Chapter 15

1. No answer required.
2. Sum integers

   // Add two unsigned integers and show if there is carry.
   #include <stdio.h>
   #include "add_two.h"
   
   int main(void)
   {
       unsigned int x, y, z, carry;
         
       printf("Enter an integer: ");
       scanf("%u", &x);
       printf("Enter an integer: ");
       scanf("%u", &y);
       carry = add_two(&z, x, y);
       printf("%u + %u = %u\n", x, y, z);
       if (carry)
           printf("** Carry occurred **\n");
   
       return 0;
   }
   

   // Add two unsigned ntegers and return 1 for carry, 0 for no carry.
   
   #ifndef ADD_TWO_H
   #define ADD_TWO_H
   unsigned int add_two(unsigned int *a, unsigned int b, unsigned int c);
   #endif
   

   // Add two unsigned integers and return 1 for carry, 0 for no carry.
   
   #include "add_two.h"
   
   unsigned int add_two(unsigned int *a, unsigned int b, unsigned int c)
   {
       unsigned int sum;
       unsigned int carry = 0;   // Assume no carry
         
       sum = b + c;
       if ((sum < b) || (sum < c)) {   // sum wraps if carry
           carry = 1;
       }
       *a = sum;
   
       return carry;
   }
   

3. Inline assembly to detect carry.

   // Add two integers and show if there is carry.
   
   #include <stdio.h>
   
   int main(void)
   {
       unsigned int x, y, z, carry;
         
       printf("Enter an integer: ");
       scanf("%u", &x);
       printf("Enter an integer: ");
       scanf("%u", &y);
   
       asm ("adds %w0, %w2, %w3\n"
           "cinc %w1, wzr, cs"
           : "=r" (z), "=r" (carry)
           : "r" (x), "r" (y));
   
       printf("%u + %u = %u\n", x, y, z);
       if (carry)
           printf("** Carry occurred **\n");
   
       return 0;
   }
   

4. Registers for variables.

   // Add two integers and show if there is overflow.
   
   #include <stdio.h>
   
   int main(void)
   {
       int x, y;
       register int z, overflow;
         
       printf("Enter an integer: ");
       scanf("%i", &x);
       printf("Enter an integer: ");
       scanf("%i", &y);
   
       asm ("adds %w0, %w2, %w3\n"
           "cinc %w1, wzr, vs"
           : "=r" (z), "=r" (overflow)
           : "r" (x), "r" (y));
   
       printf("%i + %i = %i\n", x, y, z);
       if (overflow)
           printf("** Overflow occurred **\n");
   
       return 0;
   }