Computer Architecture

Start Lecture #1

Chapter 0: Administrivia

I start at Chapter 0 so that when we get to chapter 1, the numbering will agree with the text.

0.1: Contact Information

• <my-last-name> AT nyu DOT edu (best method)
• http://cs.nyu.edu/~gottlieb
• 715 Broadway, Room 712
• 212 998 3344

0.2: Course Web Page

There is a web site for the course. You can find it from my home page listed above.

• You can also find these lecture notes on the course home page. Please let me know if you can't find it.
• The notes are updated as bugs are found or improvements made.
• I will also produce a separate page for each lecture after the lecture is given. These individual pages might not get updated as quickly as the large page.

0.3: Textbook

The course text is Hennessy and Patterson, Computer Organization and Design: The Hardware/Software Interface, 3rd edition.

• Available in the bookstore.
• The main body of the book assumes you know logic design.
• I do NOT make that assumption.
• We will start with appendix B, which is logic design review.
• Most of the figures in these notes are based on figures from the course textbook. The following copyright notice applies.

  All figures from Computer Organization and Design: The Hardware/Software Approach, Third Edition, by David Patterson and John Hennessy, are copyrighted material (COPYRIGHT 1998 MORGAN KAUFMANN PUBLISHERS, INC. ALL RIGHTS RESERVED). Figures may be reproduced only for classroom or personal educational use in conjunction with the book and only when the above copyright line is included. They may not be otherwise reproduced, distributed, or incorporated into other works without the prior written consent of the publisher.

0.4: Computer Accounts and Mailman Mailing List

• You are entitled to a computer account, please get yours asap if you don't already have one.
• Sign up for the course mailman mailing list here.
• If you want to send mail just to me, use the address given above, not not the mailing list.
• Questions on the labs should go to the mailing list. You may answer questions posed on the list as well. Note that replies are sent to the list.
• I will respond to all questions; if another student has answered the question before I get to it, I will confirm if the answer given is correct.
• Please use proper mailing list etiquette.
  • Send plain text messages rather than (or at least in addition to) html.
  • Use the Reply command to contribute to the current thread, but NOT to start another topic.
  • If quoting a previous message, trim off irrelevant parts.
  • Use a descriptive Subject: field when starting a new topic.
  • Do not use one message to ask two unrelated questions.
  • Do NOT make the mistake of sending your completed lab assignment to the mailing list. This is not a joke; several students have made this mistake in past semesters.

0.5: Grades

Your grade will be a function of your exams and laboratory assignments (see below). I am not yet sure of the exact weightings, but it will be approximately 30% midterm, 30% labs, 40% final exam.

0.6: The Upper Left Board

I use the upper left board for lab/homework assignments and announcements. I should never erase that board. If you see me start to erase an announcement, please let me know.

I try very hard to remember to write all announcements on the upper left board and I am normally successful. If, during class, you see that I have forgotten to record something, please let me know. HOWEVER, if I forgot and no one reminds me, the assignment has still been given.

0.7: Homeworks and Labs

I make a distinction between homeworks and labs.

Labs are

• Required.
• Due several lectures later (date given on assignment).
• Graded and form part of your final grade.
• Penalized for lateness.
• Most often are computer programs you must write.

Homeworks are

• Optional.
• Due the beginning of the Next lecture.
• Not accepted late.
• Mostly from the book.
• Collected and returned.
• Able to help, but not hurt, your grade.

0.7.1: Homework Numbering

Homeworks are numbered by the class in which they are assigned. So any homework given today is homework #1. Even if I do not give homework today, the homework assigned next class will be homework #2. Unless I explicitly state otherwise, all homeworks assignments can be found in the class notes. So the homework present in the notes for lecture #n is homework #n (even if I inadvertently forgot to write it to the upper left board).

0.7.2: Doing Labs on non-NYU Systems

You may solve lab assignments on any system you wish, but ...

• You are responsible for any non-nyu machine. I extend deadlines if the nyu machines are down, not if yours are.
• Be sure test your assignments to the nyu systems. In an ideal world, a program written in a high level language like Java, C, or C++ that works on your system would also work on the NYU system used by the grader. Sadly this ideal is not always achieved despite marketing claims to the contrary. So, although you may develop you lab on any system, you must ensure that it runs on the nyu system assigned to the course.
• If somehow your assignment is misplaced by me and/or a grader, we need a to have a copy ON AN NYU SYSTEM that can be used to verify the date the lab was completed.
• When you complete a lab and have it on an nyu system, email the lab to the grader and copy yourself on that message. This email must come from your CIMS account. Keep the copy until you have received your grade on the assignment. The systems support staff can retrieve your mail from their logs given your copy and from that we can verify the dates. I realize that I am being paranoid about this. It is rare for labs to get misplaced, but they sometimes do and I really don't want to be in the middle of an I sent it ... I never received it debate. Thank you.

0.7.3: Obtaining Help with the Labs

Good methods for obtaining help include

1. Asking me during office hours (see web page for my hours).
2. Asking the mailing list.
3. Asking another student, but ...
   Your lab must be your own.
   That is, each student must submit a unique lab. Naturally, simply changing comments, variable names, etc. does not produce a unique lab.

