CS 210

PROJECT #3			           Due: Noon Wednesday 04/26/06

     The purpose of this project is to introduce the student to ways in 
which application programs can modify various behaviors of the terminal 
device (i.e., the combined keyboard and display-monitor), as needed for
certain kinds of systems utilities such as the 'login' program.   


			   PROBLEM STATEMENT

     Develop a complete assembly language program (named 'password.s')
which will prompt the user to type in a secret password, and then will 
display a notice that informs the user whether further access is being
denied or granted:

		 Correct password -- access is granted.
				or
		 Incorrect password -- access refused.  

To insure that you acquire some experience with using the x86 string-
manipulation instructions (including appropriate choice of a 'repeat'
prefix and conditional-jump), this project will explicitly ask you to
employ the 'cmpsb' instruction when comparing the user's input-string
with your program's built-in password-string.  These three additional 
'user-interface' features are also requirements (intended to increase 
convenience, forgive mistakes, and improve security):

     (1) allow users to type in the required password using either
	 uppercase or lowercase letters;

     (2) allow users to reenter their password, up to a maximum of 
	 four tries, before their login attempt is finally denied;

     (3) rewrite the keyboard-input procedure so that the password
	 itself cannot be seen on the screen as it is being typed,
         but so that some neutral character (such as '*') will get
         displayed instead (to provide your user with 'feedback').

Your implementation of these features will (necessarily) acquaint you
with loop-constructs, with ascii-codes, and with some individual bits 
in specific fields within a system-defined 'termios' data-structure.

Be aware in your design of the user's experience: for example, do not
print a message saying 'access is refused' if you will then allow the
user another try at typing in the password correctly; instead, show a 
more accurate message, saying:

		Incorrect password -- please try again.

The easiest way to implement "case insensitivity" for your user-input
is for your program to convert lowercase letters to uppercase letters
before making the comparison with the correct built-in password.  

Eliminating the automatic "echo" of input-characters on the screen is
undoubtedly the most challenging part of this assignment and involves
details of the Linux 'tty' interface to be discussed in our lectures.
_____________________________________________________________________
Allan B. Cruse        University of San Francisco         Spring 2006