Question

I’m a newbie with MIPS & MARS. As a part of my program, I want to read a string from the user.

I have a simple code as follows

.globl  test
.data   0x10010000
foo: .asciiz "Input a string"
.data   0x10020000
in: .asciiz "xyz"
.text   0x00400000

test:
li  $v0, 54
add $a0, $zero, 0x1001
add $a1, $zero, 0x1002
add $a2, $zero, 3
syscall

The idea is to read a string of say length at most 3 into memory at 0x1002. Running the code gives me “Runtime exception at 0x00400010: address out of range 0x00001001”.

Why does this happen? How do I fix it? I suspect I’m doing something very silly, but cannot figure it out.

Answer 1

I think you’re mixed up with addressing. You’ve defined your buffer space at 0x10020000 but you’re using an address of 0x1002 for the syscall. Same thing with the dialog string.

Your $a0 should be the address of the string that gets shown in the dialog; I suspect this should be foo:

la $a0, foo

And your buffer should be in:

la $a1, in

Edit: these are replacements for the adds, so in the end it should look like:

test:
li  $v0, 54
la  $a0, foo
la  $a1, in
li  $a2, 3
syscall

I’ve also replaced the $a2 line with something simpler. Try to see how they’re equivalent.