Mips Overflow Detection , Printing the Result - overflow

Question

i wrote down this simple code just to verify the overflow in Mips Processor for my final test:
.data
.text
.globl main
main:
li $s0, 2000000000
li $s1, 2000000000
addu, $s2 , $s0, $s1
li $v0, 1
move $a0, $s2
syscall
j end
end:
li $v0, 10
syscall
it of course result in 0xee6b2800, in decimal -294967296, just because mips encode numbers in two's complement and it recognizes the MSB as the Sign bit( 1, and its negative).
How can i implement a loop for printing ( in the same program) the right value? ( In this case 4 Bil, 4x10^9)
Thank you very much

Answer 1

Detecting overflow is not difficult if you know both the operands and the result:
li $s0, 2000000000
li $s1, 2000000000
addu $s2 , $s0, $s1
xor $t0, $s0, $s2 # $t0 = op1 ^ result
xor $t1, $s1, $s2 # $t1 = op2 ^ result
and $t0, $t0, $t1 # $t0 = (op1 ^ result) & (op2 ^ result)
bltz $t0, overflow
Signed overflow occurs when adding two numbers of the same sign results in a number with the opposite sign.
For 32-bit numbers, if ((op1 ^ result) & 0x80000000) == 0x80000000), then op1 and result have opposite signs.
Thus, if (((op1 ^ result) & (op2 ^ result)) & 0x80000000) == 0x80000000), then that must mean that op1 and result have opposite signs, AND op2 and result also have opposite signs, therefore op1 and op2 must have the same sign. Which is exactly what we wanted to check. The bltz is just a simpler way of checking if the most significant bit (0x80000000) is set.
To print the value as an unsigned integer you probably have to write your own int-to-string conversion routine, and then print the result string using the print_string system call. That's a fairly simple operation, and you should be able to find examples if you do some searching (perhaps not for MIPS specifically, but the algorithm is the same regardless of the instruction set).