For the input x= y= 1073741824 (2^30)
programme addu.asm calculate the sum of x and y as -2147483648
while  programme add.asm calculate throw an overflow exception

the two programmes behave differently because 2^30 is 
0b0100 0000 0000 0000 0000 0000 0000 0000,
thus, the sum of x and y is 0b1000 0000 0000 0000 0000 0000 0000 0000.
As a result, the fist bit (sign bit) is changed. 

so, if we do signed addition, an overflow will occur and if we do unsigned addtion, in which case the change of value of the sign bit is not concerned, a negtive number will be given as the result.