Computer Science Department
College of Arts and Sciences
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:
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