may also find some of the programs from the bindirectory useful, such as false, and true. For getppid(), write your own (short) test program.4. Coding Practices, Submission and GradingIn this project, you can continue to work with your partner from Project 1. If you worked alone in Project1, you can pair up with another CS 471 student from your section who also worked alone in Project 1.You will need to (again) put significant time in reading the existing OS 161 kernel code. Make sure toallocate sufficient time to debugging and extensive testing.On the assignment due date:1. Commit all your final changes to the system. Make sure your partner has committed everything aswell. Make sure you Have the most up-to-date version of everything. Re-run make on both thekernel and userland to make sure all the last-minute changes get incorporated.2. Run the tests and generate scripts. If anything breaks, repeat step 1.3. Tag your Git repository; prepare the diff and submission source tree.4. Make your submission to the Blackboard by following the guidelines below.SubmissionsYour final submission should be a .tar.gz file. Please name this file uid1_uid2-asst2.tar.gz,where uid1 and uid2 are you and your partners GMU email IDs. If you work alone, it should be named asuid-asst2.tar.gz where uid is your GMU e-mail id. It should contain the following:● A copy of the complete current source code of your OS/161 version.● All of the following in a newly-created asst2 directory○ Your design document. Please use only .pdf, or.txt file formats.○ A diff between asst2-begin and asst2-end.○ A script of OS/161 running the various tt* tests successfully.○ A script of the new test or tests you added for testing your system call implementations.Note: Your design document is worth 20% of the grade for this assignment. It should contain:● A high level description of how you are approaching the problem.● A detailed description of the implementation (e.g., new structures, why they were created, whatthey are encapsulating, what problems they solve).● A discussion of the pros and cons of your approach.● Alternatives you Considered and why you discarded them.Include your answers to the code walk-through questions in the design document as well (worth another20% of the grade).11On Blackboard, under the Projects/Project 2 folder you will find a link to submit your compressed file.All members of a group must submit separately the same compressed file, and before the deadline. Makesure to coordinate.You can make as many Submissions as you like; we will consider ONLY the last submission that youmake. So in case you need to re-submit, you must make sure the compressed file contains all thecomponents listed above.Questions:Programming and system-related questions about the project should be directed to Fall 2020 CS 471OS/161 Project Piazza Page, which is monitored by the GTAs from 10 AM to 6 PM on weekdays. Yourposts to the Piazza are by default private, meaning that it is received only by the instructors and GTAs.You should NOT post public inquiries by changing those settings (occasionally, the GTAs can decide tomake some questions and answers public, if they think they are relevant for many students).But keep in mind that code-reading and exploring the OS/161 kernel files is a component of theassignment, so you should not expect detailed explanations of how the OS/161 kernel functions work.GradingThis programming Assignment accounts for 12% of your final grade. Late penalty for this assignment willbe 15% for each day. Submissions that are late by 3 days or more will not be accepted.如有需要，请加QQ：99515681 或邮箱：99515681@qq.com
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