CS1101C Lab 1 (Odd Week)

Bit Masking

The name of your C program file must be called mask.c, files with any other name will not be marked.

Preliminary

Bit masking is a technique that is frequently used in network and systems programming. Suppose we have a 4-bit binary string (i.e. a string consisting only of 0s and 1s) such as "0110", we can apply a 4-bit binary mask such as "1100" to produce a binary string "0100".

A 4-bit mask can be seen as a set of four adjacent gates in which each gate either allows the corresponding bit of the 4-bit string to pass through (i.e. 1) or blocks it (i.e. 0). A detailed explanation is provided below.

4-bit binary input  : 0110
4-bit binary mask   : 1100
--------------------------
4-bit binary output : 0100
                      ^^^^
                      ||||
                      |||+--- mask bit of 0 blocks corresponding bit 0 of input,
                      |||     thereby resulting in output bit 0
                      |||
                      ||+---- mask bit of 0 blocks corresponding bit 1 of input,
                      ||      thereby resulting in output bit 0
                      ||
                      |+----- mask bit of 1 allows corresponding bit 1 of input,
                      |       thereby resulting in output bit 1
                      |
                      +------ mask bit of 1 allows corresponding bit 0 of input,
                              thereby resulting in output bit 0

The Task

Write a program mask.c that accepts a 4-bit binary input string followed by a 4-bit binary mask, and displays the computed 4-bit binary output string. The following mask.c program has been provided for you that reads in the two input. Your task is to simply extend the program to compute and print out the masked output.

/**********************************************************************/
/* Matric Number: U051234A and U059876B                               */
/* Userid: u0501234 and u0509876                                      */
/* (For those doing pair programming, only ONE of you should submit   */
/*  your program. The other student should NOT submit.)               */
/* Lab: 1                                                             */
/* Lab Group Number: 99                                               */
/* Lab TA's Name: Who needs a lab TA when you're Bill Gates?          */
/* Lab Session Date: 13 September 2006                                */
/* Lab Session Time: 0800 - 0945                                      */
/* Title: Bit Masking                                                 */
/* Purpose: Perform 4-bit masking.                                    */
/**********************************************************************/

#include <stdio.h>

int main()
{
    /* Variable declaration.
       You can declare any other variables here. */
    int input, mask;

    /* Get user input. */
    printf("Enter 4-bit binary  : ");
    scanf("%d", &input);

    printf("Enter 4-bit mask    : ");
    scanf("%d", &mask);

    /* Compute and print result here. */

    return 0;
}

Assuming that the executable is mask, a sample run of the program is shown below. User input is denoted in bold.

$ gcc -Wall mask.c -o mask
$ mask
Enter 4-bit binary  : 0110
Enter 4-bit mask    : 1100
--------------------------
4-bit binary output : 0100

$ mask
Enter 4-bit binary  : 1010
Enter 4-bit mask    : 0000
--------------------------
4-bit binary output : 0000

$ mask
Enter 4-bit binary  : 0101
Enter 4-bit mask    : 1111
--------------------------
4-bit binary output : 0101

$ mask
Enter 4-bit binary  : 0000
Enter 4-bit mask    : 0000
--------------------------
4-bit binary output : 0000

$ mask
Enter 4-bit binary  : 1111
Enter 4-bit mask    : 1111
--------------------------
4-bit binary output : 1111

$ mask
Enter 4-bit binary  : 0111
Enter 4-bit mask    : 0001
--------------------------
4-bit binary output : 0001

$

Points to Note

• For simplicity, we assume that user input will always be within the range of permissible values, so there is no need to perform any explicit checks on the data entered by the user.
• Do not use any other data types except int. That means you are not allowed to use double, float, char, etc. You are only allowed to use the int data type and nothing else.
• The only conversion specifier allowed within printf statements is strictly %i (or %d).
• Do not use any field-width specifiers with scanf or printf. E.g. You are not allowed to use scanf("%4d", &input); or printf("%04d", input);
• Do not use any logical (&&, ||, !) or bit-wise (&, |, ^, !, <<, >>) operators.
• Do not use any selection statements (if, if-else, switch), repetition statements (while, do-while, for), nor the conditional operator "?:".
• Do not use any functions from the math library.

A Question to Ponder

• Why do we use %d instead of %i in the scanf statements?

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