hi.. can someone help me with this question

Write down a sequence of instructions that would implement the synthetic instruction addd $t0,$t2 to produce a double precision sum (i.e. 64-bit sum) of two unsigned 64-bit integers. The first 64-bit integer is in the register pair $t0 and $t1, while the second 64-bit integer is in the register pair $t2 and $t3 (the even numbered register in each pair contains the most significant bits). The result should be left in the $t0,$t1 register pair. Your solution should not modify any registers other than $t0 and $t1 ( and $1 if needed). Nor should your solution involve any type of branch or jump instruction.

I understand that synthetic instructions are like

add $t1,$t0,$a1

since the question is asking for sum of two unsigned 64 bit integers.. what does that mean? aren't unsigned integers 32 bit?

thanks