# Homework 1: Numerical Representations and Disassembly of Object Files

Assigned Friday, January 16th, 2015 Monday, January 26th, 2015 at 5:00pm (17:00) Submit a PDF file of your solutions here.

## Introduction

The purpose of written homework assignments is to get you thinking about the topics being covered in lecture and in readings in the textbook which are not represented in the hands-on, programming lab assignments. These written assignments also better prepare you for course examinations. It is worth noting that the book contains many practice problems similar to the problems we ask on these written assignments! The solutions for those practice problems are located at the end of each chapter and should give you a feel for the kind of answers we expect you to turn in for these kind of assignments.

## Logistics

Please write up your answers and submit them as a PDF file in the online dropbox. This simplifies our grading and ensures we read your answers clearly. If you don't know how to create a PDF, a quick internet search should help, or ask us.

We will provide solutions to all of the problems in the written homework assignments in a timely fashion after the assignment is due. This may be around 4 or 5 days after the due date, in general, because a student may use up to 3 late days on an assignment.

## Questions

Answer the following problems from the textbook.

1. Suppose we are given the task of generating code to multiply integer variable x by various different constant factors K. To be efficient, we want to use only the operations +, -, and <<. For the following values of K, write C expressions to perform the multiplication using at most five operations per expression. K = 21, K = -13, and K = 30.
2. Homework Problem 2.81 (pp. 126-127), parts A, B, C, D, and E.
Note: random() generates a number between 0 and Tmax (not Umax)in otherwords: 0 to 2^31 - 1. (Clarification: By "describe the underlying mathematical principles" when the given property always holds, we do not mean you need a formal proof, just an informal description of why it works.)
3. Recall that a floating point number is of the form: (-1)S x M x 2E. M is the mantissa and 2E is the exponent. Also recall that M ranges from 1.0 to (almost) 2.0. Tell us the values of M and E for the number 13.6875 and how this would be encoded in IEEE 754 representation (32 bits). Please also explain the decimal values of M and E, and the binary values of the `frac` and `exp` parts of the representation. (Recall `frac` = M - 1.0.)
4. Homework Problem 3.54 (pp. 294-295). You may use any number of temporary variables in your solution; your code does not have to match up exactly line-by-line with the given assembly code. It just needs to be C code that accomplishes the same thing.
5. For this problem you will use the jack-of-all-trades tool objdump to examine executable programs on Linux. Be sure to work on this problem on attu or the CSE home VM. You will need a copy of the code that you turned in for Lab 1 (your code does not have to be entirely correct or complete, but at least some of your puzzle functions should be filled in).
1. Run make in your Lab 1 directory to build your code. Run the command objdump -t btest | grep text and examine the output. What type of thing do some of the strings in the rightmost column of the output represent? (you you not need to identify what all of them refer to)
2. Use objdump to disassemble your btest code (review the lecture slides or check the man page to find the right flag). Find the labels in the assembly code corresponding to your bit puzzle functions. What x86 assembly instructions appear to perform right-shift and left-shift operations?
3. Most Linux distributions include several other useful programs for examining binary files containing machine code. Try out commands such as ldd, file, nm, strings and readelf, and try other flags with objdump. Report something neat, unusual, or unexpected that you find. Don't forget to use the man pages to learn more about all of these programs.
4. Extra Credit (a little): run the objdump -t command on a different program on the system - choose one in the /usr/bin/ directory. You will likely see the output "SYMBOL TABLE: no symbols". Why does objdump -t appear to work on your program, but not on others?

## Notes

Make sure you are using the 2nd edition of Computer Systems: A Programmer's Perspective, ISBN: 978-0-13-610804-7. If you're not using the right book, you might be doing the wrong problems!

## Submitting Your Work

Please turn in a PDF file containing your answers to the Catalyst Drop Box for this assignment. We will not accept submissions that are not in PDF format.