Lecture 30. Passing Arguments to a Subroutine 

At last example to spot error Result stored from first add instruction into r3 which is overwritten in second add instruction.

In the and ( question ) , there is a another mistake. Isn't 'add r3 , r0, r2' should be like that > 'add r3, r3, r2' ?

There's no 128-bits types in AAPCS. So for the return value, it only uses R0 Or R0R1.