Mips - print_string syscall doesn't work in -bare mode

Question

This code is part of a much bigger program that worked just fine. Then I realized the assignment would be tested in -bare mode. As soon as I tried running it in -bare mode my print_string syscalls went from working fine to not printing anything and I have no idea why.

Running this gives me no errors it just doesn't print anything

(also are the 2 or $0,$0,$0 necessary at the end?)

.globl main

.data
v: .space 2
w: .space 2
x: .space 2
y: .space 2
z: .space 2
inputPrompt: .asciiz "Input mathmatical expression: "

.text
main:   #main block used to read in all the necessary data

    lui $a0, 0x1001

    #prompt for expression
    addi $v0, $0, 4
    addi $a0, $a0, 10
    syscall
    or $0,$0,$0
    jr $ra
    or $0,$0,$0

score 2 · Accepted Answer · answered Oct 21 '15 at 05:52

This is the problem:

lui $a0, 0x1001

If you look at the addresses in the DATA/STACK viewer in SPIM, you'll see that when you're in bare mode the DATA section starts at 0x10000000 instead of 0x10010000. So you should change that lui to lui $a0, 0x1000.

are the 2 or $0,$0,$0 necessary

Bare mode implies that branch delay slots are simulated. So you should fill any branch delay slots with instructions that are safe to execute there. That could be NOPs, or more meaningful instructions if you manage to reorganize your code. For example, if you had:

ori $a0,$0,1
jal foo

you could've changed that into:

jal foo
ori $a0,$0,1  # will be executed in the delay slot

Note that syscall doesn't have any delay slot. From MIPS32™ Architecture For Programmers Volume II: The MIPS32™ Instruction Set:

Format: SYSCALL

Description:
A system call exception occurs, immediately and unconditionally transferring control to the exception handler

awesome, simple mistake and easy to fix in my program. Thanks — Christopher Dominessy, Oct 21 '15 at 11:57

Mips - print_string syscall doesn't work in -bare mode

1 Answers1