Computer Organization and Structure


Homework #2

Due: 2004/11/2


1.      What binary number does this hexadecimal number represent: 7fff fffahex? What hexadecimal number does this binary number represent: 1100 1010 1111 1110 1111 1010 1100 1110two? What decimal number do they represent, respectively?


2.      Implement the following C code in MIPS, assuming that set_arry is the first function called:


int i;

void set_array(int num) {

      int array[10];

      for (i=0; i<10; i++) {

           array[i] = compare(num, i);



int compare(int a, int b) {

      if (sub(a, b) >= 0)

           return 1;


           return 0;


int sub (int a, int b) {

      return a-b;



Be sure to handle the stack and frame pointers appropriately. The variable code font is allocated on the stack, and i corresponds to $s0. Draw the status of the stack before calling set_array and during each function call. Indicate the names of registers and variables stored on the stack and mark the location of $sp and $fp.


3.      Add comments to the following MIPS code and describe in one sentence what it computes. Assume that $a0 and $a1 are used for the input and both initially contain the integers a and b, respectively. Assume that $v0 is used for the output.


add   $t0, $zero, $zero

loop:    beq   $a1, $zero, finish

         add   $t0, $t0, $a0

         addi  $a1, $a1, -1

         j     loop

finish:  addi  $t0, $t0, 100

         add   $v0, $t0, $zero


4.      The following code fragment processes two arrays and produces an important value in register $v0. Assume that each array consists of 2500 words indexed 0 through 2499, that the base addresses of the arrays are stored in $a0 and $a1 respectively, and their sizes (2500) are stored in $a2 and $a3, respectively. Add comments to the code and describe in one sentence what this code does. Specifically, what will be returned in $v0?


sll   $a2, $a2, 2

sll   $a3, $a3, 2

add   $v0, $zero, $zero

add   $t0, $zero, $zero

outer:   add   $t4, $a0, $t0

         lw    $t4, 0($t4)

         add   $t1, $zero, $zero

inner:   add   $t3, $a1, $t1

         lw    $t3, 0($t3)

         bne   $t3, $t4, skip

         addi  $v0, $v0, 1

skip:    addi  $t1, $t1, 4

         bne   $t1, $a3, inner

         addi  $t0, $t0, 4

         bne   $t0, $a2, outer


5.      Show the single MIPS instruction or minimal sequence of instructions for this C statement:


x [10] = x [11] + c;


Assume that c corresponds to register $t0 and the array x has a base address of 4,000,000ten.


6.      Write a procedure, itoa, in MIPS assembly language that will convert an integer argument into an ASCII decimal string. The procedure should take two arguments: the first is an integer in register $a0; the second is the address at which to write a result string in register $a1. Then itoa should convert its first argument to a null-terminated decimal ASCII string and store that string at the given result location. The return value from itoa, in register $v0, should be a count of the number of non-null characters stored at the destination.