Nachos Operating System Simulator

This document assumes you have read the introduction to Nachos handout.

IMPORTANT NOTE: Nachos, SmartGDB, and the support tools are installed only on the cse451 Linux box. You MUST ssh (or rlogin or telnet) to this machine to use these tools at the time of this writing. DO NOT TRY TO USE THEM FROM ANY OTHER MACHINE.

I suggest using the command ssh cse451 to establish a connection to the cse451 Linux computer. Before doing this, create a file called .rhosts in your home directory with the following two lines:

    cse.unl.edu login-name
    cse451.unl.edu login-name

The Nachos source code contains nine subdirectories. Of these, you will spend most of your time working with the threads, userprog, machine, and test subdirectories. The directories are listed in rough order of importance to your work.

1. threads - this directory contains the thread code for Nachos. These routines create and maintain the individual user threads which are the processes in the Nachos operating system. This is one of the most important directories you will need to work with and understand.
   • thread.cc - most of the thread operations
   • scheduler.cc - scheduling routines
   • list.cc - a list object (for scheduling, interrupts, etc).
   • synchlist.cc - synchronization objects
   • main.cc - the startup routines
2. userprog - this directory contains most of the routines that Nachos uses to run user programs. All of the memory handling routines (address space and memory and swap management) are here. This is the other important directory you will need to work with and understand. The only executable that you will need to run directly, nachos, will be built here.
   • addrspace.cc - the address space handling routines.
   • systemcall.cc - the system call routines.
   • memmgr.cc - the memory management routines.
   • swapmgr.cc - the swap management routines.
3. machine - this directory contains the machine simulator for Nachos. The files in this directory simulate various parts of a real MIPS machine.
   • mipssim.cc - the main part of the machine simulator
   • machine.cc - the auxiliary parts of the machine simulator
4. test - this directory contains several example user programs for nachos. These programs (and those you may write on your own) will be used to test the Nachos code you provide. The start.s file contains the stub routines for the system calls as well as the equivalent of the startup routines (usually found in crt0.o).
5. lib - this directory contains a user library for Nachos. Several of the standard string handling functions are provided to make writing user programs easier. You can add routines to this directory if you wish to add more functionality.
6. bin - this directory contains the coff2noff converter. coff2noff converts from the output format gcc uses (coff) to the noff format that Nachos uses. You should not need to do anything with this directory.
7. filesys - this directory contains the file system for Nachos. Again, you should not need to do anything to this directory.
8. network - this directory contains the network code for Nachos. None of this code is functional yet, so you will not need to do anything with it.
9. vm - this directory was to contain the virtual memory subsystem for Nachos.

Running Nachos

Although Nachos has many optional arguments, you will probably only use two of them on a regular basis.

nachos -x <userprog>

../userprog/nachos -x shell

If you are in a different directory, you will need to specify the full path to the user program. For example, if you were in the top level directory (nachos) you would need to type:

userprog/nachos -x test/shell

1. exit-prog - a simple program that Write()'s a message and then calls the Exit() system call.
2. halt - another simple program that calls the Halt() system call.
3. matmult - a matrix multiplication program intended to stress virtual memory. Several versions of this program are available. The versions with numbers after matmult indicate how many copies of the program are forked (for instance, matmult8 forks 8 copies of the program).
4. shell - the nachos shell program. This program allows you to run other user programs. Simply typing a program name will start that program running just as in your normal shell (tcsh for instance). If you are not in the nachos/test directory, you will have to type the path to the user programs each time in order for the shell to find them.

   You can run programs in the background by appending an ampersand "&" to the end of the command line without any space in between the program's name and the &. When you run a program in the background, the shell does not issue a Wait() for that program, so it will not completely die until you issue an explicit "wait" command to the shell (remember that child processes do not die until their parent Wait()'s on them, they become ZOMBIE processes--try launching a program in the background and then check the status of it after it exits). The only other built-in command in the shell is "exit", a built-in command to exit the shell.

5. sort - a program to bubble sort an array of integers which is initialized in reverse order. This is another test of virtual memory ability.

../userprog/nachos -S <swap file name>

Where <swap file name> is the name of the file you want nachos to use as the swapfile (where pages that can't be kept in memory are temporarily stored--the swap). If you do not use -S, nachos will create a file called "swapfile" in the current directory. Obviously, when you are running nachos from a directory you do not have write access to, you, you will need to specify a swapfile in your own directory. For example:

../userprog/nachos -S ~/swapfile -x shell

will create a file called "swapfile" in your home directory to use as swap while running the "shell" user program.

In some cases, you will want to run Nachos without preemptive multitasking. This is accomplished using the -noswitch flag:

../userprog/nachos -noswitch

When you exit Nachos, some informational statistics will be printed including the total number of ticks used broken down into the number of idle, system, and user ticks. The number of Console reads and writes, and the paging statistics, including the number of faults, pageins and pageouts.

Other Links to Nachos Information

These are some other links to Nachos information on the web. Warning: Extensive changes have been made to our Nachos release, so some of the information on the standard release may no longer be applicable.

A Road Map Through Nachos

Nachos Home Page: Berkeley

Nachos Introduction