Question

1. Write a procedure, bfind( ), in C code using pointer-based and convert it into MIPS assembly language with clear and necessary comments. The procedure should take a single argument that is a pointer to a null-terminated string in register $a0. The bfind procedure should locate the first character b in the string and return its address. If there are no b’s in the string, then bfind should return a pointer to the null character at the end of the string. For example, if the argument to bfind points to the string "imbibe," then the return value will be a pointer to the third character of the string. Use minimum number of MIPS instructions.

C code:

char *bfind(const char *str)

{

char *p;

for(p=str;*p;p++)

   {

    if(*p==’b’)

      return p;  

            }

return p;

}

Answer 1

PROGRAM:

.data
   string: .asciiz "imbibe"
   output: .asciiz "Character at the returned address is: "
.text
.globl main
main:
   la $a0,string       # Load the address to string to pass as argument to subroutine

   jal bfind       # Jump to subroutine

   move $t0,$v0       # Store the return value
  
   li $v0,4       # 4 is the print string syscall
   la $a0,output       # Load the address of string to be printed
   syscall           # Print the string
   li $v0,11       # 11 is print character syscall
   lb $a0,0($t0)       # load $a0 with the character to be printed
   syscall           # Print the character
  
   li $v0,10       # 10 is the exit program syscall
   syscall           # Exit the program
  
# Definition of bfind subroutine
bfind:
   # Adjust the stack pointer
   addi $sp,$sp,-8
   sw $s1,4($sp)       # Store $s1 on stack
   sw $ra,0($sp)       # Store return address on stack
  
loop:   lb $t0,0($a0)       # Load a byte from string
   beq $t0,98,return   # If byte is equal to character b, return the address
   beqz $t0,return       # If byte is equal to null return the address
   addi $a0,$a0,1       # Increment the string pointer
   b loop           # Repeat the loop
  
return:   move $v0,$a0       # Load the address to be returned into $v0
   lw $s1,4($sp)       # Restore $s1 from stack
   lw $ra,0($sp)       # Restore $ra from stack
   addi $sp,$sp,-8       # Readjust stack pointer to its original position
   jr $ra           # Jump to return address (i.e., to calling function, main)

Please refer to the following screenshot of the program for indentation of the code:

OUTPUT: