Nios II/first implements a data cache memory unlike Nios II/standard and Nios II/economy.

Since the write policy of the data cache uses the write-back method, data consistency may not be obtained between the main memory and the cache memory. At this time, DMA transfer or access to shared memory such as dual port memory may lead to malfunction.

You can either write the data back to main memory or change the software to bypass the cache before performing these operations.