0.7.4: Computer Language Used for Labs

You may write your lab in Java, C, or C++. Other languages may be possible, but please ask in advance. I need to ensure that the TA is comfortable with the language.

0.8: A Grade of Incomplete

The rules for incompletes and grade changes are set by the school and not the department or individual faculty member. The rules set by CAS can be found in http://cas.nyu.edu/object/bulletin0608.ug.academicpolicies.html state:

The grade of I (Incomplete) is a temporary grade that indicates that the student has, for good reason, not completed all of the course work but that there is the possibility that the student will eventually pass the course when all of the requirements have been completed. A student must ask the instructor for a grade of I, present documented evidence of illness or the equivalent, and clarify the remaining course requirements with the instructor.

The incomplete grade is not awarded automatically. It is not used when there is no possibility that the student will eventually pass the course. If the course work is not completed after the statutory time for making up incompletes has elapsed, the temporary grade of I shall become an F and will be computed in the student's grade point average.

All work missed in the fall term must be made up by the end of the following spring term. All work missed in the spring term or in a summer session must be made up by the end of the following fall term. Students who are out of attendance in the semester following the one in which the course was taken have one year to complete the work. Students should contact the College Advising Center for an Extension of Incomplete Form, which must be approved by the instructor. Extensions of these time limits are rarely granted.

Once a final (i.e., non-incomplete) grade has been submitted by the instructor and recorded on the transcript, the final grade cannot be changed by turning in additional course work.

0.9: Academic Integrity Policy

The CS policy on academic integrity, which applies to all graduate courses in the department, can be found here .

Appendix B: Logic Design

Remark: Appendix B is on the CD that comes with the book, but is not in the book itself. If anyone does not have convenient access to a printer, please let me know and I will print a black and white copy for you. The pdf on the CD is in color so downloading it to your computer for viewing in color is probably a good idea. If you have a color printer that is not terribly slow, you might want to print it in color—that's what I did.

Homework: Read B1

B.2: Gates, Truth Tables and Logic Equations

Homework: Read B2

The word digital, when used in digital logic or digital computer means discrete. That is, the electrical values (i.e., voltages) of the signals in a circuit are treated as a non-negative integers (normally just 0 and 1).

The alternative is analog, where the electrical values are treated as real numbers.

To summarize, we will use only two voltages: high and low. A signal at the high voltage is referred to as 1 or true or set or asserted. A signal at the low voltage is referred to as 0 or false or unset or deasserted.

The assumption that at any time all signals are either 1 or 0 hides a great deal of engineering.

• A full engineering design must make sure not to sample the signal when not in one of these two states.
• Sometimes it is just a matter of waiting long enough (determines the clock rate, i.e., how many megahertz).
• Other times it is worse and you must avoid glitches.
• Oscilloscope traces are shown on the right. The vertical axis is voltage, the horizontal axis is time.
  1. Square wave—the ideal. This is how we think of circuits.
  2. (Poorly drawn) Sine wave.
  3. Actual wave.
     • Non-zero rise times and fall times.
     • Overshoots and undershoots.
     • Glitches.

Since this is not an engineering course, we will ignore these issues and assume square waves.

In English digital implies 10 (based on digit, i.e. finger), but not in computers.

Indeed, the word Bit is short for Binary digIT and binary means base 2 not 10.

0 and 1 are called complements of each other as are true and false (also asserted/deasserted; also set/unset)

A logic block can be thought of as a black box that takes signals in and produces signals out. There are two kinds of blocks

• Combinational (or combinatorial)
  • Does NOT have memory elements.
  • Is simpler than circuits with memory since the outputs are a function of the inputs. That is, if the same inputs are presented on Monday and Tuesday, the same outputs will result.
• Sequential
  • Contains memory.
  • The current value in the memory is called the state of the block.
  • The output depends on the input AND the state.

We are doing combinational blocks now. Will do sequential blocks later (in a few lectures).

Truth Tables

Since combinatorial logic has no memory, it is simply a (mathematical) function from its inputs to its outputs.

A common way to represent the function is using a Truth Table. A Truth Table has a column for each input and a column for each output. It has one row for each possible set of input values. So, if there are A inputs, there are 2^A rows. In each of these rows the output columns have the output for that input.

How many possible truth tables are there?

1-input, 1-output Truth Tables

Let's start with a really simple truth table, one corresponding to a logic block with one input and one output.

How many different truth tables are there for a one input one output logic block?

There are two columns (1 + 1) and two rows (2¹). Hence the truth table looks like the one on the right with the question marks filled in.

Since there are two question marks and each one can have one of two values there are just 2²=4 possible truth tables.

1. The constant function 1, which has output 1 (i.e., true) for either input value.
2. The constant function 0.
3. The identity function, i.e., the function whose output equals its input. This logic block is sometimes called a buffer.
4. An inverter. This function has output the opposite of the input.

