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• Clicker Questions - 02-C-Stuff-2

Clicker Questions - 02-C-Stuff-2

Please refer to the following code for all question. Assume that the machine is little endian.

#include <stdio.h>
#include <stdlib.h>

int main() {
    unsigned int p[2];
    unsigned int x;
    unsigned char *cp;
    unsigned char c;

    p[0] = 0x28f4a13e;
    p[1] = 0xbc70695d;
    cp = (unsigned char *) p;

    c = cp[0];
    printf("0%02x\n", c);

    c = cp[6];
    printf("0%02x\n", c);

    c = (p[0] >> 8);
    printf("0%02x\n", c);

    x = ((p[0] >> 16) | (p[1] << 16));
    printf("0%08x\n", x);

    return 0;
}

Question 1: What is the first line of output?
Choices are:

A. 0x28
B. 0x28f4
C. 0x28f4a13e
D. 0x3e
E. 0x3ea1f428
F. 0x5d
G. 0x82
H. 0xa13e
I. 0xbc
J. 0x13e

Answer

You can cut/paste/compile/run to confirm these answers.

Take a look at the following diagram, which shows p and cp. The ordering of cp is because the machine is little endian:

           cp[3]  cp[2]  cp[1]  cp[0]
p[0]:    0x   28     f4     a1     3e

           cp[7]  cp[6]  cp[5]  cp[4]
p[1]:    0x   bc     70     69     5d

Question 1: cp[0] is 0x3e: D

Question 2: What is the second line of output?
Choices are:

A. 0x07
B. 0x5d
C. 0x69
D. 0x695d
E. 0x70
F. 0x96
G. 0xbc
H. 0xbc70
I. 0xbc70695d
J. 0xd59607cb

Answer

You can cut/paste/compile/run to confirm these answers.

Take a look at the following diagram, which shows p and cp. The ordering of cp is because the machine is little endian:

           cp[3]  cp[2]  cp[1]  cp[0]
p[0]:    0x   28     f4     a1     3e

           cp[7]  cp[6]  cp[5]  cp[4]
p[1]:    0x   bc     70     69     5d

Question 2: cp[6] is 0x70: E

Question 3: What is the third line of output?
Choices are:

A. 0x13
B. 0x28
C. 0x28f4
D. 0x28f4a13e
E. 0x28f4a1
F. 0x3e
G. 0xa13e
H. 0xa1
I. 0xf4
J. 0xf4a13e

Answer

Now, (p[0] >> 8) will shift the least significant byte of p[0] off, and you are left with 0x28f4a1. When we store it as a byte, it takes the least significant byte, and jettisons the rest:

Question 3: c is 0xa1: H

Question 4: What is the last line of output?
Choices are:

A. 0x28f4
B. 0x28f4695d
C. 0x28f4bc70
D. 0x695d
E. 0x695d28f4
F. 0xa13e
G. 0xa13e695d
H. 0xa13ebc70
I. 0xbc70
J. 0xbc70a13e

Answer

For question 4, (p[0] >> 16) will shift the least significant two bytes of p[0] off: 0x28f4.

(p[1] << 16) will shift the least significant two bytes into the most significant positions, and put zeros in their place: 0x695d0000. When you OR them together, you get the answer:

Question 4: x is 0x695d28f4: E

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