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UB CSE 421 - CSE 421 Course Description

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Project 1 Exceptions and Simple System Calls Introduction to Operating Systems Assigned September 5 2002 CSE421 521 Due October 2 2002 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 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 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 Nachos 1 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 If we remember from class 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 In this assignment we are giving 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 Our 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 and then converted into Nachos format using the program coff2noff The only caveat is that floating point operations are not supported NOTE For this project you should NOT alter the code within the machine directory only the code within the userprog directory Phase 3 90 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 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 with 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 2 c All system calls beyond Halt will require that Nachos increment the program counter before the system call returns If this is not properly done Nachos will execute the system call forever Since the MIPS emulator handles look ahead program counters as well as normal ones you will have to emulate the program counter increment code as found in


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