EE 4720 - FAQ

Content (Lectures, Homework, etc.)

In Chapter 4 of the text (second U.S.A. edition) there are several instances of branch instructions not stalling when the branch condition is produced by the immediately preceding instruction. Is that a mistake?
On Sat, 4 Apr 1998 19:20:26 Sunil R. Atri wrote:

In page 230 of the text, in the second paragraph there is a sentence, "In our example there is a data dependence between the SUBI abd BNEZ, but this dependence does not lead to hazard for this pipeline, since forwarding eliminates the hazard."

I do not see how this is possible, as the data is ready at the end of EX stage while the data is required at the beginning of the ID stage leading to 1 stall.

I have come across a paperback second edition [purchased in India]) of this book which states that there will be a stall whereas this book [hardcover] states completely the opposite.

It turns out that the cheaper version is correct. The errata includes a correction for this error.
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In the pipeline execution diagram for the RAW hazard example presented in the 27 February 1998 lecture the reading instruction appears to complete ID several cycles before the writing instruction completes WB. Is that really what's happening?
A student who wishes to remain anonymous writes:
> In class we used the following pipelining example:

  Time/cycle:          0  1  2  3   4   5  
> 1. AND r10,r11,r12   IF ID EX MEM WB
> 2. ADD r1,r2,r3         IF ID EX  MEM WB
> 3. SUB r4,r1,r5            IF ID         EX MEM WB
> 4. XOR r13,r14,r15

> In the above example, register r1 will be written in the WB stage by
> the line-2 instruction. The instruction on line 3 uses (reads) that
> value in its ID stage, before the WB stage of line 2 even starts.
> Will this not cause a problem?

If the line-3 instruction finished ID in cycle 3, there would indeed be a problem. But the instruction remains in ID for three cycles, in cycle 5, the last, the correct value (written by the line 2 instruction) will be read.

> Should the above pipeline execution be something like this :

> line2 : IF ID EX MEM WB
> line3 :    IF        ID EX MEM WB

In line 3 the instruction remains in the decode (ID) stage for three cycles. That could be indicated in the pipeline execution diagram by writing ID for each cycle, like this:

2. ADD r1,r2,r3         IF ID EX  MEM WB
3. SUB r4,r1,r5            IF ID  ID  ID  EX MEM WB

but, for brevity we indicate it like this:

2. ADD r1,r2,r3         IF ID EX  MEM WB
3. SUB r4,r1,r5            IF ID  ----->  EX MEM WB

which is probably what you meant by this:

line2 : IF ID EX MEM WB
line3 :    IF        ID EX MEM WB
So, both of your examples are identical.
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What exactly does "a register can be written and read in the same cycle" mean? (Used in the 27 February 1998 lecture.)
A better phrasing is: "a register value can be read in the same cycle it is written." For a register file that does not have this capability a register value can be read no sooner than the cycle after it is written. Suppose at the beginning of cycle 22 register r1 holds a 014. During the cycle one instruction is writing a 980 to r1 while another instruction is reading r1. There are two possible outcomes (depending on how the register file is implemented): the instruction may read a 014 (the old value) or it may read a 980 (the new value). If the new value were read we say a register value can be written and read in the same cycle.
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Should I take this course?
If you have the background, yes!, definitely!!, it's a great course!!! But be aware that many students who registered for the course without having the proper background dropped after receiving a low mid-term exam grade. These were primarily graduate students arriving from other universities who did not have sufficient exposure to computer organization and assembly language programming.
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Course Materials

The book is too expensive for me. Do I really have to buy the book?
No. Though the text is listed in the syllabus as REQUIRED, you will never have to provide any proof that you actually bought the book. That said, students are responsible for material in the book. If purchasing a new copy would be difficult, consider these options: Try to buy a used copy of the book. Find a classmate who studies at different times than yourself, and split the cost with that person. Note that the exams will be closed book, so a personal copy is not needed for exams.
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How do I get copies of the slides presented in class?
Copies of the slides are available in my office, and via the Internet (see The Internet copies are in three forms, HTML, which can be displayed by any web browser; Portable Document Format version 3.0, which can be displayed using the Adobe Acrobat Reader application or plug-in; and gzipped PostScript, which can be displayed or printed from any web browser on a properly configured system. Most Unix systems are properly configured. PDF plugins and helper-apps can be downloaded from the Adobe site. The HTML-formated slides are a rough translation (of the TEX dvi format) and so mathematics will not come out properly (some symbols might be missing) and illustrations will not show at all.
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Grading and Exams

When is the midterm exam?
Some time in the middle of the semester. A date will be discussed in class and decided on at least two weeks in advance.
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My midterm grade is ranked X. I understand that many students with low midterm grades--especially graduate students--drop. How will that affect my letter grade?
Letter grades are based on the absolute level of work, so a student's grade would be the same whether or not other students drop. Many A's and B's are given in courses with a large fraction of graduate students because those who fear a C or lower based on the midterm exam tend to drop.
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I am thinking about dropping the course. What grade would I receive if I stay in?
My deal with the fortune tellers: I don't predict the future, they don't teach EE.
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I received the highest B in the class. Can you change my grade to an A?
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How many times does a question have to be asked for it to be considered "frequently asked" and included in this list?
Zero. For example, no one ever asked the question answered here.
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David M. Koppelman -
Modified 30 Oct 2000 8:57 (1457 UTC)
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