Introduction to Building and Using Nachos

This document is intended as a short introduction to Nachos. The commands the user would type are shown in bold face. Any comments are shown in italics. Output from commands is shown in normal type.

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
where login-name is your own login name on cse (and cse451). This file will let you establish rlogin or secure shell rlogins. the command ssh cse451 will create a secure rlogin session and redirect any X (graphic) output to your terminal.

Downloading, Building, and Using Nachos

Start by downloading a copy of Nachos.

cse451 [101]> tar -xzf nachos.tar.gz

Replace <Name of Nachos Archive> with the name of the Nachos archive that you downloaded from the assignment/project page. This extracts the archive in the current directory. All of the archives create a directory called "nachos" in the current directory. If you already have an old version of nachos that you would like to keep, you should move it out of the way (with mv nachos nachos-old, for example) first.

the command is tar -xzf nachos.tar.gz
Remember, if you have an old nachos directory, you should move it out of the way first (mv nachos nachos-hw2, for example)

cse451 [102]> cd nachos
/users/rhill/nachos

This is the top level directory of the Nachos source code. You will always build the Nachos code from here.

cse451 [103]> make

This builds the Nachos code. The output from the build is omitted due to its length.

cse451 [104]> cd test
/users/rhill/nachos/test

This is the directory where the user programs are located. Starting nachos from this directory will make it easier to specify user programs to run. The Introduction to Nachos describes the purpose of each subdirectory in the Nachos source.

cse451 [105]> ../userprog/nachos -x exit-prog

This will run the Nachos simulator with the user program "exit-prog" (located in the current directory). The -x flag to nachos tells it what user program to run when it starts. A description of the user programs is available on the Nachos introduction page.

Exit Program
Machine halting!

Ticks: total 1511, idle 1169, system 310, user 32
Context switches: 13
Disk I/O: reads 0, writes 0
Console I/O: reads 0, writes 13
Paging: faults 6, pageins 8, pageouts 0
Network I/O: packets received 0, sent 0

Cleaning up...

The exit-prog user program prints "Exit Program" and then issues an Exit() system call. When the system notices that there are no processes left to run, it initiates a system shut down (printing "Machine halting!"). During the shutdown, the statistics collected while the simulator is running are printed.
  • Ticks - the number of "simulated clock cycles". This is broken down into idle ticks which were unused processor time, system ticks which are clock cycles spent in the kernel, and user ticks which are clock cycles spent in user space.
  • Context switches - the number of context switches that occurred.
  • Disk I/O - currently unused as the disk I/O subsystem is merely a wrapper around the Unix open/read/write/close system calls.
  • Console I/O - the number of reads and writes to the console (in terms of characters)
  • Paging - the number of page faults, pages paged in and pages paged out.
  • Network I/O - currently unused as there is no network subsystem.

cse451 [106]> ../userprog/nachos -x shell
nachos> matmult
7220 <---[1]
Wait returned 1. Exit value = 7220

The matmult user program performs a matrix multiplication and prints the result as well as its process ID ("7220" is the result and "1" is the PID). The shell, after issuing a Fork() and Exec() waits for the child process to finish using the Wait() system call. When Wait() returns, it prints both the PID of the child that exited and the value that it exited with (as supplied to the Exit() system call).

nachos> exit-prog
Exit Program
Wait returned 2. Exit value = 0
nachos> exit

"exit" is a command built in to the Nachos shell. It causes the shell to exit. Since the Nachos shell was the only process running, the machine then shuts down.

Machine halting!

Ticks: total 115151387, idle 114433081, system 74950, user 643356
Context switches: 147
Disk I/O: reads 0, writes 0
Console I/O: reads 43, writes 193
Paging: faults 99, pageins 105, pageouts 0
Network I/O: packets received 0, sent 0

Cleaning up...

For more information on what the Nachos shell can do, see the Nachos introduction.

cse451 [107]>

More Information on Nachos

We have created several documents to introduce you to the Nachos system:


SmartGDB

At this point, you may want to look into SmartGDB which is a breakpoint debugger that will help you examine the Nachos code.

Notes on Modifying Nachos

Whenever you make modifications to the Nachos source code, you must issue the make command from the top level directory ("nachos") to rebuild Nachos.

If you have any questions or comments, please send them to the TA (deng@cse.unl.edu).

Steve Goddard
Last modified: Fri Sep 18 11:30:00 CDT 1998