CSC 2400: Bit Manipulation Assignment

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1 CSC 2400: Bit Manipulation Assignment The purpose of this assignment is to enhance your skills on 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. You may work in groups of two on this assignment. If you do not have a partner and would like one, please let me know. Functions to Implement The table below lists nine functions that you ll need to implement. Each of these functions manipulates sets of bits. Your solutions should be as clean and straightforward as possible. One of the goals of this assignment is to instill in you a sense of keeping things as short and simple as you can. 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. The Max Ops ' field in this table gives the maximum number of operators you are allowed to use to implement each function. Function Description Max Ops 1 bitxor(x,y) ^ using only & and ~ 14 2 isequal(x,y) x == y? 6 3 getbyte(x,n) Extract byte n from x 5 4 copylsb(x) Set all bits to LSB of x 16 5 logicalshift(x,n) Logical right shift x by n 40 6 leastbitpos(x) Mark least significant 1 bit 30 7 tmax() Largest two's complement integer 4 8 isnegative(x) x < 0? 6 In the following we describe each function in turn. 1. Function bitxor should duplicate the behavior of the XOR (^) bit operation using only the bit operations AND (&) & and NOT (~). * bitxor - x^y using only ~ and & * Example: bitxor(4, 5) = 1 * Legal ops: ~ & * Max ops: 14 int bitxor(int x, int y)

2 2. Function istequal compares x to y for equality. As with all predicate operations, it should return 1 if the tested condition holds and 0 otherwise. * isequal - return 1 if x == y, and 0 otherwise * Examples: isequal(5,5) = 1, isequal(4,5) = 0 int isequal(int x, int y) 3. Function getbyte extracts a byte from a word. The bytes within a word are ordered from 0 (least significant) to 3 (most significant). * getbyte - Extract byte n from word x * Bytes numbered from 0 (LSB) to 3 (MSB) * Examples: getbyte(0x ,1) = 0x56 int getbyte(int x, int n) 4. Function copylsb replicates a copy of the least significant bit in all 32 bits of the result. * copylsb - set all bits of result to least significant bit of x * Example: copylsb(5) = 0xFFFFFFFF, copylsb(6) = 0x * Max ops: 5 int copylsb(int x)

3 5. Function logicalshift performs logical right shifts. * logicalshift - shift x to the right by n, using logical shift * May assume that 1 <= n <= 31 * Examples: logicalshift(0x ,4) = 0x * Legal ops: ~ & ^ + << >> * Max ops: 16 int logicalshift(int x, int n) 6. 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. * leastbitpos - return a mask that marks the position of the * least significant 1 bit. If x == 0, return 0 * Example: leastbitpos(96) = 0x20 int leastbitpos(int x) 7. Function tmax returns the largest integer. * tmax - return maximum two's complement integer * Max ops: 4 int tmax()

4 8. Function isnegative determines whether x is less than 0. * isnegative - return 1 if x < 0, return 0 otherwise * Example: isnegative(-1) = 1, isnegative(0) = 0. int isnegative(int x) Testing Your Functions Write a single C program called bits.c that includes your solution to each of the functions above. In addition to the 8 functions above, your program should contain one main function that tests the behavior of each bit manipulation function. For example, after you have completed your bitxor function, test it immediately, before moving on to the next function: int bitxor(int x, int y) Code for the other functions goes here int main() int x = 0x4A; int y = 0x52; int result; Test code for bitxor: result = bitxor(x, y); if(result!= (x^y)) printf( Error in function bitxor\n ); Test code for the other functions goes here return 0;

5 Hand In Instructions Write a short readme text file that contains your identifying information (your name(s) and your Unix login name(s)), the Unix account and directory where the file bits.c is located, plus any bugs and deviations from the assignment specifications. Hand in printed copies of your readme and bits.c files, along with a sample output. Leave these two files in your Unix account. Your code will be tested on our Unix system. Your score will be computed based on the following distribution: [60%] Correctness of code running on one of the class machines. [30%] Performance of code, based on number of operators used in each function. [10%] Complete submission and style points, based on the quality of your solutions and your comments. Good Luck, and Have Fun!