MIPS Functions and arrays - arrays

Question

I can understand and use Java/c++ to a good extent, but for the life of me assembly just confuses me there are 2 functions I'm having trouble with. First:
One function that receives a string and prints it on the terminal
And another one that receives a string and converts it to integers (Strings given all made of numbers).
Any idea on where to start?
Update
On the second function, so far I got this:
main:
atoi:
li $v0, 8
la $a0, tstr
li $a1, 64
syscall
sub $sp, $sp,4
sw $ra, 0($sp)
move $t0, $a0
li $v0, 0
next:
lb $t1, ($t0)
beqz $t1, endloop
mul $v0, $v0, 10
add $v0, $v0, $t1
sub $v0, $v0, 48
add $t0, $t0, 1
b next
endloop:
lw $ra, 0($sp)
add $sp, $sp, 4
Updated code, still getting the error on 10 being an invalid operand. And about sub $v0, $v0, 48 should I just do it as sub $t1, $t1, 48?

Answer 1

For input-output stuff, you have to use system calls. For writing (zero-terminated) strings you'll use syscall #4, which wants the address of the buffer in $a0. Now just place the umber of the syscall in $v0 and execute it. For example, this snippet reads a string:
li $v0, 8 # syscall number
la $a0, theString # buffer
li $a1, 64 # length of buffer
syscall # read!
Here you can find some syscalls numbers.
For the second exercise, here's the C++ code, try to translate it :P
int result = 0;
while (*digit >= '0' && *digit <='9') { // digit is char *
result = (result * 10) + (*digit - '0');
digit++;
}
EDIT:
Ok, there are a couple of errors. First, you're not checking whether you've reached the end of the string (simply compare $t1 with 0 at the beginning). And you should first subtract '0' from $t1, then add it to $v0.
next:
lb $t1, ($t0)
# are there other character or this is the last one?
mul $v0, $v0, 10
add $v0, $v0, $t1
sub $v0, $v0, 48 # subtract 48 only from $t1, not from whole result
add $t0, $t0, 1
b next