Store -A

The sign bit is shifted right 31 bit positions. This puts it in the low-order bit of the destination register ($9 in this case). To test if $9 is zero, use branch-on-equal with register $0 (which is always zero).

## absVal.asm
##
## Calculate the absolute value of A

      .text
      .globl  main

main: 
# Get A
        lui   $10,0x1000     #  Init base register
        lw    $8,0($10)      #  Load A
        sll   $0,$0,0        #  no-op

# Is A negative?
        srl   $9,$8,31       #  Shift sign bit to position 0
        beq   $0,$9,done     #  sign bit == zero, done
        sll   $0,$0,0

# Store -A
        sub   $8,$___,$8     #  negate A
        sw    $8,___($___)   #  save it

done:   sll   $0,$0,0

      .data
A:    .word   -1

## End of file

Now calculate -A and store it back into word "A". The instruction at done is a no-op.