128 bit Avalon read/write

In the code snippet you posted, it does not appear your are checking for the Avalon "write" signal. Your code would attempt to store a word on each clock cycle even when there is no Avalon write taking place.

Personally, I would probably use 2 address bits to split the 128 bit write into four separate 32 bit writes at consecutive word addresses. Then you don't need to keep track of an index manually as in the code you posted.