1Project #1 – Exceptions and Simple System Calls Introduction to Operating Systems CSE421 Assigned: January 21, 2004 Due: February 17, 2004 11:59:59 PM The first project is designed to further your understanding of the relationship between the operating system and user programs. In this assignment, you will implement simple system call traps. In Nachos, an exception handler handles all system calls. You are to handle user program run time exceptions as well as system calls for IO processing. We give you some of the code you need; your job is to complete the system and enhance it. Phase 1: Understand the Code The first step is to read and understand the part of the system we have written for you. Our code can run a single user-level ‘C’ program at a time. As a test case, we’ve provided you with a trivial user program, ‘halt’; all halt does is to turn around and ask the operating system to shut the machine down. Run the program ‘nachos –rs 1023 -x ../test/halt’. As before, trace what happens as the user program gets loaded, runs, and invokes a system call. The files for this assignment are: progtest.cc test routines for running user programs. syscall.h the system call interface: kernel procedures that user programs can invoke. exception.cc the handler for system calls and other user-level exceptions, such as page faults. In the code we supply, only the ‘halt’ system call is supported. bitmap.* routines for manipulating bitmaps (this might be useful for keeping track of physical page frames) filesys.h openfile.h (found in the filesys directory) a stub defining the Nachos file system routines. For this assignment, we have implemented the Nachos file system by directly making the corresponding calls to the UNIX file system; this is so that you need to debug only one thing at a time. In assignment four, we'll implement the Nachos file system for real on a simulated disk. translate.* translation table routines. In the code we supply, we assume that every virtual address is the same as its physical address -- this restricts us to running one user program at a time. You will generalize this to allow multiple user programs to be run concurrently in a later lab. machine.* emulates the part of the machine that executes user programs: main memory, processor registers, etc. mipssim.cc emulates the integer instruction set of a MIPS R2/3000 processor. console.* emulates a terminal device using UNIX files. A terminal is (i) byte oriented, (ii) incoming bytes can be read and written at the same time, and (iii) bytes arrive asynchronously (as a result of user keystrokes), without being explicitly requested. synchconsole.* routine to synchronize lines of I/O in Nachos. Use the synchconsole class to ensure that your lines of text from your programs are not intermixed. ../test/* C programs that will be cross-compiled to MIPS and run in Nachos2 Phase 2: Design Considerations In order to fully realize how an operating system works, it is important to understand the distinction between kernel (system space) and user space. Each process in a system has its own local information, including program counters, registers, stack pointers, and file system handles. Although the user program has access to many of the local pieces of information, the operating system controls the access. The operating system is responsible for ensuring that any user program request to the kernel does not cause the operating system to crash. The transfer of control from the user level program to the system call occurs through the use of a “system call” or “software interrupt/trap”. Before invoking the transfer from the user to the kernel, any information that needs to be transferred from the user program to the system call must be loaded into the registers of the CPU. For pass by value items, this process merely involves placing the value into the register. For pass by reference items, the value placed into the register is known as a “user space pointer”. Since the user space pointer has no meaning to the kernel, we will have to translate the contents of the user space into the kernel such that we can manipulate the information. When returning information from a system call to the user space, information must be placed in the CPU registers to indicate either the success of the system call or the appropriate return value. Nachos gives you a simulated CPU that models a real CPU. In fact, the simulated CPU is the same as the real CPU (a MIPS chip), but we cannot just run user programs as regular UNIX processes, because we want complete control over how many instructions are executed at a time, how the address spaces work, and how interrupts and exceptions (including system calls) are handled. Nachos provided simulator can run normal programs compiled from C -- see the Makefile in the ‘test’ subdirectory for an example. The compiled programs must be linked with some special flags, then converted into Nachos format, using the program “coff2noff” Phase 3: [80%] Exceptions and IO System Calls Implement exception handling and handle the basic system calls for file IO. (All system calls are listed in syscall.h) We have provided you an assembly-language routine, ‘syscall’, to provide a way of invoking a system call from a C routine (UNIX has something similar -- try ‘man syscall’). You will need to do the following steps. NOTE : You should -ÆNOTÅ alter the code within the machine directory, only the code within the userprog directory. a) Alter exception.cc to handle all of system exceptions as listed in machine/machine.h Most of the exceptions listed in this file are comprised of run time errors, from which the user program will be unable to recover. The only special cases are no exception, which will return control to the operating system and syscall exception, which will handle our user system calls. For all other exceptions, the operating system should print an error message and Halt the simulation. b) Create a control structure that can handle the various Nachos system calls. Test your control structure by re-implementing the void Halt() system call. Make sure that this call operates in the same manner as we discussed during the Nachos walkthrough; it should cause the Nachos simulation to terminate immediately. Test the call’s accuracy with the test user program. c) All system calls beyond Halt() will require that
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