/*  Add two unsigned 64 bit

    Input:
        r1, r2: A.high, A.low
        r3, r4: B.high, B.low

    Output:
        r5, r6: C.high, C.low
*/
pub u64.add:
    add r5, r1, r3
    add r6, r2, r4
    cmp r6, r2
    jae u64.add_no_low_overflow
    add r5, r5, 1 ; overflow

    pub u64.add_no_low_overflow:
    ret