Chapter 9

Page 193

1. The compiler gives an error message:

    gcc -g -c -Wall -masm=intel inches2feet.c
    inches2feet.c: In function ‘main’:
    inches2feet.c:15:3: error: address of register variable ‘inches’ requested
      15 |   ptr = &inches;
          |   ^~~
    make: *** [makefile:12: inches2feet.o] Error 1
   

   We cannot take the address of a register variable. However, we can ask the compiler to use a register for the ptr variable:

    register int feet;
    register int inchesRem;
    int inches;
    register int *ptr;
   

2. Endianess.

    /* endian.c
    * Determines endianess. If endianess cannot be determined
    * from input value, defaults to "big endian"
    */
   
    #include <stdio.h>
   
    int main(void)
    {
        unsigned char *ptr;
        int x, i, bigEndian;
         
        ptr = (unsigned char *)&x;
         
        printf("Enter a non-zero integer: ");
        scanf("%i", &x);
         
        printf("You entered %#010x and it is stored\n", x);
        for (i = 0; i < 4; i++)
            printf("   %p: %02x\n", ptr + i, *(ptr + i));
   
        bigEndian = (*ptr == (unsigned char)(0xff & (x >> 24))) &&
                (*(ptr + 1) == (unsigned char)(0xff & (x >> 16))) &&
                (*(ptr + 2) == (unsigned char)(0xff & (x >> 8))) &&
                (*(ptr + 3) == (unsigned char)(0xff & x));
        if (bigEndian)
            printf("which is big endian.\n");
        else
            printf("which is little endian.\n");
   
        return 0;
    }
   

3. Endianess is property of CPU.

    /* endianReg.c
    * Stores user int in memory then copies to register var.
    * Use gdb to observe endianess.
    */
   
    #include <stdio.h>
   
    int main(void)
    {
        int x;
        register int y;
         
        printf("Enter an integer: ");
        scanf("%i", &x);
         
        y = x;
        printf("You entered %i\n", y);
   
        return 0;
    }
   

   Running this under gdb I got:

    (gdb) b 17
    Breakpoint 1 at 0x11d1: file endianReg.c, line 17.
    (gdb) r
    Starting program: /home/bob/GitHub/itco_x86-64/build/chapter_09/Your_Turn/endianess_CPU/endianReg 
    Enter an integer: 12345
   
    Breakpoint 1, main () at endianReg.c:17
    17          printf("You entered %i\n", y);
    (gdb) print &x
    $1 = (int *) 0x7fffffffdb04
    (gdb) print &y
    Address requested for identifier "y" which is in register $rbx
    (gdb) i r rbx
    rbx            0x3039              12345
    (gdb) x/4xb 0x7fffffffdb04
    0x7fffffffdb04: 0x39    0x30    0x00    0x00
    (gdb)
   

   This shows that the int is stored little endian in memory, but the individual bytes are loaded into the register (rbx) in the correct order.