Using alt_flash_open_dev with 'enable_reduced_device_drivers'

You probably won't manage to (sensibly) write to flash unless you just use the low level functions.

IIRC the higher level ones (probably still in the middle of the stack you are trying to use) will do a sector erase before every write - which means that you'll need a sector sized buffer (probably 64k or larger) to do writes.

The flash itself (probably) allows single word writes provided that they only change 1s to 0s.

The other fubar is that the flash 'block' always contains the boot code - even if you don't need it.

I also suspect that you can directly bit-bang the (EPCS) flash from the nios almost as fast as the hadware block accesses it.