Caches are classified as either write$-$through or write$-$back. In a writethrough cache, the data written to a cache block is simultaneously written to main memory. In a write$-$back cache, a dirty bit $($D$)$ is associated with each cache block. D is $1$ when the cache block has been written and $0$ otherwise. Dirty cache blocks are written back to main memory only when they are evicted from the cache. A write$-$through cache requires no dirty bit but usually requires more main memory writes than a write$-$back cache. Modern caches are usually write$-$back because main memory access time is so large. Suppose a cache has a block size of four words. How many main memory accesses are required by the following code when using each write policy: write through or write$-$back?

addi t$5,$ zero, $0$

sw t$1,0($t$5)$

sw t$2,12($t$5)$

sw t$3,8($t$5)$

sw t$4,4($t$5)$