CSSE232 : Computer Architecture I

Name: Box: Date:

HW8 solution

In the following assembly code fragment, registers $t1 and $t2 are initialized to N and zero respectively.
```
LOOP: slt     $t3, $0, $t1
      beq     $t3, $0, DONE
      addi    $t1, $t1, -1
      addi    $t2, $t2, 3
      j       LOOP
DONE:    
```
a. (2 points) What is the value in register $t2 for when the code reaches the label DONE:?
$$ 3N$$
b. (2 points) How many MIPS instructions are executed?
$$ 5N + 2$$
c. (6 points) If the registers $t1, $t2, and $t3 correspond to variables i, a, and t respectively, write the equivalent C code for the loop. Note: you probably won't end up with a t variable in your answer.
```
a = 0;
i = N;
while(0 < i) { //same as i >= 0
    i--;
    a = a + 3;
}
```
or
```
a = 0;
for( i=N; 0<i; i-- )
    a += 3;
```

(20 points) Below is a snippet from a Checkers game program in C.

void setKing(int row, int col, int isKing)
{
  int* rowAddr = getRowAddr(row); //returns the memory address of row
  rowAddr[col] = isKing;
}

Fill in the missing portions of the setKing function below, adhering to the MIPS procedure call conventions. You should assume setKing is called according to MIPS convention.

setKing:   #entry point to setKing procedure

addi $sp, $sp, -12   #allocate space on stack
sw $a1, 0($sp)       #backup needed arg values, since they
sw $a2, 4($sp)       # would be lost after jal
sw $ra, 8($sp)       #backup return address, since we do jal

jal     getRowAddr  #call to getRowAddr

lw $ra, 8($sp)       #restore values from stack
lw $a2, 4($sp)
lw $a1, 0($sp)
addi $sp, $sp, 12    #deallocate space on stack

sll $a1, $a1, 2      #multiply col by 4 to get ready for address
add $v0, $a1, $v0    #build address from base and col offset
sw $a2, 0($v0)       #store king value at row+col location

jr    $ra   #return form setKing