CS 105 Fall 2005 Lab 1 Manipulating Bits See class calendar for lab and due dates September 7 2005 6 55 PM Geoff Kuenning geoff cs hmc edu is the lead person for this assignment Introduction The purpose of this assignment is to become more familiar with bit level representations and manipulations You ll do this by solving a series of programming puzzles Many of these puzzles are quite artificial but you ll find yourself thinking much more about bits in working your way through them Logistics You MUST work in a group of at least two 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 the course Web page We strongly recommend that you and your partner brainstorm before coding Handout Instructions The materials for the data lab are on the Web at http www cs hmc edu geoff cs105 labs lab01 data datalab handout tar Start by downloading datalab handout tar to a protected directory in which you plan to do your work Then give the command tar xvf datalab handout tar This will cause a number of files to be unpacked in the directory The only file you will be modifying and turning in is bits c 1 The file btest c allows you to evaluate the functional correctness of your code The file README contains additional documentation about btest Use the command make btest to generate the test code and run it with the command btest The file dlc is a compiled binary that you can use to check your solutions for compliance with the coding rules NOTE dlc is used by the auto grader so be sure to check your solution file prior to submission The remaining files are used to build btest Looking at the file bits c you ll notice a C structure team into which you should insert the requested identifying information about the individuals comprising your programming team Do this right away so you don t forget The bits c file also contains a skeleton for each of the 15 programming puzzles Your assignment is to complete each function skeleton using only straightline code i e no loops or conditionals and a limited number of C arithmetic and logical operators Specifically you are only allowed to use the following eight operators A few of the functions further restrict this list Also you are not allowed to use any constants longer than 8 bits See the comments in bits c for detailed rules and a discussion of the desired coding style Evaluation Your code will be compiled with GCC and run and tested on Wilkes Your score will be computed out of a maximum of 50 points based on the following distribution 40 Correctness of code running on Wilkes 7 5 Performance of code based on number of operators used in each function 2 5 Style points based on your instructor s subjective evaluation of the quality of your solutions and your comments The 15 puzzles you must solve have been given a difficulty rating between 1 and 4 such that their weighted sum totals to 40 We will evaluate your functions using the test arguments in btest c You will get full credit for a puzzle if it passes all of the tests performed by btest c half credit if it fails one test and no credit otherwise Regarding performance our main concern at this point in the course is that you can get the right answer However we want to instill in you a sense of keeping things as short and simple as you can Furthermore some of the puzzles can be solved by brute force but we want you to be more clever Thus for each function we ve established a maximum number of operators that you are allowed to use for each function This limit is very generous and is designed only to catch egregiously inefficient solutions You will receive 5 points for each function that satisfies the operator limit Finally we ve reserved 2 5 points for a subjective evaluation of the style of your solutions and your commenting Your solutions should be as clean and straightforward as possible Your comments should be informative but they need not be extensive 2 Name bitNor x y bitXor x y isNotEqual x y getByte x n copyLSB x logicalShift x n bitCount x bang x leastBitPos x Description x y using only and using only and x y Extract byte n from x Set all bits to LSB of x Logical right shift x by n Count number of 1 s in x Compute x without using operator Mark least significant 1 bit Rating 1 2 2 2 2 3 4 4 4 Max Ops 8 14 6 6 5 16 40 12 30 Table 1 Bit Level Manipulation Functions Part I Bit manipulations Table 1 describes a set of functions that manipulate and test sets of bits The Rating field gives the difficulty rating the number of points for the puzzle and the Max ops field gives the maximum number of operators you are allowed to use to implement each function Function bitNor computes the N OR function That is when applied to arguments x and y it returns x y You may only use the operators and Function bitXor should duplicate the behavior of the bit operation using only the operations and Function isNotEqual compares x to y for inequality As with all predicate operations it should return 1 if the tested condition holds and 0 otherwise Function getByte extracts a byte from a word The bytes within a word are ordered from 0 least significant to 3 most significant Function copyLSB replicates a copy of the least significant bit in all 32 bits of the result Function logicalShift performs logical right shifts You may assume the shift amount n satisfies 1 n 31 Function bitCount returns a count of the number of 1 s in the argument Function bang computes logical negation without using the operator Function leastBitPos generates a mask consisting of a single bit marking the position of the least significant one bit in the argument If the argument equals 0 it returns 0 Part II Two s Complement Arithmetic Table 2 describes a set of functions that make use of the two s complement representation of integers Function tmax returns the largest integer Function isNonNegative determines whether x is less than or equal to 0 Function isGreater determines whether x is greater than y Function divpwr2 divides its first argument by 2n where n is the second argument You may assume that 0 n 30 It must round toward zero 3 Name tmax void isNonNegative x isGreater x y divpwr2 x n abs x addOK x y Description largest two s complement integer x 0 x y x 1 n absolute value Does x y overflow Rating 1 3 3 3 4 3 Max Ops 4 6 24 15 10 20 Table 2 Arithmetic Functions Function abs is equivalent to the expression x 0 x x giving the absolute value of x for all
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