bitwise operations, C (gcc)

bitwise operations

/*Simple program showing how to read an integer and printing it to output in different formats.*/

#include "stdio.h"

// write a function that gets an integer and a mask and returns the result  of using the mask to reset some bits to 0
int ResetBits(int a, int mask) {
  return (a & mask);
}

//converts decimal value to a string that represents the binary number
//note that decimal is already represented in binary in the memory
//all we need is an array of characters 0 and 1 that show the number in binary
int DecimalToBinary(int decimal, char *binary) {

unsigned int temp = decimal;
    int i;
    for (i=0; i<32; i++) {
    
       //ASCII code for '0' is 48 and for '1' is 49
       
       //the bit we calculate in this loop is Least Significant Bit (LSB), so the index is 31-i
       binary[31-i] = 48+ temp % 2;
       
       //dividing by 2, simply shifts the number to right and allows us to process the next bit 
       temp = temp / 2;
       }
       
    //an end is put at the end of binary string. Again this is not a number, but an array of '0's and '1's   
    binary [32] = 0;
 }

int main(void) 
{
   int a;

printf("Please input an integer value: ");
   scanf("%x", &a);        //reading integer from standard input: "%d" will look for a decimal, &a gives it the address of a-puts input in a
   printf("\nDecimal: %d\n", a);
   printf("Hex: 0x%x\n", a);
   
   char binary[33];
   DecimalToBinary(a, binary);
   printf ("Binary: %s\n", binary);
   
   //gives the least significant byte of a as an integer or char
   //a = 0xabcd1234   output = 0x34
   //a = -1 = 0xffffffff    output = 0xff
   //imagine a = 123 and I am asking for lowest digit
   int temp;
   temp = a % 256;        //use 256 since you want to divide by 2^8 to get remainder
   printf("Least significant byte: Ox%x\n" ,temp);
   
   
   //give four bits starting from bit position 3 in integer form
   temp = a >> 3;                //shift to the right 3 times so that bit position 3 is in the lsb
   temp = temp & 0xf;    //this number is called mask-used to isolate the bit positions you want
   //or could do temp = (a >> 3) & 0xf;
   printf("Four bits from position 3: %x\n", temp);
   
   //& operation to force some bits to 0
   //the mask that you need has 0s on those bits and 1s for the rest
   
   //set the bits 4, 5, and 6 to 1 and leave the rest of the bits unchanged
   temp = a | 0x70;
   DecimalToBinary(temp, binary);
   printf("Setting bits 4-6: %s\n", binary);
   
   
   
   return 0;
}


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