absolute value in MIPS

Question

Do you have any simple ways to make a value in a register in MIPS as an absolute value?

Answer 1

The easiest way would just to do a bit of binary math on the values.

http://en.wikipedia.org/wiki/Signed_number_representations describes how various systems store their negative numbers. I believe MIPS uses a two's complement scheme to store signed numbers. This makes it a bit harder than a bit flag, which could just be turned off by ANDing the number with 0b01111111, but it is still doable.

Answer 2

Here is a pretty simple way to do it.

#assume you want the absolute value of r1
        ori $2, $zero, $1      #copy r1 into r2
        slt $3, $1, $zero      #is value < 0 ?
        beq $3, $zero, foobar  #if r1 is positive, skip next inst
        sub $2, $zero, $1      #r2 = 0 - r1
foobar:
#r2 now contains the absolute value of r1