Question

Write MIPS code that converts a string containing a number into an actual number. The function to do this conversion, atoi (Array To Integer) should have a single input (the string) and output a single number (the integer). The Java/C code is as follows:

atoi(a0: array) {

int digit, number, i = 0;

while( (array[i] >= '0') && (array[i] <= '9') )

{ digit = array[i] - '0';

number = 10 * number + digit;

i++;

}

return number;

}

Answer 1

Please find the code below:::

MIPS

.globl main
.data
prompt: .asciiz "Enter the number : "
prompt2: .asciiz "Converted number is : "
prompt3: .asciiz "Error.... Non numeric character found!!! "

input : .space 30
.text   
main:
li $v0,4
la $a0,prompt #it will print prompt
syscall
li $v0,8 #get string
la $a0,input
li $a1, 30
syscall

jal str2int # call the procedure str2int
move $s0,$v0 # move the return value into $s0
li $v0,4
la $a0,prompt2 #it will print prompt
syscall
move $a0,$s0
li $v0,1
syscall # print the result

li $v0, 10
syscall # Exit the program

# register $v0(return value)   

str2int:
# Check for sign
lb $t0,0($a0) # load the first byte into $t1
beq $t0,'-',negint # if sign is -,goto negint
beq $t0,'+',posint # if sign is +,goto posint
j convert
negint: li $t1,1 # set flag $t1 to 1(represents negative)
add $a0,$a0,1 # goto next ASCII character
j convert # goto convert
posint: li $t1,0 # set flag $t1 to 0(represents positive)
add $a0,$a0,1 # goto next ASCII character
convert:
li $s0,0 # sum=0
loop:
lb $t0,0($a0) # load the ASCII character into $t1
beqz $t0,exitloop # if the character is null, exit the loop
beq $t0,10,exitloop #if end of string space came
blt $t0,'0',fail # if $t1 is a non-digit character,return -1
bgt $t0,'9',fail
sub $t0,$t0,48 # convert ASCII digit to decimal digit
mul $s0,$s0,10 # multiply the previous sum with 10 and
add $s0,$s0,$t0 # the converted digit to sum
add $a0,$a0,1 # goto next ASCII character
j loop # goto loop
exitloop:
beq $t1,0,copy # if sign is +, goto copy
neg $s0,$s0 # if the sign is -, take negation of integer
copy: move $v0,$s0 # store the converted integer into $v0
j return
fail: li $v0,-1

li $v0,4
la $a0,prompt3 #it will print prompt
syscall
li $v0,10
syscall #exit

return: jr $ra # return $v0 to main

output: