Performance Issues

Unfortunately you are.

all of these profibuschips have both memory and registers with a single chipselect. the data to and from the profibus is stored inside the chip so this chip-memory area must be a memory inside the nios datamaster that the software can use 8/16/32 Bit access and even memcopy(move) operations without takeing care about that this device is a 8 bit device. (all available softwarestacks need this and we do not want to re-write such an stack an certify it) on the other hand with the same chipselect you gain access to the registers and some of them monitor a read access, so if you want to read a 8 bit register, but executed is 4 read cycles, then you do in real life a read from 3 registers that are not accessed by the software.

the crazy thing is that the byteenables indicate which byte is requested. so the only workaround is to insert a statemachine between avalon and the profibuschip that fetches only these byte that are requested by the software. this works fine here.

so for the software it is neccesary to have a memory modul instead of a register modul to get rid of the gaps in the memory. but no additional (wasted) read cycles.