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CMU CS 15213 - Homework Assignment H2

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CS 213, Fall 1998Homework Assignment H2Assigned: Sept. 10, Due: Wed., Sept. 16, 11:59PMChris Colohan ([email protected]) is the lead person for this assignment.The purpose of this assignment is to learn Alpha assembly language and to become familiar with how Ccode is translated into assembly. You will do this by looking at a sequence of assembly language sequencesand decompiling them to find the C source code that produced them. Reverse engineering this code willimprove your understanding of both the C constructs and the assembly code.IntroductionIn this assignment you will be given the assembler dump for a function, and it is your goal to derive the Csource code that produced it. To help you I will give you hints about which operators and constructs wereused in the function. The further in the assignment you go, the fewer hints you will have to work from.If we look at a very simple function then we expect the compiler to produce very little code. For example:int increment(int arg){return arg + 1;}produces the code (compiled with gcc -O):0x0: addl a0, 0x1, v00x4: ret zero, (ra), 1From this simple example we can see that the argument arg is passed in register a0, and the return valueis passed in register v0.When you are trying to figure out what a given functions does, try creating a small example like this oneto see what code the compiler emits. If you can create a series of small functions that produce part of theanswer, you can then piece them together to create a solution.LogisticsYou may work in a group of up to 2 people in solving the problems for this assignment. The only “hand-in” will be electronic. Any clarifications and revisions to the assignment will be posted on Web pageassigns.html in the class WWW directory.1All files for this assignment are in the directory:/afs/cs.cmu.edu/academic/class/15213-f98/H2First create a (protected) directory to work in, and copy our template code using the command tar -xvf/afs/cs.cmu.edu/academic/class/15213-f98/H2/H2.tar. Thiswill create the files Makefile,TEAM_MEMBERS,ques[1-10].c, and ques[1-10]_ref.S.In this assignment you will only modifyand hand in the files ques[1-10].c, as well as TEAM_MEMBERS. Please edit the TEAM_MEMBERS filenow so you do not forget.These problems are self-testing. To check the solution for question 1, run the command make test1.This will compile ques1.c into an object file and disassemble it into ques1.S. If the disassembly isidentical to ques1_ref.S then your solution is correct. If it is not, then it is incorrect.The C functions which created the reference files are only a few lines long (except for the switch statement,which is longer but repetitive), and use just the variables declared in ques[1-10].c. If you keep yourprogram simple it is more likely to match the code given.EvaluationEach question is worth 3 points, and will be graded on two aspects: correctness and simplicity. If your codeproduces the same assembly code as the reference code, then it will get on correctness. Many of theprograms can be expressed correctly using low level primitives but are still hard to read. If your solutioncontains a while() loop with an initialization and an increment, try turning it into a for() loop. If itcontains a whole series of if() statements, see if you can express it as a switch() statement. Often asimple integer expression will be turned into a more complex one by the compiler as well — see if you canfind the simpler one. If your function contains simple code, it will getfor simplicity. If I tell you thefunction is a loop and you implement it as a series of if() statements, goto commands, and bit operationsthen you will getfor simplicity.The problems are all weighted equally. The earlier problems are easier than the later problems, so planaccordingly. Since there arequestions, the assignment will be graded out of .ProblemsQuestion 1: Integer ExpressionThis function implements an integer expression that uses +, - and *. You will notice that after the retinstruction is some no-ops, these are to align the next function in the program.Question 2: Integer DivisionDivision by a constant is handled in a clever way by the compiler. The compiler uses tricks to handle boththe positive and negative case, and emits tuned code to handle dividing by a power of two. More complexdivision is handled through a series of table lookups. This question contains a number of integer divisionsby a power of two constant, as well as other operations.2Question 3: Bit OperationsWhen I first tried solving the bang() question on Homework 1, I did it using a tree of shifts and ors. Thisis a simplified version of my solution. It uses the operators >>, |, & and ˆ.Question 4: ConditionalsIf your code contains if statements then the compiler will generate branches. This code contains a branch.Question 5: LoopsLoops are just like conditionals, but the branch goes backwards. The compiler inserts nops into the code tomake the cache perform better.Question 6: Small Switch StatementIf there are a small number of cases in a switch() statement then the compiler turns it into a series of ifstatements. Find the switch() that produced this code.Question 7: Large Switch StatementIf there are a large number of cases in a switch() statement and they are consecutive then the compilerbuilds a jump table. In this question you can not see the entries in the table, but you can guess what they arefrom the code.Question 8: Function CallThis function calls another function. This makes it necessary to set up the stack for the called function.Question 9: Recursive Function CallIf a function calls itself then it can use this knowledge to skip some of the function prologue. Once you havedeciphered this function it should be familiar to you.Question 10: Grab BagThis question combines a number of the operations and constructs listed above.Hand InMake sure you have edited the file TEAM_MEMBERS to identify who is in your programming team. If yourun the command make handin NAME=yourname, where yourname is replaced with the Andrew Idof the first person on your team, a tarfile will be created and copied to the handin directory.3You only have write permission to this directory. If you make a mistake, e.g., by giving the file an incorrectname, send a copy of the correct file via email to the contact person for this assignment along with anexplanation.You may only hand in an assignment once. Make sure your code is in its


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CMU CS 15213 - Homework Assignment H2

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