Question

A byte is made up of 8 bits although a byte is rarely represented as a string of binary digits or bits. Instead, it is typically presented as a pair of hexadecimal digits where each digit represents 4 bits or half a byte or "nibble." We have been reading in hex digits and outputting their decimal value, but what we really want to do is create a byte from a pair of hex digits read from input.

Create a C program that:

1. Reads a pair of hexadecimal characters from input. For example: 3B or CF.
2. Converts each of them into their decimal value. For example: 3 (0011 in binary) and 11 (1011 in binary)
3. Packs the two nibbles into a single byte. For example, 3 and B packed into an 8 bit byte would be 0011 1011.
4. Outputs the merged byte in hexadecimal to verify that the byte was packed correctly.

The printf() function supports outputting an integer as a hexadecimal number using the %x (where the hexadecimal letters are presented in lower-case) and %X (where the hexadecimal letters are presented in upper-case).

To pack two integers that have values represented with only 4 bits, you need to perform two operations with the integers:

1. Shift the integer holding the decimal value of the first hexadecimal digit in the pair to the left 4 bits.
2. OR the integer holding the decimal value of the second hexadecimal in the pair with the shifted integer.

After the shift and the OR, the first integer will not have the lower 8 bits (byte) set to the values represented by the two characters read from input.

Answer 1

// C program to convert a pair of hexadecimal characters to BYTE

#include <stdio.h>

#include <string.h>

int main()

{

    char hexa[2], byte[8] = "";

    int i,n=0,l;

    /* Enter a pair of hexadecimal digts from user */

    printf("Enter a pair of hexadecimal digits: ");

    x:gets(hexa);

    l=strlen(hexa);

    if (l>2)

    {printf(" Please enter only two hexadecimal digits");

    goto x;

     }

    /*loop to extract digit and find binary of each hex digit */

    for(i=0; hexa[i]!='\0'; i++)

    {

        switch(hexa[i])

        {

            case '0':

                strcat(byte, "0000");

                break;

            case '1':

                strcat(byte, "0001");

                break;

            case '2':

                strcat(byte, "0010");

                break;

            case '3':

                strcat(byte, "0011");

                break;

            case '4':

                strcat(byte, "0100");

                break;

            case '5':

                strcat(byte, "0101");

                break;

            case '6':

                strcat(byte, "0110");

                break;

            case '7':

                strcat(byte, "0111");

                break;

            case '8':

                strcat(byte, "1000");

                break;

            case '9':

                strcat(byte, "1001");

                break;

            case 'a':

            case 'A':

                strcat(byte, "1010");

                break;

            case 'b':

            case 'B':

                strcat(byte, "1011");

                break;

            case 'c':

            case 'C':

                strcat(byte, "1100");

                break;

            case 'd':

            case 'D':

                strcat(byte, "1101");

                break;

            case 'e':

            case 'E':

                strcat(byte, "1110");

                break;

            case 'f':

            case 'F':

                strcat(byte, "1111");

                break;

            default:

                n=1;    

}

    }

if(n==0)

    {

    printf("Hexademial Pair = %s\n", hexa);

    printf("Byte Conversion = %s", byte);

    }

else

    printf("Wrong hexadecimal input");

    return 0;

}