UMBC CMSC 313, Computer Organization & Assembly Language, Fall 2004

Project 1: ROT13

Also available in PDF: project1.pdf.

Due: Thursday, September 23, 2004


This project is a finger-warming exercise to make sure that everyone can compile an assembly language program, run it through the debugger and submit the requisite files using the systems in place for the programming projects.


The ROT13 format is used on USENET newsgroups to mask potentially offensive postings, movie spoilers, etc. The idea is that readers who think they might be offended by a controversial remark will simply not "decode" the posting and thus not be offended. Many news readers and email clients support ROT13.

The encoding is very simple. The characters 'a' - 'm' are mapped to 'n' - 'z' and vice versa. Upper case letters are transformed analogously. All other characters (e.g., digits and punctuation marks) are left alone. For example, "There was a man from Nantucket" becomes "Gurer jnf n zna sebz Anaghpxrg" after ROT13 transformation. To decode a message in ROT13, you simply apply the ROT13 transformation again.


For this project, you must do the following:
  1. Write an assembly language program that prompts the user for an input string and prints out the ROT13 encoding of the the string. A good starting point for your project is the program toupper.asm (shown in class) which converts lower case characters in the userŐs input string to upper case. The source code is available on the GL file system at: /afs/
  2. Using the UNIX script command, record some sample runs of your program and a debugging session using gdb. In this session, you should fully exercise the debugger. You must set several breakpoints, single step through some instructions, use the automatic display function and examine the contents of memory before and after processing. The script command is initiated by typing script at the UNIX prompt. This puts you in a new UNIX shell which records every character typed or printed to the screen. You exit from this shell by typing exit at the UNIX prompt. A file named typescriptis placed in the current directory. You must exit from the scriptcommand before submitting your project. Also, remember not to record yourself editing your programs --- this makes the typescript file very large.

Turning in your program

Use the UNIX submit command on the GL system to turn in your project. You should submit two files: 1) the modified assembly language program and 2) the typescript file of your debugging session. The class name for submit is cs313_0101, the project name is proj1. The UNIX command to do this should look something like:

submit cs313_0101 proj1 rot13.asm typescript


Additional help on running NASM, gdb and making system calls in Linux are available on the assembly language programming web page for this course.

Recall that the project policy states that programming assignments must be the result of individual effort. You are not allowed to work together. Also, your projects will be graded on five criteria: correctness, design, style, documentation and efficiency. So, it is not sufficient to turn in programs that assemble and run. Assembly language programming can be a messy affair --- neatness counts.

Last Modified: 24 Sep 2004 14:14:53 EDT by Richard Chang
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