In order to appreciate the progress that computer technology has experienced over the last few decades, it may be helpful to set up an analogy to compare ``computer speeds'' with (say) ``airplane speeds''. Moore's Law (suggested some 33 years ago) mentioned in Chapter 1 of the textbook and explained in the lecture states that ``The speed of a silicon chip of the same size would double about every 18 months.'' The analogy for airplanes would be that ``The speed of an airplane of the same size would double about every 18 months.'' Let's consider 100-seat airplanes manufactured 33 years ago. Let's assume that they flew at speeds that allowed them to travel around the world in 72 hours (i.e. 3 days). How long would an airplane manufactured today take to go around the world, if Moore's Law were to apply to airplanes?

As explained in class, multi-user (timesharing) operating systems give to their users the illusion of having the computer ``all to themselves'' thanks to a trick called ``time slicing''. Using this technique, the computer gives each user a ``time slice'' during which the computer performs whatever operations are necessary to satisfy the user's need. When that ``time slice'' expires, the computer shifts its attention to another user. This process goes on until all user are serviced and then it starts all over again. Obviously, given the very high speeds at which computers operate and the slow speed at which humans interact with computers (e.g. through typing), this process is usually transparent, unless the number of users becomes large enough, in which case the computer will ``seem'' a bit slow. This happens because as the number of users in a timesharing system increases, the time it takes the computer to come back to a user becomes longer.

To understand this, assume that a user can type up to 60 words per minutes and that (on the average) a word consists of 5 characters. Also, assume that it takes the computer 2 millisecond to process a single character (or in other words, the computer can process 500 characters in a second). How many users would it take to make the computer seem ``slow'' (i.e. users have to wait to see what they type appear on the screen)?

To be able to use a multi-user (timesharing) computer system, you must have an account on that system. In this problem, you will go through the exercise of applying for an account on CSA, the computer system that we will be using for CS101. If you already have an account on ACS, you still have to open an account on CSA for this course. Follow the steps highlighted in the lecture (and section) and open your course account. You should remember the following:

• The terminal room is in MCS B24 (this is in the basement of 111 Cummington street).
• In addition to the course number, your name, and your id, the program to open an account will ask you to enter a login name and a password.
  • The login name (also known as your user name) you pick will be used throughout this semester as your identification. To make an analogy, your login name is similar to your bank account number. It remains associated with you as long as your account is still active. Usually, computer users choose their last names as their login names. You are free, however, to choose any name (e.g. your first name, your nickname, ...) Since login names are used for identification purposes, the computer will not accept a login name that somebody else is using.
  • The password you pick is a secret sequence of characters that you will be using to gain access to your account. This is similar to the password you use when you use teller machines. Your password should be at least 6 characters. It should not be anything that is easy to guess (like your first name, or your birthday, or any word in the dictionary, ... etc.). It must have both, letters and numbers, in it. Unlike your login name, your password can be changed once your account is opened. Notice that whenever the computer is expecting you to enter a password, it will not display what you type. This is necessary to ensure secrecy.
  • If you have another account on campus (e.g. on ACS), you could use the same login name for CSA, but it is a good idea to have a different password. This would protect your account if your password on the other account is compromized.

• The process of opening an account must be verified by one of the TAs (Terminal Assistants) in the terminal room. After you are finished with steps a. through d., ask one of the TAs to verify it. The TA will need to see your BU ID with a valid current registration sticker, so make sure to have yours!
• Once your application for an account is completed, it will be processed and approved within few hours following your application. Only then will you be able to proceed with the solution of the next problems as well as the ones in the following homeworks.
  

There is nothing to hand in for this problem. We will be able to check automatically whether or not you succeeded in opening your account or not.

In this problem you are asked to log in and out of the system.

Logging in: To start a work session on CSA you need to identify yourself to the computer system. At the login: prompt, enter the login name you chose when you applied for the account. Now, the computer will ask you to enter your password:. Type it in. (Notice that nothing will be displayed on the screen as you type in your password for security reasons.) Now, the computer will display a welcoming message. Now, you are done with logging in, and UNIX, the operating system of the system is ready for your orders! To tell you that it is waiting for your instructions, a UNIX prompt consisting of your login name and a % sign will be displayed.

Logging out: To finish a work session on CSA all you have to do is to inform UNIX that you want to ``log out''. To do this, just type logout at the UNIX prompt.

There is nothing to hand in for this problem. We will be able to check automatically whether or not you succeeded in logging in and out of your account.

Created on: 1997.08.01
Updated on: 1997.09.02
Maintainer: Azer Bestavros
            best@cs.bu.edu