2-input, 1-output Truth Tables

Three columns (2+1) and 4 rows (2²).

How many are there? It is just the number ways can you fill in the output entries, i.e. the question marks. There are 4 output entries so the answer is 2⁴=16.

Larger Truth Tables

How about 2 in and 8 out?

• 10 cols.
• 2²=4 rows.
• 4*8=32 question marks.
• 2³² (about 4 billion) possibilities.

3 in and 8 out?

• 11 cols.
• 2³=8 rows.
• 2^8*8=2⁶⁴ possibilities.

n in and k out?

• n+k cols.
• 2ⁿ rows.
• 2^k*2n possibilities. This gets big real fast!

Boolean algebra

We use a notation that looks like algebra to express logic functions and expressions involving them.

The notation is called Boolean algebra in honor of George Boole.

A Boolean value is a 1 or a 0.
A Boolean variable takes on Boolean values.
A Boolean function takes in boolean variables and produces boolean values.

Four Boolean function are especially common.

1. The (inclusive) OR Boolean function of two variables. Draw its truth table on the board. This is written + (e.g. X+Y where X and Y are Boolean variables) and often called the logical sum. (Three out of four output values in the truth table look like the sum.)
   
   
2. AND. Draw its truth table on the board. And is often called the logical product and written as a centered dot (like the normal product in regular algebra). I will offen write it as a period in these notes. As in regular algebra, when all the logical variables are just one character long, we indicate the product by juxtaposition, that is, AB represents the product of A and B when it is clear that it does not represent the two character symbol AB. All four truth table values look like a product.
   
   
3. NOT. Draw its truth table on the board. This is a unary operator (i.e., it has one argument, not two as above; functions with two inputs are called binary operators). Written A with a bar over it. I will use ' instead of a bar as it is easier for me to input in html.
   
   
4. Exclusive OR (XOR). Draw its truth table on the board. Written as ⊕, a + with a circle around it. True if exactly one input is true. In particular, remember that TRUE ⊕ TRUE = FALSE.

Homework: Draw the truth table of the Boolean function of 3 boolean variables that is true if and only if exactly 1 of the three variables is true.

Some manipulation laws

Remember this is Boolean Algebra.

• Identity (recall that I use . for and):
  • A+0 = 0+A = A
  • A.1 = 1.A = A
  
  
• Inverse (recall that I use ' for not):
  • A+A' = A'+A = 1
  • A.A' = A'.A = 0
  • The name inverse law is somewhat funny since you Add the inverse and get the identity for Product or Multiply by the inverse and get the identity for Sum.
  
  
• Commutative Laws:
  • A+B = B+A
  • A.B = B.A
  • Due to the commutative laws, we see that both the identity and inverse laws contained redundancy. For example from A+0 = A and the commutative law we get that 0+A = A without stating the latter explicitly.
  
  
• Associative Laws:
  • A+(B+C) = (A+B)+C
  • A.(B.C)=(A.B).C
  • Due to the associative law we can write A.B.C without parentheses since either order of evaluation gives the same answer. Similarly we can write A+B+C without parentheses.
  
  
• Distributive Laws:
  • A(B+C)=AB+AC
  • A+BC=(A+B)(A+C)
  • Note that, unlike the situation for ordinary algebra, both distributive laws are valid.
  • Note also that like the situation for ordinary algebra, multiplication has higher precedence than addition if no parentheses are used.
  
  
• DeMorgan's Laws:
  • (A+B)' = A'B' (NOT has the highest precedence
  • (AB)' = A'+B'

How does one prove these laws??

• Simple (but long). Write the truth tables for each side and see that the outputs are the same. Actually you write just one truth table with columns for all the inputs and for the outputs of both sides. You often write columns for intermediate outputs as well, but that is only a convenience. The key is that you have a column for the final value of the LHS (left hand side) and a column for the final value of the RHS and that these two columns have identical results.
• Prove the first distributive law on the board. The following columns are required: the inputs A, B, C; the LHS A(B+C); and the RHS AB+AC. Beginners like us would also use columns for the intermediate results B+C, AB, and AC.

Homework: Prove the second distributive law.

Homework: Prove DeMorgan's laws.

Let's do (on the board) the example on pages B-6 and B-7.

Consider a logic function with three inputs A, B, and C; and three outputs D, E, and F defined as follows: D is true if at least one input is true, E if exactly two are true, and F if all three are true. (Note that by if we mean if and only if.

1. Construct the truth table. This is straightforward; simply fill in the 24 entries by looking at the definitions of D, E, and F.
   
   
2. Produce logic equations for D, E, and F. This can be done in two ways.
   
   
   1. Examine the column of the truth table for a given output and write one term for each entry that is a 1. This method requires constructing the truth table and might be called the method of perspiration.
      
      
   2. Look at the definition of D, E, and F and just figure it out. This might be called the method of inspiration.
      
      For D and F it is fairly clear. E requires some cleverness: the key idea is that exactly two are true is the same as (at least) two are true AND it is not the case that all three are true. So we have the AND of two expressions: the first is a three way OR and the second the negation of a three way AND.