How to get ADC-data into on-chip memory?

Hello,

i am trying to get my adc's parallel data into my Nios II. The first try resulted in using a PIO to read the data from a register outside the system, triggered by an interrupt. Though this solution worked well, it is very slow (my adc is generating data with a frequency of 4MHz and reading PIO registers and writing it to on-chip memory takes some time). So my second guess is to write the adc data directly into the on-chip memory (something like shared memory). But I don't have a clue how to do this. Can anyone of you push me in the right direction?

Thanks in advance,

Nightfall