a$.$ There are many ways to compute Hamming weight. We could test the most significant bit $($bit $31)$ of s$0.$ For example, extract bit $31$ with an AND instruction, and compare it with $0.$ This is similar to the method in the code given. However, we can save one instruction. If we treat s$0$ as a $2$s complement number, s$0$ is less than $0$ if and only if bit $31$ in s$0$ is $1.($In other words, instead of extracting MSB bit by making a mask first and then extract, it would be faster to directly check whether it is a positive number or a negative number.$)*$Typically$,$ the lowest$-$order bit is called "bit zero".

Using this method, write RISC$-$V instructions to compute the Hamming weight of s$0.$ Explain your method in comments. We can start with the following two instructions. How many instructions are executed if s$0$ is $0$xFF$00$FF$00?$

addi s$1,$ x$0,0$ # s$1=0$

add t$0,$ x$0,$ s$0$ # make a copy so s$0$ is not changed