GSP 215 Week 3 Homework Representing and Manipulating Information NEW

05 Jun GSP 215 Week 3 Homework Representing and Manipulating Information NEW

GSP 215 Week 3 Homework Assignment

 

Week 3 Homework—Representing and Manipulating Information

 

Structures are a mechanism for creating a data type to aggregate multiple objects into a single unit. All the components of a structure are stored in a contiguous region of memory and a pointer to a structure is the address of its first byte. To access the fields of a structure, the compiler generates code that adds the appropriate offset to the address of the structure.

The example on the book on page 242 shows the following structure.

struct rec {

            int i;

            int j;

            int a[3];

            int *p;

};

This structure contains four fields: two 4-byte int’s, an array consisting of three 4-byte int’s, and a 4-byte int pointer giving a total of 24 bytes. j is offset 4 bytes.

    0                          4                          8                     16                      20                      24

i

j

a[0]

a[1]

a[2]

p

 

Assuming variable 4 is of type struct rec * and is in register %edx, the following code copies element r->i to element r->j.

movl  (%edx), %eax    // Get r->i

movl %eax, 4(%edx) //Store r->j  To store into the field j, the code adds offset 4 to the address of r

Consider the following structure declaration containing a structure within a structure, and answer the following questions.

struct person{

            struct size{

            int height;

            int weight;

            }s;

            int *hp;

            int games[2];

}person1, person2;

1.      How many total bytes does the structure require?

height	weight	hp	games[0]	games[1]

 

2.      What are the offsets in bytes of the following fields?

s.height: ______________

      hp:_______________

   games[1]:______________

 

3.      The compiler generates the following assembly code for the body of str_init (shown below).

 

movq    16(%rbp), %rax     //Get p1 into register %rax

movl    4(%rax), %edx      //Get p1->s.weight store in register %edx

movq    16(%rbp), %rax     //Get p1 into register %rax

movl    %edx, (%rax)       //Store in p1->s.height

movq    16(%rbp), %rax     //Get p1 into register %rax

leaq    4(%rax), %rdx      //Compute address of p1->s.weight in register %rdx

movq    16(%rbp), %rax    //Get p1 into register %rax

movq    %rdx, 8(%rax)     //Store in p1->hp

 

4.      On the basis of this information, fill in the missing expressions in the code for str_init.

   void str_init(person *p1)

   {

      p1->s.height = _____________;

      p1->hp = _________________;

}

5.      How would you call str_init with the structperson1 passed to it?