Read file character by character using MIPS in MARS IDE

Question

How can I loop until the end of file without hardcoding the the value of $t0

        addi $t0, $t0,1000

loop:   beq $t0, $0, loopend
        #this code reads in a character from the file
        jal readchar


#print the character to the console
        li $v0,11
        move $a0,$t1
        syscall

        addi $t0,$t0,-1
        j loop
loopend:

readchar:   li $v0,14
            move $a0,$s6
            la $a1,inchar
            li $a2,1
            syscall
            lb $t1,inchar
            jr $ra

Because when I use $a2 it doesn't work, but it says here http://courses.missouristate.edu/kenvollmar/mars/help/syscallhelp.html

that read from file

li $v0,14
$a0 = file descriptor

$a1 = address of input buffer

$a2 = maximum number of characters to read $v0 contains number of characters

And when I use this as substitute for the hard coded ($ addi $t0, $t0,1000) $a2 is still 0

li $v0,14
move $t0,$a2
sycall

Thank you!

Answer 1

I'm not sure what purpose moving $a2 to $t0 before the system call would serve. $a2 is an argument to the system call where you specify the maximum number of characters to read. The return value - ie the number of characters actually read - is available only after the system call, and will be in register $v0 .

So if you put 1 in $a2 before the system call and $v0 contains zero after the system call, then you've probably reached the end of the file.