Need help : implement Gigabit Ethernet

hi

i began from zero to do my own architecture

i've successfully write on a character lcd, and use different pio for controling some vhdl functions around to count some frame and display the count value in the lcd

now, i would like to control a memory to store pixels values inside and then make the nios processor able to read these values

i heard about the dma but haven't yet understtod how to use for controling an external memory as a SRAM one

any idea how to do ?