Project 1: exceptions and simple system calls

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 : defines all file operations

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 making the corresponding calls to the UNIX file system directly. Because the calls are made directly, it’s necessary 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 like 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 byte-oriented and allows incoming bytes to be read and written at the same time. 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

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.