Nios II exception handler

You won't find an easy solution to that one. Some options:

1) Add software to verify that x[3] doesn't change.

2) Use a mmu and make x[2] be the end of a memory page, and make sure the following page is never allocated.

3) Use on-chip debugger facilities to trap on writes to specific locations (only a few traps will ever be supported, not sure the nios has any).

4) Use signaltap to detect writes to x[3].

5) Wrap all the array accesses in functions and verifu=y the index.

6) Write the code carefully!