CSC231 Lab 1 2014

From dftwiki3
Revision as of 09:46, 10 September 2012 by Thiebaut (talk | contribs) (→‎Mini Challenge of the Day)
Jump to: navigation, search

<meta name="keywords" content="computer science, assembly language, pentium, exercise, machine language, intel" /> <meta name="description" content="Dominique Thiebaut's Web Page" /> <meta name="title" content="Dominique Thiebaut -- Computer Science" /> <meta name="abstract" content="Dominique Thiebaut's Computer Science Web pages" /> <meta name="author" content="thiebaut at cs.smith.edu" /> <meta name="distribution" content="Global" /> <meta name="revisit-after" content="10 days" /> <meta name="copyright" content="(c) D. Thiebaut 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,2008" /> <meta name="robots" content="FOLLOW,INDEX" /> --D. Thiebaut 19:55, 7 September 2010 (UTC)




This lab is the first lab of this course and its purpose is to bring everybody up to speed with the assembly process, and have a tiny bit of fun at the same time. It will not be graded. It is self-paced, but you should work with somebody else in pair programming mode.




Creating a program from scratch

  • Find the skeleton program on the class Web page (or open it here)
  • Login to beowulf with your new 231a-xx account (see the Troubleshooting section at bottom of this page in case of trouble)
  • Create a file called skel.asm in your account, and save the skeleton program in it. At the Linux prompt, type:
  emacs skel.asm
Use the Emacs Quick-Reference page for the most often used commands.
  • Save it by typing Control-X followed by Control-C in emacs. You should be back at the Linux prompt.
  • Make a copy of skel.asm and call it lab1.asm
  cp skel.asm lab1.asm
  • Edit lab1.asm
  emacs lab1.asm
  • Add a string variable in your data section:
             msg     db     "Welcome to csc231", 10
             MSGLEN  equ    $-msg


  • Add some code to output the string in the text (code) section, right below the _start: label


             mov    eax, 4
             mov    ebx, 1
             mov    ecx, msg      ; use the same name as the string in the data section
             mov    edx, MSGLEN   ; # of chars in string
             int    0x80


  • Save your file by typing Control-X, Control-C while in emacs.


Assembly and Linking steps

  • You should be back at the Linux prompt. Assemble your program
    nasm -f elf -F stabs lab1.asm
  • Fix any errors you may get by starting emacs again and figuring out where the error is.
  • Link your program
    ld -melf_i386  -o lab1 lab1.o
You shouldn't get any errors at this stage.

Execution

  • Run your program:
   ./lab1
It should run and display the string.

Playing around with the program

  • Modify the msg string to make it look like this:
             msg     db     10, 10, "Welcome", 10, "to", 10, "csc231", 10, 10
             MSGLEN  equ    $-msg
  • Reassemble, link and run the new version of your program.
  • Something different. Modify your program once more, as shown below:
             msg     db     10, 10
                     db     "Welcome", 10
                     db     "to", 10, "csc231"
                     db     10
                     db     10
             MSGLEN  equ    $-msg


  • Reassemble, link and run the new version of your program.


Mini Challenge of the Day

QuestionMark3.jpg


  • Modify your program so that its output looks like this:
                    *********************************
                    * Welcome to CSC231             *
                    * Home of the Assembly Language *
                    *********************************


The lines of stars contain 33 *-characters. Each line is thus 34-character long (to account for the line-feed character). Can you find a solution where the number of characters in your data section is not 4 x 34 = 136 characters, but 3 x 34 = 102, instead?

Test it out!

Version 1
Make the data section as short as possible. That means do not replicate the second line of dashes.
Version 2
Make the code section as short as possible. That means print only one string of characters.

Notice that there is a trade-off... what is it about? Have you experienced something similar before with Python, Java or some other language you may have used? (We'll revisit this trade-off many times this semester.)

Submit your Solution (optional)

Enter your name or names in the header of the program, and at the Linux prompt, type: 

   rsubmit lab1 lab1.asm

Troubleshooting

It is possible that if you are using ssh to connect to Beowulf the first time, and if you are using a Mac, you get a message of this form: 

ssh 231a-xx@beowulf.csc.smith.edu
==================================================
   WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     
==================================================
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
bf:db:14:e6:a4:e0:f3:3d:d8:87:35:66:a9:35:68:fb.
Please contact your system administrator.
Add correct host key in /Users/alex/.ssh/known_hosts to get rid of this message. 
Offending key in /Users/xxxxx/.ssh/known_hosts:1
RSA host key for beowulf.csc.smith.edu has changed and you have requested strict checking. 
Host key verification failed.

In this case, use the editor of your choice and edit the file ~/.ssh/known_hosts and remove the lines that contains the word beowulf. Be careful, the lines are very long and wrap around to form blocks of 4 or 5 lines on your screen, so deleting one line will require a big block to disappear. Bold text








Retrieved from "http://dominiquethiebaut.com/dftwiki3/index.php?title=CSC231_Lab_1_2014&oldid=15010"