CSC231 Lab 1 2014

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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

             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

• 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!

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