C to MIPS Decimal to Binary Converter Using Recursion - c

Question

I am new to MIPS and I wrote a basic format of what I think the code from the C file I wrote is the equivalent to of the MIPS.
My assignment is to convert the following C file which I wrote into a direct translation of what the MIPS is supposed to be. My current C code is :
#include <stdio.h>
int d2b(int d)
{
if(d == 0)
{
return;
}
else
{
return (d %2 + 10 * d2b(d/2));
}
}
int main()
{
int d = 99;
int b;
b = d2b(d);
printf("Input => %d \n", d);
printf("Output => %d ", b);
return;
}
So far I have the following :
.data
msg1 .asciiz “Number is “
msg2 .asciiz “\nConverted to \n“
.text
.globl main
main:
li $v0, 4
la $a0, msg1
syscall
li $v0, 5 #Exit syscall
syscall
add $a0, $v0, $zero
jal fact
add $a0, $v0, $zero
li $v0, 1
syscall
li $v0, 10
la $a0, msg2
syscall
fact:
li $t0 0 #load 0
beq $a0, $t0, skip #test n
li $v0 0
jr $ra
skip:
subu $sp, $sp, 32
sw $ra 20($sp)
sw $fp, 16($sp)
addiu $fp, $sp, 28
sw $a0, 0($fp) #save n
li $t1 2 #load 2
divu $a0 $t1 #n / 2
mfhi $t2 #remainder
mflo $t3 #quotient
move $a0, $t3 #n = quotient
addi $v0, $a1, 10
jal fact
lw $a0, 0($fp) #restore n
multu $v0, $a0
lw $ra, 20($sp)
lw $fp, 16($sp)
addiu $sp, $sp, 32
jr $ra
My main problem is not knowing how to use syscall and not really understand a recursive function in MIPS. Please point out my mistakes and errors!

Answer 1

your MIPS syscalls are in this section
li $v0, 4
la $a0, msg1
syscall
li $v0, 5 #Exit syscall
syscall
add $a0, $v0, $zero
jal fact
add $a0, $v0, $zero
li $v0, 1
syscall
li $v0, 10
la $a0, msg2
syscall
They are commented incorrectly
In a MIPS syscall $v0 holds the "syscall function" or in English, the service you want the operating system to perform. There's a table of them here.
$a0 will hold the first parameter passed to the call. To set this parameter, one technique is to add the input value to zero storing the result into $a0 That's why you have so many lines like this
add $a0, $v0, $zero # this adds $v0 to the number zero and storing in $a0
Finally, the syscalls you are using are (4 => print String, 5 => read integer, 1 => print integer, and 10 => exit)
So a properly commented example of your code would be
la $a0, msg1 # load string as parameter
li $v0, 4 # load operation "print string"
syscall # request "print string" for msg1
li $v0, 5 # load operation "read integer"
syscall # request "read integer"
add $a0, $v0, $zero # load the read integer into $a0
jal fact
add $a0, $v0, $zero # load the value of $v0 into $a0
li $v0, 1 # load operation "print integer"
syscall # request "print integer"
As you can see, my confusion comes not from your ability to use syscalls, but from your description of what you think you are using the syscalls to do.
You state you want to print a binary number like 01001010 from a decimal input. This typically involves breaking the decimal number down, in a loop, printing out a zero or one in each of the binary number places. As this would require a loop for each placeholder in the binary number, it doesn't seem that a single call to "print integer" would be possible (unless the input was limited to only the decimal '1' and '0', or the input is limited to such a small number that it's binary representation, represented in decimal format is less than max_int).
So for an input of 5, the desired output would be 101, and that would be 3 calls to print, in the order of '1', '0', '1'. I believe this approach of printing the digits in a loop will give you greater success, and will permit you to print every positive decimal number inputted.
In short, I think your command of syscall is fine, but you're still struggling with how to do loops and solve problems in assembly. Try to figure out how you would determine the digits in the needed order by hand, using a pencil and paper, and then attempt to code that into your program.